On a 2kw Axpert I have two gel batteries in series, 120a each.

With a puny load of 100w after an hour or so the voltage plummets out of nowhere from 24.6v to 22.4v and the low voltage alarm starts and shortly after the inverter switches off.

If I turn the inverter off a multimeter shows the voltage goes back to 24v, turn the inverter on and back down to 22 we go, confirmed by a multimeter. These batteries are 3 months old, are gel batteries really that useless or is something wrong here?

Have you had a long outage where the batteries have had to service a load (even if not that large) for a long time? Or a situation where there hasn't been time for the batteries to charge fully between load sheds?

The reality of that type of battery is that once they are discharged past 50% they start taking real punishment and their effective capacity drops quite rapidly.

Also although what you are backing up is only 100W, the Axpert (or any other inverter) also consumes some load to keep itself up and running. 

Usually those batteries can't be charged as quickly as lithiums can be. So especially when the load shedding stage is high, you can get a situation where the battery discharges for 2, sometimes 4 hours, and then there's another outage 3.5 to 4 hours later and the batteries have not fully recovered, so under heavy load shedding the chance of the batteries not having been fully recharged, and then dropping to too low a SOC increases.

Another thing I have seen with this type of set up: Batteries are not just batteries. They all look the same and have similar numbers, but they are designed for different applications. So get batteries from a business that deals in back up systems, has a track record, and who knows what type of battery to use. Knowledgable specialists offer real extra value. 

Edited April 13, 20232 yr by Bobster.