apologies if i can't ask this in this stack. My 24V sine inverter runs on 12V x 2 lead acid old batteries (left with ~50% capacity). I want to buy two new leadacid batteries but still try to configure such that i can use the left life of my old batteries till they die. I was thinking doing a old 12v x 2 parallel with new 12V x 2 config which gives 24V and inverter probably will be happy. can it?
The inverter will be happy, but the batteries won't. The new ones will be constantly trying to recharge the old ones and wasting mucho power, maybe ruining your nice new batteries in the process. You need to isolate the two battery packs from each other. The simplest way is to incorporate a nice high current diode with each battery pack to allow the electricity to flow out of the batteries only. You don't mention what you have in the way of a charging system. If the batteries are charged in place as in a UPS, then this will break that charging. A separate charge feed would need to be provided to each battery pack.