About paralleling batteries

[Norman Bernstein]

I wrote this in response to a thread down in the Bilge, but figured that those who prefer to avoid the space below the floorboards might find it worthwhile.

---------------

OK, time for a minor dissertation.

Paralleled batteries are a terrible idea.... but I'll freely admit that there are circumstances and installations where it's the only thing you can do. However, in the context of the cruising sailor, at least, I can demonstrate why it's not the preferred way to go.

We'll start with the technical reasons. If you place two brand new identical batteries in parallel, you now have a bank which has the combined capacity of each. For example, a pair of 4D batteries (typically 180 amp-hours each) results in a bank with 360 amp-hours of capacity.

This works fine, as long as the electrochemical balance of the two batteries remains identical... and they will... for a while, at least. However, as the batteries age, the electrochemical balance within each battery may NOT remain equivalent to the other one; due to manufacturing tolerances, etc., one will eventually become weaker, and current from the stronger one will flow into the waker one, in an attempt to equalize them... fruitlessly, of course. Eventually, the imbalance becomes worse and worse... and, in some situations, disaster can result.

In the worst case scenario, the weaker battery can overheat, rupture, and spew corrosive gasses. While it doesn't happen often, I've seen at least one situation where the outgassing completely ruined the entire interior of a Catalina 30... all woodwork, headliner, carpeting and floorboards, all cushions, etc... to say nothing of the acid mess in the battery compartment, which dripped into the bilge.

Will this inevitably happen? NO.. but the possibility exists.

Now for the strategic reason... and this has to do with recharging.

Recharging a lead-acid battery is a highly non-linear process. When the battery is nearly depleted, it can 'accept' a very high charge, but as it approaches a recharged state, the current it can 'accept' reduces. 'Acceptance' means the rate at which you can recharge the battery without the terminal voltage rising to excessive levels.

Back in the old days, a typical recharger was a 'constant-voltage' circuit, i.e., it would provide as much current as the battery could accept without the voltage exceeding some preset point. Of course, no battery charger can supply infinite current, so the voltage would typically sag when the charger was providing it's maximum current, and the terminal voltage would eventually reach the regulation point, where the current would begin to taper exponentially as the battery approached full charge. 'Ferroresonant' chargers worked that way, and they employed a devilishly clever magnetic principle to achieve regulation, avoiding any active circuitry. In some respects, they were, and still are, the best kind of charger.... if you can find one anymore, that is.

Nowadays, charger manufacturers trumpet their 'multiphase' charger circuits. These modern chargers recognize that it's OK to permit the battery terminal voltage rise to a level somewhat higher than the nominal open circuit voltage, while the battery is receiving the bulk of it's charge... and as the 'acceptance' drops, the terminal voltage can be reduced back towards the nominal value. The result is a faster charge (albeit not without some drawbacks). Much of the literature is pure 'hype'; the battery itself has no knowledge of multi-phase charging, and there's no such thing as a perfect charging algorithm. The drawback to a multiphase charger is somewhat reduced battery life. Some years back, I researched this extensively, and queried a number of battery manufacturers... almost without exception, they agreed that a constant voltage charge was kinder to their batteries than an agressive multi-phase charging system.

When you're on a cruising boat, and not connected to shorepower for recharging, your typical option is to recharge off the engine alternator (I'm excluding boats with gensets... lucky stiffs!). The alternator is often rated for a certain number of amps.... for example, the one on my Yanmar engine is rated for 72 amps... but this rating is widely misunderstood. The alternator will only put out 72 amps for a very brief time, when operated at a very high RPM... as the windings heat up, the output drops precipitously... and you wouldn't be redlining your engine just for the sake of recharge. So, as a practical matter, you can forget about the alternator rating; think maybe half that, or one third that, operating at a moderately higher RPM while recharging.

Now, imagine two scenarios: one where we have two large batteries in parallel, being recharged, and one where we recharge each battery separately. In the case of the paralleled batteries, we'd like to recharge at a rate no higher than C/5, where 'C' is the amp-hour rating of the battery. Assuming our previous example, we'd need 72 amps to reach the C/5 level (360 amp-hr / 5) to reach the nominal maximum charging rate. However, we would be absolutely maximizing the load on the alternator, resulting in a lot of heat, which results in a lowered output level.

If we instead charged each battery separately, we'd need only 36 amps to reach the C/5 level... resulting in substantially less heat in the alternator, and more output at reasonable RPM's. The net result: we could recharge both batteries faster, if we did them one-at-a-time, than if we wired them in parallel.

This makes even more sense when we consider sizing the batteries to the load. This example is admittedly specific, relating to a typical cruising sailboat, so your situation may vary.

On my own boat, we typically anchor every evening while cruising, and the batteries are used for refrigeration, lights, pumps (shower, bilge, etc), a TV and/or stereo, etc. The only really significant load is the refrigeration, which pulls about 5 amps, with a 50% duty cycle, resulting in a total draw of 60 amp-hours per day. All the rest of the loads add up to well under 20 amp-hours, so let's presume 80 amp-hours per day. If I was using a single 4D battery, I could last about two days on it... but more like a day and a half, if I wanted to limit the depletion to around 65% of capacity (good for longer battery life).

