I’m tired of buying a new 12 V battery every 1-2 years. I have about 4 small (rated ~ 300 W [not VA]) UPSes with 7 Ah, 12 V, maintenance free batteries.

I’m thinking about replacing them with one, powerful unit. But the more powerful the unit, the more it costs - non-linear.

Do you have experience with some DIY solutions? Like 12V DC to 230V AC inverter, battery charger, some UPS controller (simple relay?)?

Or maybe you have some other idea that is cheap(ish)?