Voltage variance in 24V AGM battery bank.

[Simonb]

Hi,
My DIY solar off-grid system has been operating successfully for 2 years at around 700 cycles going to float each time. The SOC has never dropped below 80%. The battery system consists of four 6V 400Ah East Power AGM batteries in series for 24V.
I have been checking the batteries directly (I have a Shunt for normal operation) every 6 months and putting them in parallel to balance for 24 hours. The recent balance operation has shown one battery having a lower resting voltage than the others.
Resting at 100% soc at 22 degrees celsius.
Battery 1 = 6.49v
Battery 2 = 6.48v
Battery 3 = 6.49v
Battery 4 = 6.42v
I have individually charged battery 4 to 6.75v and allowed to rest for 8 hours and it settled at 6.42v.
I have reassembled the batteries into series at 24v and operated with the solar charge controller and checked the voltages at Float.
At Float 100% soc at 23 degrees celsius;
Battery 1 = 6.80v
Battery 2 = 6.80v
Battery 3 = 6.78v
Battery 4 = 6.66v
Total Float = 27.1v

So my question is whether the difference in voltage in battery 4 is something to worry about given that the battery always rests at the lower voltage? Is this under performing battery putting extra strain on the other normal batteries.

In buying the battery bank I was aiming for a 5 year lifetime before upgrading to lithium batteries once prices reduce further. So will I get to year 5 with the current voltage difference within the battery bank? Best regards!

 

[snoobler]

Putting your 4 batteries in parallel for 24 hours is a waste of time UNLESS you're also charging the batteries while paralleled. The act of paralleling and sitting transfers almost no charge between them, but it can give the appearance of leveling the voltages. Once reconfigured for 24V and a load is applied, you will see the original discrepancy return.

6.75V is not fully charged. Most AGM in cyclic use take 7.2-7.4V to fully charge them. Check your battery specs for the proper bulk/apsorption and float voltages. Here... I found it:

https://www.ablesolar.co.nz/site/ablesolar/ED6-400.pdf

7.20-7.35V for full charge per the above for cyclic use. 6.75V is only for standby use, i.e., as backup power where they sit floated for extended periods of time between uses one would expect for a backup power situation. With 700 cycles in 2 years, that qualifies as cyclic use.

Recommend you parallel CHARGE your batteries through a full bulk/absorption and float cycle at the proper voltages.

Reassemble at 24V and resume normal operation ensuring charge parameters: 28.8-29.4V absorption and 27V float. Take voltage readings at absorption and float voltages on the next cycle and then take bottom of the range voltages, i.e., after overnight use and before charging starts in the morning.

With the above readings, you can better characterize the relative health of the batteries.

Also consider that after 700 cycles to "80%", your battery capacity has likely decreased to 80-85% of rated, so if you're discharging 20% of rated, or 80Ah/day, you are now cycling deeper. 80% of 400 is 320Ah. 80Ah is 25% of 320Ah. Thus your cyclic life is deteriorating more rapidly due to greater depths of discharge.

It would be prudent to adjust your capacity in your battery monitor from 400Ah to 320Ah to better reflect utilization.