If I carried two of these batteries, I'd have roughly 3 days on the hook before I needed to recharge... although we almost never stay three days in one place, and more likely are 'moving' (power or sail) every other day.

So, a single 4D battery is actually a very sensible solution, well matched for my loads and habits. I can devise a very sensible schedule which keeps my batteries charged, and always leaves me with reserve power should I need it. Here's how it works: at the end of the day, when we drop the hook, we assume that we've done at least enough motoring (using battery #1) for it to be fully recharged. As soon as I drop the hook, I switch to battery #2. This leaves me with a full battery in reserve, guaranteeing an engine start in the morning (assuming I didn't have a separate starting battery). The next night, I switch to battery #1... and so on. Used in this way, you get the following result:

1) You get the fastest recharge, since you're only recharging a single battery, and not overloading your alternator

2) You always have one fully charged battery in reserve

3) Each battery satisfies your needs for at least 24 hours.

Obviously, there will be times when we either stay in one place longer, or use more battery power.... so we'll run the engine at around 1500 RPM for an hour or so, which puts back enough charge for maybe 12 hours, if necessary.

So, there you have it... a brief explanation of why paralleled batteries are a lousy idea. OK, I fully admit, not everyone's needs are the same... and there are certainly circumstances where paralleled batteries are necessary... but at least you now know why it's a tradeoff.

[wizbang 13]

2 smalls are easier to physically handle.Thoses years go by quickly.

[Ian McColgin]

Having two banks of two essentially car batteries, I already figured out that I need to replace at least a pair at a time, if not all four. I now see that my habit of charging one bank at a time - just intuitive - is a good thing since I don't have one of those smart regulators that can do different things for different banks.

I guess for me, since this is battery year, the choise is two big or 4 regular.

Thank you Norm for adding some considerations.

[Norman Bernstein]

I guess for me, since this is battery year, the choise is two big or 4 regular.

Well, from a 'handling' point of view, the biggest battery you can lug with reasonable ease is a group 31 battery... rated for around 90-110 amp-hours. Economically, a 4D is somewhat better: 160-180 amp-hours, although it's pretty heavy and you'll need to do some huffing and puffing.

Here's a tip for saving some money, though. For conventional wet cell lead acid batteries, Exide manufactures and markets batteries for both the truck/industrial market, and the marine market. The batteries are exactly identical.... except for the terminals. The truck version has posts, while the marine version has studs.

The important fact is that the truck version is at least 40% cheaper! For a few bucks, you can buy post-to-stud conversion terminals. If you can find an Exide dealer, he can probably order you the truck versions.... you can save a bunch of bucks.

[htom]

Excellent post, Norm. With three, you can cycle each of them through using -> charging -> waiting -> using, and always have a full one waiting.

[willmarsh3]

This is a very insightful article. I'd want a low voltage sense alarm on the house batteries to make sure they didn't go completely flat before switching to the fresh battery since running a battery completely flat will drastically shorten its life.

[Dan McCosh]

Only two things missing--We use golf cart batteries, with two six-volt in series, rather than two 12-volt in parallel. Also, many marine-rated alternators do handle high-amperage continuous charging, and would much rather have a higher load.

[willmarsh3]

I've read about some smart alternator voltage regulators that limit the current going into a battery so as to not over tax the alternator. Does anyone here have practical experience with these? Can one really get past the C/ 5 limit with these?

[Norman Bernstein]

That is why myself and most of the world who uses battery's look at the above as about the silliest bunch of tripe ever written about battery's. It certainly doesn't come from experience as this ex-submariner would know.

"...silliest bunch of tripe ever written about battery's."

Well, that was pretty harsh... I guess my 36 years of experience as an electrical engineer doesn't count for much.

Regardless, for the sake of others, allow me to explain. Yes, as I stated in the OP, there are circumstances and situations where one cannot avoid paralelling batteries.... there are limits to the size of an individual one. The US Navy indeed needs to parallel batteries in submarine applications... and has a staff of sailors, with the appropriate equipment, to very regularly monitor those batteries and replace defective cells. Cost or time or labor is NOT an issue, in a submarine.

For the average coastal cruiser, that sort of constant attention and maintenence might be a bit beyond what a person is willing to do... and my comments about charging efficiencies and alternators still apply.

But, what the hell, don't take MY word for it... as long as you're lucky enough to not have happen to you, what happened to a local Catalina 30 owner. The replacement of the entire interior of his boat was pricey.

[Norman Bernstein]

Only two things missing--We use golf cart batteries, with two six-volt in series, rather than two 12-volt in parallel. Also, many marine-rated alternators do handle high-amperage continuous charging, and would much rather have a higher load.

Unquestionably, you can buy much better alternators with higher outputs, less copper loss, etc.... Balmar makes quite a few, and I've known several boaters who have installed them. You still don't get the 'rated' output, unless they're cold and running at high RPM, but you DO 'move the curve' somewhat in your favor. They're not cheap, though.

