Measuring 12v Battery Capacity

[SailingFish]

I was wondering if anyone could help with the following question. I am using a small deep cycle 12v battery to run a depth finder, lights and baitwell pump. My boat has no motor and I was wondering what I could use to measure the capacity left in the battery?

[Donn]

This is what I use in my skiff:

http://a1460.g.akamai.net/f/1460/1339/6h/www.cabelas.com/cabelas/en/content/Item/01/42/10/i014210sq01.jpg

"Power Station Battery Box
This unique battery box features a built in battery meter that allows you to determine battery condition at the touch of a button. External connectors provide easy accessory hook-up and eliminate the hassle of opening the box to charge the battery or access the terminals. A plug-in Power Port socket allows use of common 12-volt accessories. The circuit breaker removes the annoyance of fuses."

$40 at Cabelas (http://www.cabelas.com/cabelas/en/templates/home/home.jhtml), under boating/boating accessories/battery accessories.

Or, for less $$:

http://a1460.g.akamai.net/f/1460/1339/6h/www.cabelas.com/cabelas/en/content/Item/01/38/59/i013859sq01.jpg

$23, same place

[kc8pql]

For longest life, it's best not to discharge a deep cycle battery below 50% of it's capicity. The simplest way to keep track of this on a small boat is to install a volt meter. When the voltage drops to 12.2 volts, it's time to recharge. This is only a semi-accurate way to do it, but it works well enough.

Ken

[Norm Bernstein]

There is only one true way to measure the state of charge of a 12V wet cell battery, and that is a hydrometer, which measures the specific gravity of the electrolyte. Obviously, it's a pain in the butt to do, but don't think that voltmeters are really an adequate substitute; a deeply discharged battery may still indicate over 12 volts with no load attached.

The best compromise? Use the voltmeter, but only with a load attached (i.e., baitwell pump and lights running). Anything below 11 volts or so (with a load attached) is probably the point at which I'd want to recharge.

A slightly better solution? 1) Buy a battery big enough to provide the capacity you need for your longest trip, 2) pick up a simple, inexpensive solar panel (the kind sold for automotive and RV purposes, with a low output, like 50mA) and use it to recharge while you're not using the boat.

Norm Bernstein

[frank undewood]

Norm: If you discharge a 100 amp hour battery 50% that results in having to find about 50 amp hours from a charger. A 50mA charger would take about 1000 hours of sunlight. That a lot of sun. Perhaps your decimal point was two off. To get enough solar panel to charge this 50 amp hrs, taking over 100 hours, will cost about $150.00. Solar panels are usually used to augment regular charging systems, unless you get a 5 amp charger and that costs around $600.00.

[Norm Bernstein]

Frank: the original query didn't state what the battery size was, nor did it say much about the actual amp-hour usage. From the description (depthfinder, bait livewell, lights) I was presuming a very small boat used for fishing, and guessed that the total amp-hour consumption in the course of a day's fishing might be modest. The depthfinder might run 0.5A, the bait livewell might be perhaps two amps for maybe 4 hours, and the lights could possibly be 2 amps for an hour or less (running back to the slip at the end of the day). Totalled, that's around 10 amp-hours or less. I believe I slipped just one digit on the solar panel: 500mA, times perhaps 5 sunny hours per day (in a sunny climate, of course) times 5 days, would total up to 12.5 amp-hours of recharge. A panel that could produce 500mA is only 6 watts or so, a pretty small and relatively inexpensive panel. Of course, this presumes just one day's usage per week, so all of these assumptions might just be making an ass out of u and me! *grin*

[Bob Perkins]

I have an 8 Watt solar panel on my sailboat. If I putit out on a weekend - the battery is 100% by the next weekend.

The lower wattage panels make in less expensive because if you buy one that has too much power - then it needs a regulator to prevent over charging the battery. (per something I read somewhere...)

Take Care,
Bob

[Norm Bernstein]

Bob,

I believe the rule of thumb is that if a panel represents less than 1% of the total battery amp-hour capacity, you don't need any regulation; I think the rule is based on the notion that an ordinary wet cell battery will self discharge at nearly that rate, so you never quite reach an overcharge condition).

Therefore, a 6 watt panel (500mA or thereabouts) could be used, without any regulation, on a battery of 50aH capacity... so it would be safe to use with any battery that is group 24 or larger.

[ahp]

Once upon a time there was a lead acid battery maker, I wish I could remember who, that made batteries with a transparent case. Each cell had three little colored balls in tubes molded into the case, bult-in hygrometers. When all balls were up it was charged. When all were down you were discharged, etc.

I had one. It worked. Can someone with a better memory help out.

[Frank Wentzel]

ahp

That would be Exide, also known as The Electric Storage Battery Company and ESB, Inc., at various times. Those batteries were primarily for stationary service rather than automotive or traction (fork lift etc.) service.

/// Frank ///

[Venchka]

OK, so a fellow has a battery or two with no measurable electrolyte. Said batteries being gel cells or AGM. A voltmeter is the only way to measure remaining life/charge? Is the 11 volt with a load figure correct?

Wayne
Very curious in the Swamp. :D

[Frank Wentzel]

JTA

Note that these voltages are with no load after at least a 24 hour stand with no load or charge applied.

