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Thread: How many amp hours do my batteries have?

  1. #1
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    Default How many amp hours do my batteries have?

    Last year I bought two 12 volt Interstate lead acid deep cell batteries for my electric powered launch. At the time, I couldn't afford the expensive AGM batteries I wanted.
    These are rated as dual purpose, meaning they can be used for running electric motors like mine or used as starting batteries for gas engines.
    There is no amp hour rating listed on the battery and, when asked, the salesman gave a wild guess at 100 aH.
    These two 12 volt batteries are hooked together to make a 24volt setup.
    The markings on the battery are:
    SRM-29
    675CCA
    845MCA
    645 Cranking amps
    210 Reserve

    I find a lot on the internet on amp hours but not how to determine what aH are in my batteries from the information provided.
    I also assume that the aH are the same as a 12v as two hookup up as a 24v.
    Next year I hope to update to some nice AGM batteries, but for now I'd like to know what I've got.
    Anyone out there savvy with these things?
    I was born on a wooden boat that I built myself.

  2. #2
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    Default Re: How many amp hours do my batteries have?

    The reserve capacity is usually slightly higher than double the amp hour rating. To be safe, I would assume the are 80-85 amp hours.

  3. #3
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    Default Re: How many amp hours do my batteries have?

    Great info Sean! I didn't know that.

    Thing is Rich - these batteries really aren't deep cycle. Oh, probably better than a regular car battery, but no where near the capacity of true deep cycle. Yes, AGM are expensive, but you can also find conventional lead-acid deep cycle batteries.

    The Interstate store in Williston often has "blems" (somehow not pretty) that they sell at reduced cost. Also check online - as even with shipping you can often find some better prices.

    However - if you come up to Williston, you could go out to lunch with Grant & me!
    "If it ain't broke, you're not trying." - Red Green

  4. #4
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    Default Re: How many amp hours do my batteries have?

    Also, Amp hour figures are a bit "it depends" on how many amps you want over how short a time period. The lowest amp hour figure being when you want a continuous high draw, vs intermittent lower draw. So flat out you might get two hours out of them, and half throttle, being where you are drawing exactly half the amps, might be 2.5x as long, rather than the 2x you'd expect... unless there's severe Murphy's law type consequences of course, then it's 1.5
    2019: returning from being sidelined with medical probs, crossing fingers worst is over, still in "armchair enthusiast" mode for time being.

  5. #5
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    Default Re: How many amp hours do my batteries have?

    Quote Originally Posted by Garret View Post
    Great info Sean! I didn't know that.

    Thing is Rich - these batteries really aren't deep cycle. Oh, probably better than a regular car battery, but no where near the capacity of true deep cycle. Yes, AGM are expensive, but you can also find conventional lead-acid deep cycle batteries.

    The Interstate store in Williston often has "blems" (somehow not pretty) that they sell at reduced cost. Also check online - as even with shipping you can often find some better prices.

    However - if you come up to Williston, you could go out to lunch with Grant & me!
    Might take you up on that in the spring!
    I was born on a wooden boat that I built myself.

  6. #6
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    Default Re: How many amp hours do my batteries have?

    Lead acid batteries are about 15 watt hours per pound.

  7. #7
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    Default Re: How many amp hours do my batteries have?

    The "official" Ah rating is more or less meaningless for electric propulsion with lead acid batteries since it is a discharge rating over 20h. Because of the Peukert factor what you need to know is the capacity at your average A draw. This is usually obtained from the manufacturer as a C table and you must translate it.

    The reserve capacity as stated on your batteries is defined as the average number of minutes you can draw 25A before the battery drops below 10.5V. That 210 reserve means you can draw 300W for 3.5h. That makes 600W at 24V for 3.5h or 2.1kWh. This stresses the battery considerably so realisticly speaking around 2 hours at 25A should be enough if you want reasonable life out of them. That reasonable life will be shorter than with true deep cycle batteries because the plates are thinner and they are actually made to be discharged only 10% but the upside is they have better Peukert and will handle power bursts much better.

    Basicly it comes down to your usage profile. How many amps do you draw at your chosen cruising speed under normal conditions, for how long do you want to do it, and what is your attitude vs. speed bursts and depth of discharge.

  8. #8
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    Default Re: How many amp hours do my batteries have?

    Quote Originally Posted by Rumars View Post
    The "official" Ah rating is more or less meaningless for electric propulsion with lead acid batteries since it is a discharge rating over 20h. Because of the Peukert factor what you need to know is the capacity at your average A draw. This is usually obtained from the manufacturer as a C table and you must translate it.

    The reserve capacity as stated on your batteries is defined as the average number of minutes you can draw 25A before the battery drops below 10.5V. That 210 reserve means you can draw 300W for 3.5h. That makes 600W at 24V for 3.5h or 2.1kWh. This stresses the battery considerably so realisticly speaking around 2 hours at 25A should be enough if you want reasonable life out of them. That reasonable life will be shorter than with true deep cycle batteries because the plates are thinner and they are actually made to be discharged only 10% but the upside is they have better Peukert and will handle power bursts much better.

