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Chad Brassard avatar image

SOC vs. Battery Voltage

I have 2x6 V batteries with 232 Ah capacity setup in parallel to give me 12v in our travel trailer. I'm fairly sure I have overcharged them in the past and believe their actual capacity to be less than the 232. I installed the BMV 702 to both understand their actual capacity and guard against over or under charge going forward.

I've been testing the setup at home before we go dry camping. Running the heater and fridge and some lights over 8 hours or so, I pulled 23 Ah at a max draw of 5-6A. The battery got down to about 12.35V and the SOC showed 93%. My limited understanding is that 12V batteries shouldn't cycle below around 12.3V. If this is the case then it seems 93% is not correct. It also seems like my batteries are actually compromised in that they come nowhere near the 232 Ah.

I may be missing some factor in this and would appreciate your insight.

Thanks

battery system voltage
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Chad Brassard avatar image
Chad Brassard answered ·

Thanks for the detailed responses Gents. I had a 50:50 chance to get it right - they are indeed in series. Each battery lists a capacity of 232 Ah and that is what I have entered into the BMV, I will let them rest and see what kind of voltages I am getting and I like the idea of the testing the listed capacity at the 20-hr rate of 11.6 A and then re-calibrating the BMV with that capacity. I'll just have to figure out what device(s) I can run at 11.6 A. I don't have an inverter in the trailer so will have to connect an external one to be able to connect something more than the furnace, fridge, lights etc that can draw at a constant 11.6.

Re: #5, I hope you're right JohnC - the trailer is second hand and I didn't realize my converter was single-phase only (was missing a $30 part) and so I boiled off electrolyte last summer and ended up with probably 1/4" of plates exposed and seems like a slight swelling of the side of the battery....

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No problem.

If you want to run a proper capacity test, don't use a fridge as that will cycle on & off - so wont provide a constant load (that's required).

If you have nothing handy, just go to an auto wrecker and buy a few old style incandescent 12v headlight / taillight bulbs - then mix & match until you get a setup that's close to 11.6A - as verified by the BMV.

Most taillight bulbs are around 20W, so about 1.6A, so about 7 wired in parallel should be close.

Mark avatar image
Mark answered ·

Firstly, I think that you have an error with your configuration description. You mention that the 2x 6v batteries are wired in parallel, but they would have to be wired in series to provide 12v. When wiring in series, the capacity of each individual battery does NOT increase, only voltage does.


Assuming that 232Ah is the 20h constant discharge rating, that means that the batteries would take exactly 20h to be fully depleted (starting at 100% state of charge) with a CONSTANT load of 11.6A (232Ah / 20h).

A lead acid based battery is considered fully depleted when the voltage drops to ~1.75 to 1.80v per cell. In a 12v battery setup there are 6 cells, so this equates to ~10.5v to 10.8v (check the data sheet for your particular battery).

If you want to truly 'test' the condition of your batteries in their current degraded state, then I recommend to firstly find a load / combination of loads that will draw exactly 11.6A, then FULLY charge the batteries, then connect the constant 11.6A load and start timing while monitoring the voltage. When the voltage drops to ~10.5 to 10.8v stop timing & stop the test (recharge the batteries ASAP after this test).

Then multiply the REAL time (in hours) by 11.6A to obtain the current battery capacity (including any degradation). I would then recommend to use this new/corrected capacity in the BMV going forward & assess the new corrected/real capacity against your requirements.


Now if your batteries were new & performed as per manufacturers specifications, then if you were to draw a load LOWER than the 20h constant load (<11.6A) then you would actually get MORE than the 20h capacity rating of 232Ah. Conversely if you were to draw a higher load (>11.6A) then you would get LESS that the 20h capacity rating of 232Ah.

The 'Peukert Exponent' entered into the BMV (normally 1.25 for lead acid based batteries) will attempt to compensate for this normal performance behavior - so the SOC % should be as accurate as reasonably possible (based on the parameters entered into the BMV). This general behavior will still apply to your current batteries with 'degraded' performance.


As per advice from JohnC, if you want to compare battery voltage to SOC % (preferably from the manufacturers data sheet for your particular battery) then only do so after the batteries have had a chance to properly 'rest' with no loads or charge sources connected for a good few hours (preferably 6h+), otherwise the voltage can be artificially high or low.

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JohnC avatar image
JohnC answered ·

Hi Chad. Factors/insights, which may/may not apply to you.. :)

1. Your 232Ah is likely a C20 spec, and you'll likely never see that from them in practice. C10 or even C5 spec may be a more useful indicator of what you can get from them.

2. 232Ah should be the value entered in the BMV. You wouldn't be the first to double it because you have 2 batts, but Ah should always be stated at a V.

3. Your 12.35V is quite meaningless unless you've let the batts stand with no load/charge on them for a few hours - overnight is good. Tis why the BMV exists (among other reasons).

4. SOC is a calculated figure, and you can fine-tune it for best results.

5. Your batts may be fine. Don't write em off yet.

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