As for GC batteries, I think they are the best bargain in wet cell lead-acid batteries today... two in series are 225 amp-hours, often at less than the cost of a single 180 amp-hour 4D battery (I'm told you can get them for around $55 each at Sams' Club or Costco). The only reason I don't use them is that I haven't got the height, in the battery ocmpartment of my boat... they're a lot taller than what I'm using.

[Norman Bernstein]

I've read about some smart alternator voltage regulators that limit the current going into a battery so as to not over tax the alternator. Does anyone here have practical experience with these? Can one really get past the C/ 5 limit with these?

C/5 is an arbitrary limit, recommended by battery manufacturers. furthermore, the usual alternator-mounted regulator is a simple constant-voltage design. If you are recharging a badly depleted battery, the charging voltage will drop due to the load, but the maximum current out of the alternator will also drop... when you get closer to fully charged, the regulator kicks in, limiting the voltage... hence the 'CVVC' (constant voltage variable current) designation of that type of regulator.

'Smart' chargers use a combination algorithm... it starts out at 'constant current, constant voltage' (if it can) and then switches to 'constant voltage variable current' at a later point in the charging cycle.

[willmarsh3]

I have three banks off battery's that supply the homestead. Been working like that going on twenty years. Proper and timely maintenance prevents all the issues raised above and if you are a forget it till it fails kind of person the above title post applies.
It takes nothing beyond some sleight effort to locate a bad cell and pull that battery. I usually find one or two a couple of times a year.
Most often I can recondition it and put it back in service as I have battery's past ten years old still going strong.

That is why myself and most of the world who uses battery's look at the above as about the silliest bunch of tripe ever written about battery's. It certainly doesn't come from experience as this ex-submariner would know.

What is your procedure for checking the batteries? Do you measure the voltage on each one? How often? Does your homestead use individual cells? Details please.

[paladin]

In a submarine or similar situation of multiple cells forming a complete battery, or multiple batteries forming a larger one, there are controllers/equalizers/chargers that individually monitor each cell, control the individual charging voltage/current per cell and allow the circuits to operate as an efficient single battery. I have such a system installed using multiple batteries for backup systems here, and they are about 9-10 years old. The basic charger is either solar panels or input from a rectifier that produces 50 amps at 14.5 volts, the regulator/charger/equalizer does the rest.

[George.]

Thanks, Norman. There is smoke coming out of my ears, but I can't understand if what you wrote applies to charging batteries from a solar panel, as opposed to an alternator. Bear in mind I have two panels on the boat as well as in the jungle camp.

[willmarsh3]

Alternators tend to put out as much current as the batteries will accept even if it leads to overheating - unless some sort of current limiting smart charge regulator is used. I fallowed Norman's C/5 rule instead of investing in a smart charge controller.

Solar charge controllers put out up to what the panels will generate. As long as the panel installation is sized smaller than the rated current for the charge controller it will work fine regardless of the size of the battery bank. I use a solar panel rated for 48 watts (4 amps) and a charge controller rated for 10 amps (smallest I could get at the time). It charges a battery rated for 100AH. It is working fine.

[Bill Perkins]

Norman ; thanks for the interesting post .I have 2 wet cell batteries on my boat ,charged by an alternator on the outboard . It's inconvenient to charge the batteries where the boat is stored ,so I try to put her on the rack with the batteries fully charged . In this situation can I get significantly more juice stored per engine hour by charging one at a time ,half the run time for each ?

Should I not routinely use both when starting ? Just use a single even if I choose to run with both on for charging ?I start and run with both on now , at least this keeps them balanced I guess ?If I start charging them separately,does it follow that I should start using them separately (except in emergency).

[Concordia 33]

Testing the density of a lead acid battery is fine as long as you don't have AGM's or Gel Cell's. Any advice about checking the quality of these batteries when in parallel?

[GregW]

.... Balmar makes quite a few..

Norman, thanks for the post.
One little point to be made. Balmar does not make altenators. They simply re-brand altenators (white paint) from any number of manufactures, call them "marine" and add on a zero to the price. :)

[Norman Bernstein]

Norman ; thanks for the interesting post .I have 2 wet cell batteries on my boat ,charged by an alternator on the outboard . It's inconvenient to charge the batteries where the boat is stored ,so I try to put her on the rack with the batteries fully charged . In this situation can I get significantly more juice stored per engine hour by charging one at a time ,half the run time for each ?

I couldn't say for sure, Bill.... it's a function of the alternator output, the size of the batteries, and the state of charge of the batteries. If one or the other, or both, batteries are substantially depleted, then yes, you're definately better off charging them one at a time... or, more accurately stated, you'll recharge quicker IF you're alternator-limited. The problem is that there's really no good way to determine when the charge is complete. A hydrometer would be best, but it's a pain. If you have some sort of ammeter in series with the charging circuit, then you can actually see when the acceptance rate falls... when it falls to a low level, just a few amps or so, you're close to full charge. Luckily, i have a switchable ammeter on my boat, so I can see when the charging rate drops, indicating near completion of the charging cycle.


