My setup: two 12V 20Ah AGM batteries in series for 24V. Typically discharged to about 50% depth on average about 3 times per week (less often in winter, batteries are kept indoors). The current set of batteries is about 3 years old, so are likely reaching the end of their lifespan (average of about 150 cycles/year, so around 450 half-depth cycles now.)
I have a 24V 2A smart charger that is relatively new (about 2 months old), since my old charger started making weird sizzling noises. The new charger has worked fine for the time I've had it. Normally, the batteries are always connected to the charger when not in use, and the charger will show a green "Charged" indicator, which according to its specs means it's delivering an appropriate float charge.
Yesterday morning, I connected the batteries to the charger as usual, and it went into charge mode. I left it as usual. This morning, about 24 hours later, I noticed the charger was still in "charge" mode. The LED was still blinking red (indicating charging) and the transformer was quite warm to the touch (typical when it's delivering its full load). The batteries were also quite warm to the touch - not uncomfortably warm, but definitely above room temperature.
Which is more likely?...
- that the cells just reached the end of their lifespan and thus their voltage characteristics during charge differed enough to confuse the charger, or
- that the charger itself is defective and kept charging the batteries even after their voltages reached the appropriate cutoff point?
The cells were functioning OK during the last cycle, in other words I didn't see any significant capacity drop or voltage drops. Obviously though, I now need to assume these batteries are probably either seriously degraded or otherwise useless. Basically, we have a 20Ah cell that was discharged to about half its capacity, then charged at a full 2A for the next 24 hours (48Ah). That extra energy went somewhere - and I think it's a safe bet that even though it's a sealed AGM cell, it probably lost some of its electrolyte.
But the reason for this question is to figure out - do I need to also replace the charger? I'd hate to buy a brand new set of cells and then have them also immediately damaged by the charger.
Perhaps a more technical direct question: do the voltage characteristics during charge change significantly in an AGM cell such that, when the cell is nearing end of life, the voltage might not reach the level that chargers will read as a full battery?