Greetings BITOG Council, I come seeking your thoughts and advice. 3 weeks ago, I scored a pair of OEM VWAG Varta AGM batteries in H8 form factor with a 105Ah capacity. They were manu'ed in 2017 and sat on a pallet for a couple of years I assume. They will be used in UPS backup service. Now the problem became apparent right off the bat while doing the initial equalization charge. One battery takes a charge well and as the SOC rises so too does the internal resistance and thus the voltage from the 'taper current' charger. Perfectly normal and expected. It also does not heat up. The second battery is the strange one. It will sit there and eat current (low internal resistance) and warm up. Not hot, but rather warm.
Using transformer chargers (taper current), at 2amps, the voltage will not exceed 13-13.1V, and will drop to 12.9V if another AC appliance on the same mains as the charger turns on. At 6amps the battery will not exceed 13.2V. At 10amps the voltage will not exceed 13.8V and at 16amps the voltage will not exceed 14.2V. The battery only warms up. Could this be hard sulfation? A sidebar observation is that when I put the 16amps on it (thus higher voltage), the DVM seems to swing up and down as though plate sulfation was being reversed by it.
Otherwise, that weird battery performs well. It'll take 1400watt loads like a champ, it's resting voltage is pretty normal but it's way too different to be series up with the other one and charged by the UPS. The good ends up near 15V (briefly before i disconnected it) while the weird battery sits at below 13V.
What should I do? I'm leaning towards taking it back and finding a match for the other one, maybe with a closer serial number. Or should I stick it out and try and 'rehabilitate' it. They're brand new batteries so the differences (in chemistry) is surprising.