question

vulture avatar image

Are my SLA batteries ruined?

I've had them for almost two years. They are generic alibaba type SLAs. It seems like the capacity is getting really bad, such than the SOC is more than 25% off compared to the rated capacity. I never let them go lower than 60% according to the original SOC calculation. I'm not in an area with a lot of daylight hours currently. They're supposed to be 12v 250AH SLAs, and I have 4 of them in a 24v configuration to be 24v at 500AH. Here are my graphs and charge/BMV settings, as you can see it goes to 24v at like 80% SOC. 100% SOC sync happens when the batteries are getting like 300watts. Midpoint looks okay so shouldn't be a bad cell. Is there anything I can do aside from halving the BMV amp hours?


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SOCAGM Batterybattery capacitylead-acid
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2 Answers
klim8skeptic avatar image
klim8skeptic answered ·

Adaptive absorption, or fixed time?


Peukert exponent of 1.10 :(.

The charged voltage could be higher if you are only charging with solar.

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snoobler avatar image
snoobler answered ·

@kim8skeptic has it.


To answer your original question, since the BMV and the battery have been out of sync, it's possible you've deeply discharged the batteries thinking you were above 60% while they were actually much lower, especially since you're in an area without a lot of daylight hours currently.


On the graphs you provided, I do not see indications of a false sync. Those are indicated by an abrupt jump to 100%, so you may really be dealing with degraded capacity.


Lastly, 2 years out of cheap Chinese SLA may be all they're good for, and they may never have met ratings.


Recommend the following changes:


BMV:

1642166730284.png


The idea behind setting SoC to 50% is to ensure that the next time you see 100%, it's because it's actually met the full charged criteria.



MPPT:

1642167066757.png



You should really consider generator charging if it's an option.


Other concern:


While not outrageous, your midpoint variation is of concern. Bottom voltage is clearly always a little higher than top voltage. I assume they're wired in a standard 2S2P configuration, and you're just monitoring bottom voltage on one string of 24V. Recommend you measure all 4 12V near the end of their absorption cycle and shuffle them in the bank. Match the two highest voltages in a 24V string and the two lowest voltages in a 24V string and then parallel the two strings at the ends. Also, make sure you're main leads are connected across the bank, i.e., (+) is on one string and (-) is on the other string.


Lastly, you may want to correlate resting voltage with SoC as follows:


Take the system completely offline first thing in the morning before charging begins. Allow the battery to sit at rest (no charge or discharge) for 2 hours. Measure system voltage and 12V votlages. Compare to this chart:


1642168184092.png


Hopefully you'll see them matching within 10%.


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Thank you so much for the great advice. I will implement all these suggestions. Fingers crossed.