Should I not routinely use both when starting ? Just use a single even if I choose to run with both on for charging ?I start and run with both on now , at least this keeps them balanced I guess ?If I start charging them separately,does it follow that I should start using them separately (except in emergency).

Whoa, too many variables! Once again, I couldn't say, without knowing 1) your charging capacity, 2) the size of the batteries, and 3) the loads you need to drive (lights, electronics, refrigeration, etc).

Switching batteries into parallel to start an engine definately has it's uses. Back when I owned a sailboat with a Westerbeke engine, I learned that when hot, the Westerbekes were hard to re-start... and I often had to switch the battery selector to the 'both' position. Thankfully, my Yanmar 4JH3-TE in my present boat starts just fine on a single group 27 battery, under ANY conditions.

Incidentally, it does no harm to run both batteries in parallel while charging, since each battery will draw whatever acceptance rate it can from the alternator.... I just wouldn't leave them in parallel without the engine running. A shorted cell isn't a common or likely occurrence, but if it DOES happen, it can be a horribly expensive mess to clean up.

[Norman Bernstein]

Norman, thanks for the post.
One little point to be made. Balmar does not make altenators. They simply re-brand altenators (white paint) from any number of manufactures, call them "marine" and add on a zero to the price. :)

Hahahaha!!!! I wasn't aware of that, Greg.... but, then again, I never installed one. I knew they were very pricey.

It's one of those circular escalations: you add more batteries to your boat, so you need to add a bigger alternator... and then you need to add a bigger shore power charger... and so on, and so on. Personally, I'd just rather use the system designed for the boat, intelligently, and it's never let me down. In 26 years of sailing cruising boats, I've never been without adequate electrical power.... despite NOT having a Balmar alternator, multi-phase charge regulator, monster batteries, etc :)

[Norman Bernstein]

Testing the density of a lead acid battery is fine as long as you don't have AGM's or Gel Cell's. Any advice about checking the quality of these batteries when in parallel?

Beats me. I always felt that AGM's and gel cell batteries simply weren't worth their cost. Yeah, I know, they are position-independent... but I figure that if you have to worry about your batteries tiliting and spilling acid, you've got bigger problems than just the battery.

As for their quicker recharge, I havenh't ever found recharging times to be a problem, in the cruising life. The simple fact of the matter is that cruisers motor a LOT more than they'd like to admit, so they've got plenty of time to recharge.

[willmarsh3]

You may want to search about parallelling batteries on Electric Boats Yahoo group.

[Breakaway]

Thank you for the post. May I ask how the smart chargers/regulators hurt battery life?

Thanks

[Norman Bernstein]

Thank you for the post. May I ask how the smart chargers/regulators hurt battery life?

It'd a 'wear and tear' sort of thing. 'Smart' charging regulators permit a higher charging level (and a higher terminal voltage) when they sense that a battery is fairly depleted.... for a while; this is often called 'bulk charging'. As the acceptance rate declines, they throttle back the current to maintain a more reasonable terminal voltage, and then finally drop the terminal voltage back to a level which could be considered 'float charging'. The net result is a faster recharge.

The battery manufacturers will tell you, when you've got them cornered, that a slow and gentle charging rate is best for the battery... the harder you try to charge it, the more long term damage you end up doing. The best charge would be a 14.3 volt (adjusted for temperature) constant voltage charge. I can't tell you specifically what the damage is... I'm a circuit designer, not an electrochemist... but I've polled the battery manfacturers, and they agree.

It's not as if you can do anything about this, these days. The marketing hype being what it is, and with customers intent on the shortest possible recharge time, it's hard (or impossible) to find a constant voltage charger anymore... to be competitive, charger manufacturers all provide for multiphase charging circuits. 10 or 20 years ago, you could buy a ferroresonant charger; these things were rally cool.... no circuitry at all! They used a very clever scheme involving the magnetic saturation of a transformer to regulate voltage, and this resulted in a nice, gentle, constant voltage charge. Their only drawback: lots of 60 Hz hum... so noisy, I used to have to shut mine off while I was aboard, at the dock. From what I can see, the only place you can buy these anymore would be at flea markets and yard sales.

As a practical matter, the accelerated wear and tear may not make a lot of difference to most boaters. I ordinarily would change my batteries at the first sign of diminished capacity... based on 5 years in my current boat, and six years in my previous one (both with multiphase chargers), I can get about 3-4 years from a set of ordinary wet cell lead-acid batteries, so the amortized cost is pretty low. If I was using constant voltage charging, maybe I'd get another year or two out of them... but the difference simply isn't worth it, which is why you shouldn't worry about it.

