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The battery may overheat because:
- it is getting accidentally overcharged
- inverter running for more duration than you require it
- there is problem in the battery electrolyte
The above problems are quite common for older backup domestic power systems. I would request you to check for inverter problems with the inverter maker. Do the same for the battery also. Also replace your battery once you have used it for approximately 5 years, as battery life tends to decrease after