Only three months wouldn't wash over here. Here, things have to be fit for purpose and give reasonable life.. manufacturers can't get out of it by imposing a short warranty. I'm surprised they can get away with that. I don't know whether they make small batteries, but I have Trojan batteries in my off grid home.. they are absolutely amazing.. Really good performance, good durability, long lived.. Check Trojan.. American company. And as for agm, flooded or lithium.. old fashioned lead acid flooded are known to be the most tolerant of cold, heat, over and undercharging, followed by agm then lithium last. Agm and lithium can stand vibration, and lithium they say holds more power, but that's only true until the first time it's flattened too far, or over charged slightly, or gets too cold. Lead batteries are also the most recyclable.. if that's any consideration..