One thing I'd recommend avoiding is the use, or, at least, the excessive use, of any 'equalization' feature on your battery charger. This feature lets the battery charger intentionally overcharge a battery by permitting a very high terminal voltage for a short period of time... several hours or so. The presumption is that using this feature will help rebalance the electrochemistries of the individual cells in the battery, when the battery is nearing end of life... and thereby extend their lives a bit further. I consider it a 'desperation' procedure, which unquestionably boils out more electrolyte (and definately demands that you check the electrolyte levels in the cells during and after the procedure!). But frankly, if your batteries are in such bad shape that you're contemplating hitting the 'eqaulization' button, it's probably time to bite the bullet and change the batteries. I won't leave for a long cruise if I have ANY doubt about the condition of the batteries.... changing them in a 'foregn' port is truly a pain in the butt (I had to do it in Rockland, ME, in 2007, when my batteries began to die with very little advance warning... I paid a premium price for the replacements because I couldn't shop around and had to buy them from the local chandlery there).

[Popeye]

.. multiple cells forming a complete battery, or multiple batteries forming a larger one ..

yup , the word 'battery' properly refers to a collection of cells

[Syed]

I have three banks off battery's that supply the homestead. Been working like that going on twenty years. Proper and timely maintenance prevents all the issues raised above and if you are a forget it till it fails kind of person the above title post applies.
It takes nothing beyond some sleight effort to locate a bad cell and pull that battery. I usually find one or two a couple of times a year.
Most often I can recondition it and put it back in service as I have battery's past ten years old still going strong.

That is why myself and most of the world who uses battery's look at the above as about the silliest bunch of tripe ever written about battery's. It certainly doesn't come from experience as this ex-submariner would know.

If I may ask;
What kind of batteries are those? Lead Acid, Ni Cad, ...........?

[willmarsh3]

It'd a 'wear and tear' sort of thing.


One thing I'd recommend avoiding is the use, or, at least, the excessive use, of any 'equalization' feature on your battery charger. This feature lets the battery charger intentionally overcharge a battery by permitting a very high terminal voltage for a short period of time... several hours or so. The presumption is that using this feature will help rebalance the electrochemistries of the individual cells in the battery, when the battery is nearing end of life... and thereby extend their lives a bit further. I consider it a 'desperation' procedure, which unquestionably boils out more electrolyte (and definately demands that you check the electrolyte levels in the cells during and after the procedure!). But frankly, if your batteries are in such bad shape that you're contemplating hitting the 'eqaulization' button, it's probably time to bite the bullet and change the batteries. I won't leave for a long cruise if I have ANY doubt about the condition of the batteries.... changing them in a 'foregn' port is truly a pain in the butt (I had to do it in Rockland, ME, in 2007, when my batteries began to die with very little advance warning... I paid a premium price for the replacements because I couldn't shop around and had to buy them from the local chandlery there).

It seems like a good job for a solar panel which can work all day and supply a slow, gentle trickle charge. The charge controllers that I use don't say anything about three stage charging.

About equalization: do cells in a 12 volt battery tend to get out of balance during normal charge and discharge cycles or do they tend to self correct as long as the battery is fully charged?

[Norman Bernstein]

It seems like a good job for a solar panel which can work all day and supply a slow, gentle trickle charge. The charge controllers that I use don't say anything about three stage charging.

Yeah, solar panels are outstanding, in that regard... a nice slow trickle is exactly what you'd want, in circumstances where you can tolerate a slow recharge.


About equalization: do cells in a 12 volt battery tend to get out of balance during normal charge and discharge cycles or do they tend to self correct as long as the battery is fully charged?

They do get 'out of balance', because the performance of each cell depends on how efficiently the lead dioxide is converted into lead sulfate, and back again, and this is affected by manufacturing tolerances, etc. As I said before, I'm not an eletrochemist, but if you have an interest in chemistry, what is going on is basically this:

Anode (http://www.woodenboat.com/wiki/Anode) (oxidation (http://www.woodenboat.com/wiki/Oxidation)):

http://upload.wikimedia.org/math/2/3/b/23bcf5ef07be251292c9785fcba39ba9.png

Cathode (http://www.woodenboat.com/wiki/Cathode) (reduction (http://www.woodenboat.com/wiki/Redox)):

http://upload.wikimedia.org/math/c/6/7/c6771ae270401599ee0718abd7b70576.png

Over time, the efficiency of the process drops... growths, called dendrites, appear on various spots... flaking lead drops to the bottom of the battery case, etc. From what I understand, it's far worse when the battery is left uncharged, which is why it's never a good idea to let a depleted battery just sit there for a long time. For one thing, a depleted battery has had it's electrolyte diluted (because the sufur compounds have left the acid to form lead suplhate), so the freezing point of the battery rises, and can freeze and be ruined in the winter.

I'm guessing at this point, but I think the argument for equalization is that the overcharge forces the above equations into hyperdrive, in an attempt to restore the cell into equalibrium.

In the worst case situation, a cell can short out.... and if that battery is paralleled with another, the other battery will dump huge amounts of current into the battery with the shorted cell, resulting an an expensive disaster if the battery case ruptures. I'm not saying it will always happen.... I'm only saying it CAN, and it HAS.

[Breakaway]

Thanks Norman. And I agree, based simply on experience, not all of it good, that changing out batteries regularly is a good idea. Cheap insurance versus trying to get "one more year."