    Basicly it comes down to your usage profile. How many amps do you draw at your chosen cruising speed under normal conditions, for how long do you want to do it, and what is your attitude vs. speed bursts and depth of discharge.
    Thanks. That's a lot of useful info. I usually only cruise for an hour at a time at a speed of 4 mph and the batteries are 10% depleted at the end of that time. I'll have to check the amp usage next time I'm out. Your explanation is all the more reason for me to get the good AGM deep cell batteries next year.
    I was born on a wooden boat that I built myself.

  9. #9
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    Default

    Rich,
    Sounds like your battery bank is just fine for your use. You're not coming close to using more than half the capacity. Why change?

    Kevin




    Sent from my iPhone using Tapatalk Pro
    There are two kinds of boaters: those who have run aground, and those who lie about it.

  10. #10
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    Default Re: How many amp hours do my batteries have?

    Quote Originally Posted by Breakaway View Post
    Rich,
    Sounds like your battery bank is just fine for your use. You're not coming close to using more than half the capacity. Why change?

    Kevin




    Sent from my iPhone using Tapatalk Pro
    I like the way you think! I just keep hearing from the makers of electric motors that you're best off with very expensive lithium batteries, or at the very least, AGM batteries.
    Perhaps I will just keep my present batteries for a couple more years. I was curious about the amp hours they have. From the answers here, that's really not that important with the short time cruises I take. I can't see myself out there for more then two hours under power.
    I was born on a wooden boat that I built myself.

  11. #11
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    Default Re: How many amp hours do my batteries have?

    Might note that lead-acid batteries in general have the highest energy storage capacity. AGM is a derivative of lead-acid, which has the advantage of higher amperage, quicker recharge times, and greater resistance self-discharge. Lithium ion stores more energy per pound of battery. In a boat, lead-acid make the most sense, since the weight disadvantage is either negligible, or even an advantage. Submarines use lead-acid. For an electric boat, IMO, the way to go would be golf-cart batteries, often used in fork-lift trucks. They are robust and designed for strenuous charge/discharge cycles. Also usually inexpensive.

  12. #12
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    Default Re: How many amp hours do my batteries have?

    Quote Originally Posted by Rich Jones View Post
    I like the way you think! I just keep hearing from the makers of electric motors that you're best off with very expensive lithium batteries, or at the very least, AGM batteries.
    Perhaps I will just keep my present batteries for a couple more years. I was curious about the amp hours they have. From the answers here, that's really not that important with the short time cruises I take. I can't see myself out there for more then two hours under power.
    Then definitely keep what you have until they won't do it any more. As long as your voltage is high enough (voltage below the minimum specified by the motor mfr is bad for it), electricity is electricity & the motor does not know or care whether it came from a lead acid or LI battery.
    "If it ain't broke, you're not trying." - Red Green

  13. #13
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    Default Re: How many amp hours do my batteries have?

    Definetly keep them until dead. The manufacturers recomend lithium or very good AGM because sooner or later some customer is going to use the motor at full capacity. The problem goes as follows: 1.9kW motor 83% efficient (probably motor+controller combined) means that at 24V it is going to pull 95A out of the battery. Depending on how the controller is set it can pull even more for a very short period of time, like 200A for 5s. To pull that sort of amps continuously plus the bursts you need either a chemistry that supports it or do tricks like rolled thin plate AGM. It is possible to take such amps out of flooded lead acid traction batteries but the bulk and weight would be prohibitive for small boats.
    Example: the popular Trojan T105, 6V, 225Ah, 62lbs. 810cuin, 1.50kWh of total energy. The manufacturer says that at 75A draw it has a reserve of 115 min (1.9h). At 95A draw it will be more like 1h. So to go two hours at full bore you need 8. This would put you somewhere in the 80% discharge zone so to be safe you buy 12. Now you have to store 744lbs and 9720cuin somewhere safe and you are taking 18kWh in the boat to be able to take out 4.5kWh safely in 2h.

    If you don't use the motor to capacity you have no reason to buy a lot of expensive batteries. Your instruments will show you what you need. If you use 500W to cruise at 4mph that is a draw of 20.8A at 24V. For the T105 that is the C10 rate, meaning they will last 10 hours at that rate. Cruising 2h leaves you with confortable reserves for hard manouvering and battery life. Still more then double the weight than what you have now, but much more easy on the wallet than lithium or AGM.
    Next time you go out make note of the amps drawn at different speeds and of the total energy used during the trip. Then you will know how much battery you need.

    To go back to your existing batteries a 210 minutes reserve means that in 3.5h they can put out 1050Wh of energy (25Ax12Vx3.5h). Given that these are thin plate starting batteries that should be pretty close to total stored capacity. If we asume a 1200Wh total capacity and discharge it in 20h with 5A the nameplate would be 100Ah. At 1050Wh the nameplate would say 86Ah. That's about what your batteries are, approx. 90Ah.

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