/// Frank ///

[NormMessinger]

The latest edition of "Ocean Navigator" has an article on battery care and maintenance with emphasis on AGM batteries.

[frank undewood]

Norm B. I fully agree with your calculations. I just didn't want anyone to believe they could pump up a 100 amp hour battery with a low cost solar panel. Farther down I see a table which seems pretty accurate. I try not to take a deep cell battery below 60% of its rated capacity. I have never seen 10.5 volts at load (what load) as a measurement of capacity left. I know you can start most engines with a battery at 11 volts, but you better make it on the first try.

[kc8pql]

Originally posted by frank undewood:
I try not to take a deep cell battery below 60% of its rated capacity. I have never seen 10.5 volts at load (what load) as a measurement of capacity left. I know you can start most engines with a battery at 11 volts, but you better make it on the first try.Which is why I suggested above that, with a simple system, if you recharge at 12.2 v. you will get the most life from the battery with the least chance of doing any damage.

Ken

[ 11-22-2004, 09:00 PM: Message edited by: kc8pql ]

[Paul Fitzgerald]

Norm, I'm not too sure about that 1% rule for regulators. I managed to boil out 150Ah of battery with a 20 watt unregulated panel (about 1.5 Amps)in the Sydney sun. I have since used a regulator on several boats, with excellent results. They are generally quite cheap (around $50Aus) and my batteries now last much longer as they are constantly trickle charged and dont tend to sulphate. Most of the regulators I have seen also manage to boost the charging voltage in low light, and extend the charging time in early mornings and evenings. I would use a regulator in any fixed solar system.

[ion barnes]

There is a point when things get over anylized.

I helped my neighbour with har vehicle a little while ago. She had all the doors open while she clean it out. Went to start it, nothing! Call Ion.

Turn the key on and the volt meter show 'full charge' but not enough juice to run an interior light. Gave it a jump start, and the ampmeter indicated a charge, but it was good to go indicated by the voltmeter.

Sum of the story is that the battery was dead. She had been driving around with a battery that could barely start the vehicle and was sunning on the altenator output.

My point is that for most of us the ampmeter and voltmeter should be ample to determine the battery condition, but we sometimes fail to recognize the symptoms and what they mean.

Voltmeter should be considered as the quantity of charge. Glass full - glass enpty

Ampmeter is the rate of the glass being filled compared to the rate that it is being emptied.

[RodB]

How about a digital voltmeter (Blue Sea) monitoring the battery then, for the first couple of trips use a hydrometer to get an accurate read now and then while at the same time constantly recording what the digital meter reads as the batter discharges to the max you want to allow. Do this just a few times and then you would have a good fix on exactly where the digital meter will read when the battery was at different levels of discharge.

Not a cheap method, the Blue Sea voltmeter costs over $100...Just a suggestion to calibrate a quality voltmeter to your battery. THe digital voltmeter is way more accurate than the cheap analog ones...

RB

[kc8pql]

Originally posted by RodB:
How about a digital voltmeter (Blue Sea) monitoring the battery then, for the first couple of trips use a hydrometer to get an accurate read now and then while at the same time constantly recording what the digital meter reads as the batter discharges to the max you want to allow.

RBThe problem is that the battery will lose capacity as it ages and goes through discharge/charge cycles. The readings you get when new won't be accurate over time.

[Bruce Hooke]

The battery on my father's sailboat has been charged only with a small solar panel since the system was installed in 1987. I think its about a 20 watt panel. There is no regulator on the charging system other than whatever basic regulartor is built into the panel iteself. We are on at least our second battery since the system was installed but we use pretty basic Sears deep cycle batteries because we are not putting a lot of demand on them, so I'm not surprised that we have used up a battery or two in 17 years.

That said, I just noticed that West Marine sells a basic regulator for $32, so it hardly makes sense not to install one. Back in 1987 it seemed like regulartors were much more expensive so it was more worthwhile to try to do without.

For measuring the charge, I started by using a simple $5 hydrometer from a car parts store to keep track of the battery charge and later added an $12 analog voltmeter to the circuit to help me keep track of things more easily. However, the first line of defense against a dead battey is paying attention to how much power you are using. $100 digital voltmeters are nice but they are by no means necessary!

[ion barnes]

Rapid charging is the death of all bateries but not using it also destroys a battery. And in view of all that, it is a disposable item - it wont last forever.

If you want to spend the beans, get a Link battery monitor. Its a very small computer that looks like a gauge (2 1/8" dia) but when initially set up will track the gradual decline of the battery capacity, the # of charges its taken and a whole bunch more that I dont know.

Cheap analog auto voltmeter and ampmeter will do the job of telling you what you need to know before you spend bucks. If you are going to only charge the battery at home and discharge when out there, then the only things to note are; the begining state of charge and the average rate of discarge per hour and the final voltage at the end of the trip. Compare that with the Amphour rating of the battery divided in half and you you have a good forecast.For example;

If the battery is 75AHr, divided by 2 equals 37.5 AHrs available. Round it to 35 just for the mental math. Your consumtion is 3 Amp per hour, so you have 35 divided by 3 equal 11.6 hours.