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Old 11-07-2016, 17:52   #1
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How Do You Calculate Battery Bank Total Amp Hour Capacity?

Ok I have a 12 Volt House Bank of 12-6 Volt Deka Pro Master Flooded Cell Batteries. I thought the total capacity was the sum of each battery's "Amp Hour Rating" (mine are 230s) times the batteries in the bank (12). But I have been told that the true number is roughly half of that. What calculators or formulas do you use to determine the Total Banks Capacity? I need the number for my Maretron N2K system to accurately keep track of my State of Charge/Amount of tie remaining in my house battery bank.

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Old 11-07-2016, 18:13   #2
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Re: How Do You Calculate Battery Bank Total Amp Hour Capacity?

Two 6 volt batteries in series increases the voltage to 12 volts, but does not increase the amp-hrs (capacity). So your 6 volt battery has 230 amp-hrs, but won't do you any good in a 12 volt system. Two 6 volt batteries in series make 12 volts, but is still 230 amp-hrs. Now you start adding the pairs of batteries in parallel, which does not increase voltage, but you add the amp-hrs.
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Old 11-07-2016, 18:37   #3
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Re: How Do You Calculate Battery Bank Total Amp Hour Capacity?

1380 ah
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Old 11-07-2016, 18:56   #4
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Re: How Do You Calculate Battery Bank Total Amp Hour Capacity?

It's not 1380. 2 - 230 amp 6 volt batteries connected together to give you 12 volts = 230 amp hours. hook those together with two more 6volt batteries you'll get 460 amps.
Go to the Trojan battery website. they've got great diagrams and information. I just finished upgrading my system to 4 - T125's and i've got 460 amp hours.
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Old 11-07-2016, 18:58   #5
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Re: How Do You Calculate Battery Bank Total Amp Hour Capacity?

guess i should have read the first post more thoroughly....12 - 6 volt Deka's....wow! my little CD36 would list HARD with that weight.
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Old 11-07-2016, 19:05   #6
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Re: How Do You Calculate Battery Bank Total Amp Hour Capacity?

1380 Amp/Hours but you should use no more than half that, or 690 AH, 50%.
On my boat I used 70% as a target and got long lives out of the house bank: Same batteries, Deka Golf Cart, 6 Volt, Deep Cycle, etc.
6 years was my best results.
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Old 11-07-2016, 19:07   #7
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Re: How Do You Calculate Battery Bank Total Amp Hour Capacity?

Amp hours is dependent on voltage. The actual unit of energy is Watt hours.

Just remember that Amps = Watts/Volts (or Watts = Amps x Volts).

If you have a 6 Volt x 230 Amp Hour battery, that is 1380 Watts hours (or 1.380kWh ) of energy.

You have 12 of these batteries, so the total is 16,560 Watt hours.

Since you have them wired up to provide 12 Volts, that 16560 Watt hours = 1380 Amp hours.

If you wired them up in parallel, you would have 2760 Amp hours at 6 Volts.

If you wired them up as a 24 Volt system, you would have 690 Amp Hours at 24 Volts.

If you connect them to an inverter and feed 110 Volts to AC equipment, you have about 12.5 Amp hours available.

Bottom line, Amps and Amp Hours are voltage dependent - not absolute measures of energy. If working with different voltages, calculate everything in Watt hours and convert back to Amp hours at the end.
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Old 11-07-2016, 21:51   #8
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Re: How Do You Calculate Battery Bank Total Amp Hour Capacity?

In other words 1380 Amp Hours @ 12 volts
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Old 12-07-2016, 08:25   #9
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Re: How Do You Calculate Battery Bank Total Amp Hour Capacity?

Quote:
Originally Posted by StuM View Post
Amp hours is dependent on voltage. The actual unit of energy is Watt hours.

Just remember that Amps = Watts/Volts (or Watts = Amps x Volts).

If you have a 6 Volt x 230 Amp Hour battery, that is 1380 Watts hours (or 1.380kWh ) of energy.

You have 12 of these batteries, so the total is 16,560 Watt hours.

