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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?

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    \$\begingroup\$ Measure the voltage on each battery. Do both get hot during charging? Those extra Ah went up as heat and boiled off water. Good sealed AGM batteries will recombine and recover most of it. \$\endgroup\$ – winny Mar 27 '18 at 15:44
  • \$\begingroup\$ Voltage is reading normal - each battery open-circuit is reading about 12.7V. I don't know how warm the batteries get during "normal" charging (i.e. when they were obviously working fine) but they were definitely hot when I noticed this morning that the charger was still in constant-current mode after 24 hours. \$\endgroup\$ – fdmillion Mar 27 '18 at 15:45
  • \$\begingroup\$ What’s the float voltage while on charger? \$\endgroup\$ – winny Mar 27 '18 at 15:46
  • \$\begingroup\$ I'm not sure. I didn't measure the batteries when they were on fast-charge just now. The charger specifies it floats at 13.5V, I haven't directly verified that. I will let the batteries cool off then I can try measuring the fast-charge voltage and see where it's at - as I understand it, most SLA chargers will switch to float at 14.4v (28.8v for the 24v charger)? \$\endgroup\$ – fdmillion Mar 27 '18 at 15:48
  • \$\begingroup\$ If it really does 14.4 -> 13.5 V, you have nothing to worry about charger-wise. Measure it on your next charge cycle. A shorted cell could have caused this too, but then you would not see 12.7 V open circuit on both. It’s a mystery without further measurement. \$\endgroup\$ – winny Mar 27 '18 at 16:01

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