[willmarsh3]

I'm not so much interested in a degree in electrochemistry as a practical understanding of what works, what can go wrong so I can develop and improve a maintenance plan.

I'm guessing that the mechanics of how the disaster unfolds would be as follows:

1) One cell deteriorates quicker than the others because of manufacture variagions, sulfation, dendrites and so forth as stated above. It accepts less charge than the other cells.

2) When the battery bank is discharged the bad cell becomes completely discharged e.g. flat.

3) The voltage put out by the bad battery drops to 10 volts or less even though the good cells still have a lot of charge.

4) The good battery starts dumping current into the battery with the bad cell in an attempt to equalize the voltages between the two batteries. Under a heavy load this situation can develop quickly. Examples of heavy load include starting the engine, raising the anchor with an electric winch, running a trolling motor.

5) The battery with a bad cell gets too high a charge on the other cells which causes the electrolyte to give off hydrogen and oxygen. It could cause overheating and even melting of the battery case.

6) In the worst case the hydrogen explodes inside the cell or battery compartment. I've seen a battery with the top blown off because of over charging followed by hydrogen explosion.

Tell me if any part of this is wrong.

[Dan McCosh]

It'd a 'wear and tear' sort of thing. 'Smart' charging regulators permit a higher charging level (and a higher terminal voltage) when they sense that a battery is fairly depleted.... for a while; this is often called 'bulk charging'. As the acceptance rate declines, they throttle back the current to maintain a more reasonable terminal voltage, and then finally drop the terminal voltage back to a level which could be considered 'float charging'. The net result is a faster recharge.

The battery manufacturers will tell you, when you've got them cornered, that a slow and gentle charging rate is best for the battery... the harder you try to charge it, the more long term damage you end up doing. The best charge would be a 14.3 volt (adjusted for temperature) constant voltage charge. I can't tell you specifically what the damage is... I'm a circuit designer, not an electrochemist... but I've polled the battery manfacturers, and they agree.

It's not as if you can do anything about this, these days. The marketing hype being what it is, and with customers intent on the shortest possible recharge time, it's hard (or impossible) to find a constant voltage charger anymore... to be competitive, charger manufacturers all provide for multiphase charging circuits. 10 or 20 years ago, you could buy a ferroresonant charger; these things were rally cool.... no circuitry at all! They used a very clever scheme involving the magnetic saturation of a transformer to regulate voltage, and this resulted in a nice, gentle, constant voltage charge. Their only drawback: lots of 60 Hz hum... so noisy, I used to have to shut mine off while I was aboard, at the dock. From what I can see, the only place you can buy these anymore would be at flea markets and yard sales.

As a practical matter, the accelerated wear and tear may not make a lot of difference to most boaters. I ordinarily would change my batteries at the first sign of diminished capacity... based on 5 years in my current boat, and six years in my previous one (both with multiphase chargers), I can get about 3-4 years from a set of ordinary wet cell lead-acid batteries, so the amortized cost is pretty low. If I was using constant voltage charging, maybe I'd get another year or two out of them... but the difference simply isn't worth it, which is why you shouldn't worry about it.

One thing I'd recommend avoiding is the use, or, at least, the excessive use, of any 'equalization' feature on your battery charger. This feature lets the battery charger intentionally overcharge a battery by permitting a very high terminal voltage for a short period of time... several hours or so. The presumption is that using this feature will help rebalance the electrochemistries of the individual cells in the battery, when the battery is nearing end of life... and thereby extend their lives a bit further. I consider it a 'desperation' procedure, which unquestionably boils out more electrolyte (and definately demands that you check the electrolyte levels in the cells during and after the procedure!). But frankly, if your batteries are in such bad shape that you're contemplating hitting the 'eqaulization' button, it's probably time to bite the bullet and change the batteries. I won't leave for a long cruise if I have ANY doubt about the condition of the batteries.... changing them in a 'foregn' port is truly a pain in the butt (I had to do it in Rockland, ME, in 2007, when my batteries began to die with very little advance warning... I paid a premium price for the replacements because I couldn't shop around and had to buy them from the local chandlery there).

My experience with a multi-stage charger has been that it has greatly extended the useful life of the house bank. The one I use,however is undersized--20 amps for a 225 amp/hour bank. This may put it in the low-current category. The maintenance cycle seems to work very well, as I leave it on pretty much year-round at the dock. When I went to this from a single-voltage charger, the battery life roughly tripled. Oddly, the low-amperage, three-stage charger will recharge from a fairly deep discharge pretty quickly as well. I think overcharging with the old charger resulted in greatly shortened battery life, and the regulation of the three-stage was the main reason for the improvement.

[Syed]

............................................... as I have battery's past ten years old still going strong. .................................................. .................................................. .....


I must greet you for making a world record (IMHO) on lead acid accumulators.

[Breakaway]

.... as I have battery's past ten years old still going strong.

I too have some batteries of that vintage that are still serviceable, but I just keep them around for a handy source of 12-volts and/or as spares. But I dont trust them in full time service. As I understand it, each time a battery charges, some plate material sheds off and sits in the bottom of the case(the plates are elevated above the case bottom for this very reason). When enough shedding occurs, the shedded material pile rises, and shorts out the plates. Shedding is also exacerbated by the motion of a boat--no suspension system like a truck or car. So I dont trust em in the boat, though they do prove useful to have around.

