SLA and charge balancing

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sbslider's picture
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SLA and charge balancing

This barely fits in this forum category, being it is rechargable Sealed Lead Acid (SLA) batteries. I have an electric lawnmower which uses 3 in series. Its 8 years old, and the originals won’t make a trip around the yard any longer. I purchased a set of 3 to replace them, and found they did not work much better than my 8 year old batteries, they also could not complete a trip around the yard. Maybe because the grass was really long? Anyway, I did some light load testing just to measure capacity. If found that indeed, the new batteries were not really much better than the old ones. I returned them, and the manufacturer said 2 of the 3 were defective.

I purchased a new set, and am running them the batteries individually into a resistive load, set for about a 0.05C rate. When I charged the batteries, there was one that was about 0.2V higher than the other two. I tested this one first, and it got to 18 hours with some remaining capacity, so 20 hours was within reach and I stopped the test. Now testing the second one, and likley due to the 0.2V lower starting voltage, I’m seeing this battery is a couple hours behind the first in terms of voltage vs time into the same load. Now I’m wondering if I had charged the batteries individually to a given voltage, if I would get the same run time out of each.

That makes for good experiments, but long term I want to charge the 3 batteries in series and with the charger that came with the mower and forget about it. Is there any sort of load balancing circuits for SLA batteries, or if I just get them “set” to the same voltage to start, will they stay balanced (enough) for several years like my original set?

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