These used UPS AGM batteries are certainly good for house battery duty.
Some of them might not do very good under high inverter loads, some others are as good as the regular AGMs in this department.
One can get an indication either way by how fast voltage drops under high loads, in excess of 70 amps per battery.
Many are batteries designed to power cell phone towers and such in a power outage, and they get replaced at a set time, even if the power never went out and the batteries were never even cycled, but held at their float voltage the whole time, the latter being very easy on them and they retain a large part of their original capacity.
On some other RV forums their owners are very satisfied with them and are often called used telecom batteries.
Their condition at the time of purchase is likely affected by how many power outages they endured where they were deeply cycled, and the average temperature of the storage building where they resided, and of course how long they resided there.
Some of them seem to not be able to accept high recharge amperages, which is a trait commonly touted as a big advantage to AGM, in some uses, like when one is spinning an powerful alternator very quickly for limited duration to return as much charge as possible in limited time.
I do not really know what little idiosyncrasies they might have as to ideal recharging, or how to determine when they are full and reduce voltage from absorption to float. Most AGMS are considered full when the amps required to maintain absorption voltage taper to a certain amount. Lifeline dictates 0.5 amps per 100Ah of capacity at 14.4v and at 77F.
Northstar Odyssey and Deka AGMS indicate 0.3 amp per 100Ah at absorption voltages of 14.4 to 14.7.
Seeming to find these recommendations from the telecom battery manufacturers is also not readily apparent, but like any deeply cycled lead acid battery, recharging to full as soon as possible, as often as possible, will increase the total number of cycles and the total AMP hours the batteries are able to deliver before capacity diminishes to the point where one removes them from service.
If I were to obtain some of these tellycom batteries, I would drain them to about 12.0 volts and recharge them at 20 amps per 100Ah of capacity upto 14.4v, and then hold 14.4v until amps taper to 1 amp (per100ah of capacity), while looking to see the amount of temperature rise and to see if the sides of the casing were bulging outward. No bulge and no obvious temp increase, I would then continue to hold 14.4v until amps just no longer dropped any further, and use that number of amps at absorption voltage as the determinant for full at that same temperature from then on out.
Another 50% discharge and similar recharge might behave a bit different too depending on the battery and its usage in the time it spent as a UPS battery.
Obviously this takes effort and special equipment, but I personally would not just rely on resting voltage to determine when the battery is indeed full. Different batteries and different temperatures and different charge rates will greatly affect what voltage is held and how long it takes for surface charge to subside.
Much easier to just ensure they spend a few hours at 14.4v each recharge cycle, and they last however long they last.
They should provide good service as long as they get to full charge once every 10 discharge cycles.
ps.. anyone have experience with this brand?