[Dan McCosh]

I did once have a set of batteries that lasted for about 10 years, then were dead at the beginning of a new season. I was impressed with the long life, and attempted to replace them with identical units. Unfortunately, the manufacturer had gone out of business.

[Popeye]

I was impressed with the long life, and attempted to replace them with identical units. Unfortunately, the manufacturer had gone out of business.

pfffftt .. that's what you get for building stuff to last

[Norman Bernstein]

My experience with a multi-stage charger has been that it has greatly extended the useful life of the house bank. The one I use,however is undersized--20 amps for a 225 amp/hour bank. This may put it in the low-current category.

I'd certainly say so! C/10 is a pretty low charge rate... and with that limitation, your charging regulator probably never gets to actually go 'multi-phase' at all... if your battery is meaningfully depleted, the charger doesn't have enough current output to boost the terminal voltage up to the level used for the 'bulk charging' phase! :)

I'd respectfully suggest that the 'extended life' you've experienced with that charger is a consequence of the limited output current... and has nothing to do with the 'multiphase' attribute.


The maintenance cycle seems to work very well, as I leave it on pretty much year-round at the dock.

I do the same... my boat sits at the dock all week long, on shorepower, with the charger on.


When I went to this from a single-voltage charger, the battery life roughly tripled.

Maybe your single voltage charger was simply a bit too 'hot'? The voltage regulation on older chargers was pretty crappy, and not well calibrated.... the Charles Marine chargers, IMHO, were especially crappy... I had to junk TWO of them over the years.


Oddly, the low-amperage, three-stage charger will recharge from a fairly deep discharge pretty quickly as well. I think overcharging with the old charger resulted in greatly shortened battery life, and the regulation of the three-stage was the main reason for the improvement.

Possibly, but unless you were regularly testing your cells with a hydrometer, and reading terminal voltages, I'd respectfully suggest that you have no basis to know. I do know this: if your charger has a max output of 20 amps, connected to a 225 amp-hour battery bank, and it's been 'fairly deeply discharged', then you probably never actually got to the 'bulk phase' of charging... you were current-limited.

No matter... if it's all working very well for you, then you're certainly doing it right! :)

[Norman Bernstein]

Tell me if any part of this is wrong.

I think you've got it precisely right.

[George.]

I must greet you for making a world record (IMHO) on lead acid accumulators.

Syed, meet Dutch. Please don't feed the troll in the upper decks. You may amuse yourself with him in the Bilge, however... :D

[v10builder1]

Folks,

Two comments. First, on Altamira, we have dockside power, a ferroresonant high amperage charger and a 3 stage charger of modest capacity - 10 amps. I just bought the 3 stage last winter. I have an 8D start, an 8D deep cycle (house bank 1) and 2 golf carts in series (house bank 2) and a start/house bank combiner. The advantage of the 3 stage charger is that you can just leave it on all the time and the banks will always reach 100% charge - this can take several days depending on the bank capacity. By the time the ferro charger gets the banks to 100%, you will have to add water - this I guarantee. Also, I don't see anything other than opinion to convince me the 3 state charger is reducing the life of the batteries. If you look into the batteries on 3 stage float charge (13.4V for my system), you will see just a bit of bubbling, and this is necessary to get the banks to 100% (charger off 12.67V on a quality (but uncalibrated) voltmeter.
Second, I dont doubt there was a problem on the Catalina, but the analysis is not correct. If you dont believe me, try this experiment for yourself. The idea that connecting a fully charged battery to a discharged battery causes a large current folw into the discharged battery is baloney. The only way this can happen is if the problem with the discharged battery is actually one or more shorted cells. IT IS NOT CORRECT TO THINK OF THE DISCHARGED BATTERY AS A LOAD ON THE CHARGED BATTERY. The charged battery sees the discharged battery as a very high resistance connection - and only trivial current will flow. There is not enough voltage in the charged battery anyway! How long do you think it would take to charge a battery if the charger only put out 12.4-12.6 volts (best case)? This misconception likely arose from all our experiences jump starting automobiles - the load(s) put on the donor car in these cases is the electrical loads in the dead vehicle, not any appreciable current flowing from battery to battery. Remember, the donor auto also has the alternator on, raising the charge voltage higher, and causing more current to flow. If the original poster wants to charge his batteries the way he wants to, no great harm will come to his batteries, but the ferro will boil them dry if left on for extended periods. Don't ask how I know this. BTW, I am an electrical engineer also. For reference , read up on DC theory - source and load resistances. I really enjoy these forums and the way wooden boat builders do things. Please keep 'em coming.

