The batteries in this UPS lasted less than a year. It was because those battery were "no name" batteries, or because they were connected in series directly to the AC after the transformer? Shouldn't they have a rectifier and condenser before that? It seems that in this way the battery is connected to AC 24/7
Transformers change voltage, but they don't change AC to DC; you need a rectifier to do that.
It looks like they put that on negative side of the batt.