Since you have them wired up to provide 12 Volts, that 16560 Watt hours = 1380 Amp hours.

If you wired them up in parallel, you would have 2760 Amp hours at 6 Volts.

If you wired them up as a 24 Volt system, you would have 690 Amp Hours at 24 Volts.

If you connect them to an inverter and feed 110 Volts to AC equipment, you have about 12.5 Amp hours available.

Bottom line, Amps and Amp Hours are voltage dependent - not absolute measures of energy. If working with different voltages, calculate everything in Watt hours and convert back to Amp hours at the end.
Very useful!
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Old 12-07-2016, 08:40   #10
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Re: How Do You Calculate Battery Bank Total Amp Hour Capacity?

I used to be close to Trojan Battery, and visited their plant south of Los Angeles on two occasions. Bill Brecht, who was the VP of Engineering, provided some battery life data for their 6V batteries which I always found fascinating.

The T-105 batteries (105 reserve minutes) were capable of cycling 700 times before reaching 50% of their stated battery capacity. That was at a discharge of 25A I believe. The capacity for each cycle was below 105 minutes initially as the battery matured, and then stayed very close to 105 minutes for several hundred cycles, then finally diminished beyond about 500 cycles.

The test, as I recall, was to fully charge the battery, then apply a 25A load at around 80 deg F (water bath) until the voltage reached 5.25V (since it was a 6V battery). After discharge, the battery was charged immediately to full charge, then the cycle was repeated.

Note that each cycle was intense (high rate of discharge) which was far easier than using a 5A load that would decrease the battery's state of charge far more at the end of the cycle. But that's the official BCI test.

My point is that we have this mythical idea that a 50% discharge is so vital for optimum battery longevity, but I don't think there a lot of evidence that this is so. At 100% depth of discharge, these classic batteries go 700 cycles.

Putting it another way, if you discharge to 40% SOC, you get 20% more energy from a fully-charged battery, right? Is it possible that you'd suffer less than a 20% reduction in cycle life? I think it might be so.

Cheers,

Chuck Hawley
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Old 12-07-2016, 09:31   #11
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Re: How Do You Calculate Battery Bank Total Amp Hour Capacity?

Quote:
My point is that we have this mythical idea that a 50% discharge is so vital for optimum battery longevity, but I don't think there a lot of evidence that this is so. At 100% depth of discharge, these classic batteries go 700 cycles.
Mythical....?
Lead acid batteries are pretty old technology with 100 years + plus of data and knowledge.

It is well explained in this book: A previous version had 67 pages on just battery selection, technology and mainteance.

https://play.google.com/store/books/...4IZA&gclsrc=ds

Myth, don't think so..
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Old 12-07-2016, 09:35   #12
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Re: How Do You Calculate Battery Bank Total Amp Hour Capacity?

I believe that the Lifeline manual has a chart that depicts depth of discharge and life.
It's a pretty dramatic line, look it up.
50% is a number I believe that was picked as a happy medium is all, someone thought the life limit cycling at 50% was acceptable is all.
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Old 12-07-2016, 14:38   #13
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Re: How Do You Calculate Battery Bank Total Amp Hour Capacity?

Quote:
Originally Posted by Chuck Hawley View Post
I used to be close to Trojan Battery, and visited their plant south of Los Angeles on two occasions. Bill Brecht, who was the VP of Engineering, provided some battery life data for their 6V batteries which I always found fascinating.

The T-105 batteries (105 reserve minutes) were capable of cycling 700 times before reaching 50% of their stated battery capacity. That was at a discharge of 25A I believe. The capacity for each cycle was below 105 minutes initially as the battery matured, and then stayed very close to 105 minutes for several hundred cycles, then finally diminished beyond about 500 cycles.

The test, as I recall, was to fully charge the battery, then apply a 25A load at around 80 deg F (water bath) until the voltage reached 5.25V (since it was a 6V battery). After discharge, the battery was charged immediately to full charge, then the cycle was repeated.