[Norman Bernstein]

Folks,

Two comments. First, on Altamira, we have dockside power, a ferroresonant high amperage charger and a 3 stage charger of modest capacity - 10 amps. I just bought the 3 stage last winter. I have an 8D start, an 8D deep cycle (house bank 1) and 2 golf carts in series (house bank 2) and a start/house bank combiner. The advantage of the 3 stage charger is that you can just leave it on all the time and the banks will always reach 100% charge - this can take several days depending on the bank capacity. By the time the ferro charger gets the banks to 100%, you will have to add water - this I guarantee.

Sounds to me like your ferrorresonant charger was simply rinning a bit too 'hot' (high voltage). I used one for eight years (1989-1996) and never had that problem.


Also, I don't see anything other than opinion to convince me the 3 state charger is reducing the life of the batteries.

It wasn't MY opinion... it was the opinion of a half dozen battery manufacturers that I polled, back when I was doing professional research on this issue. Furthermore, none of them said that the reduced life was all that significant a factor... just that the more aggressive charge was harder on the batteries than a gently tapering charge.


Second, I dont doubt there was a problem on the Catalina, but the analysis is not correct. If you dont believe me, try this experiment for yourself. The idea that connecting a fully charged battery to a discharged battery causes a large current folw into the discharged battery is baloney.

A discharged battery has a lower terminal voltage.... raising that terminal voltage can only be achieved by supplying current to it. The transfer curve of a lead acid battery is HIGHLY nonlinear, like a silicon junction (although not quite that severe, of course)... small changes if voltage result in large changes of current.

In any event, the danger of paralleled batteries is related to shorted cells. Without a shorted cell, paralleled batteries won't 'blow up'.... but you WILL waste a small; amount of charge energy, if a fully charged one is connected to a discharged one.


There is not enough voltage in the charged battery anyway! How long do you think it would take to charge a battery if the charger only put out 12.4-12.6 volts (best case)?

A very long time, presuming the battery wasn't damaged or defective in some way. That's why chargers put out more voltage.


If the original poster wants to charge his batteries the way he wants to, no great harm will come to his batteries, but the ferro will boil them dry if left on for extended periods.

Strange, that in 8 years, my ferroresonant charger didn't boil any batteries, despite being left on continuously. It has to do with the setpoint voltage. Since it's not adjustable, in a ferroresonant charger, the design must be carefully done... if you've boiled batteries out with one, I suspect it was poorly designed.

[Norman Bernstein]

I'd like to modify my comments a bit, and apologize, at least partially, to v10builder1.

He is correct in one respect: without a shorted cell, he is right that a fully charged battery, in parallel with a severely depleted battery, will not dump any HUGE current from the charged battery to the discharged one... I don't think I made that fully clear. SOME current will flow, but not a large amount.

The danger of paralleling, in the 'technical' sense, only really occurs in the event of a shorted cell. In that case, a battery with one shorted cell is essentially a 5 cell battery with a nominal 10.5 volt terminal voltage... and the fully charged battery will indeed start to dump large amounts of current into the damaged one. It's more or less equivalent to connecting a 6 cell battery to a 15.1 volt source, i.e., a substantial overcharge.... 20% greater voltage than the nominal terminal voltage. Certainly, the battery with the shorted cell will boil it's electrolyte vigorously.... and not all the liberated hydrogen and oxygen is guaranteed to recombine beneath the cell cap.

With two shorted cells, it's a setup for a case rupture, severe electrolyte leak, sulfuric acid fumes, etc.

If anything, my points are a good argument for replacing batteries long before they ever get to the point where flaking lead from the plates and build up enough to short out a cell.

I still stand by my comments about the 'strategic' reason not to parallel batteries.

[willmarsh3]

Strange, that in 8 years, my ferroresonant charger didn't boil any batteries, despite being left on continuously. It has to do with the setpoint voltage. Since it's not adjustable, in a ferroresonant charger, the design must be carefully done... if you've boiled batteries out with one, I suspect it was poorly designed.

Ferro resonant chargers seem to be passe. Perhaps it is due to these variable quality and calibration issues.

[paladin]

I have a couple of old, "garage style" ferro chargers at home at granddad's place. They have been in use since the 40's at least and possibly much longer. They still work, are used periodically to charge up batteries in a barn and for a tractor, and to my knowledge have never failed or boiled a battery. Grandad started hauling away the old battery cells in use at the local telco backup center and storing them and getting a few years life from them and then selling them off as scrap. Free energy for a couple or three or four years. They are connected to an automotive regulator and a generator driven by the windmill that pumps water for the critters. There is a single 12 volt bulb in the barn.

[Norman Bernstein]

Ferro resonant chargers seem to be passe. Perhaps it is due to these variable quality and calibration issues.

I suspect there are multiple reasons why ferroresonant chargers went out of style. For one thing, silicon is cheaper than magnetics these days. A ferroresonant charger needs an expensive transformer, carefully calibrated, and a matched oil-filled capacitor (no longer inexpensive, either). Another issue: they're noisy, with lots of 60Hz hum (not surprising, since the transformer is always operated at the edge of magnetic saturation). Their only virtue, when properly designed, was that they produced a good constant voltage output, calibrated for a gentle tapered charge... good for the batteries. I'm not sorry they're gone.... the noise alone was a good reason to get rid of them.