Note that each cycle was intense (high rate of discharge) which was far easier than using a 5A load that would decrease the battery's state of charge far more at the end of the cycle. But that's the official BCI test.

My point is that we have this mythical idea that a 50% discharge is so vital for optimum battery longevity, but I don't think there a lot of evidence that this is so. At 100% depth of discharge, these classic batteries go 700 cycles.

Putting it another way, if you discharge to 40% SOC, you get 20% more energy from a fully-charged battery, right? Is it possible that you'd suffer less than a 20% reduction in cycle life? I think it might be so.

Cheers,

Chuck Hawley
There are 2 related but different concepts here that are being confused.

State of Charge generally refers to the amount of battery capacity that is available to use. If we start with a battery with 100 amp hour capacity and draw 10 amps for 5 hours, we now have a battery with 50 amp hours available or a 50% SOC. Charge that battery properly and it will return to 100% SOC.

That is different from an older battery that is reaching its end of life. As batteries are discharged and recharged the total capacity of the battery declines. Higher quality batteries can experience more discharge/charge cycles than poorer quality batteries. How the battery is charged is also important. Improper charging causes a battery to age more rapidly with a subsequent loss of capacity. But this is not SOC.

Going back to the 100 amp/hour battery. Eventually it will no longer be a 100 AH battery. At some point it will be effectively a 90 AH battery, and then an 80 AH battery and so on.

If we consider the battery when its capacity is 90 AH and then discharge it at 10 Amps for 5 hours we will have used 50 AH and the battery will be at 44% SOC. Do the same with the battery at 80 AH capacity and the SOC of the battery will be 37.5% SOC. When properly charged the battery will return to 100% SOC, but because the battery is older and has used some of its discharge/charge cycles, at 100% SOC the battery will only have 90 or 80 AH available.

For more information take at look at RC's site: Welcome To MarineHowTo.com Photo Gallery by Compass Marine How To at pbase.com

John at Affordable Adventure Cruising has also written about this.
https://www.morganscloud.com/

And finally, Steve D'Antonio also writes about electrical issues
Ask Steve | Steve D'Antonio Marine Consulting
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Old 12-07-2016, 17:41   #14
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Re: How Do You Calculate Battery Bank Total Amp Hour Capacity?

Quote:
Originally Posted by Chuck Hawley View Post
The test, as I recall, was to fully charge the battery, then apply a 25A load at around 80 deg F (water bath) until the voltage reached 5.25V (since it was a 6V battery). After discharge, the battery was charged immediately to full charge, then the cycle was repeated.

...
My point is that we have this mythical idea that a 50% discharge is so vital for optimum battery longevity, but I don't think there a lot of evidence that this is so. At 100% depth of discharge, these classic batteries go 700 cycles.

Putting it another way, if you discharge to 40% SOC, you get 20% more energy from a fully-charged battery, right?

Surely, that is only 10% more energy compared to discharging to 50% SOC?

Is it possible that you'd suffer less than a 20% reduction in cycle life? I think it might be so.
Here's a typical curve. You're correct - you get about 10% more energy per cycle going down to 40% SOC and you get about 10% reduction is cycles - so it's a wash.



But you're operating at the bottom of the log curve there.

You shouldn't be regularly discharging to either 40 or 50% SOC. That is the minimum you want to be hitting and only occasionally.

You get twice as many cycles at 30% DOD compared to 50% DOD. And from there on up, it just keeps getting better and better.

Which is why I have a big battery bank and try to keep my batteries above 70% SOC as much as possible.

I am OK with going to 50% or less when necessary, but that is a rare occurrence, so the effect of doing so is not doing a lot to reduce my battery life.
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Old 12-07-2016, 18:59   #15
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Re: How Do You Calculate Battery Bank Total Amp Hour Capacity?

Quote:
Which is why I have a big battery bank and try to keep my batteries above 70% SOC as much as possible.
Great minds think alike



Quote:
. On my boat I used 70% as a target and got long lives out of the house bank: Same batteries, Deka Golf Cart, 6 Volt, Deep Cycle, etc.
6 years was my best results.
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