My team once installed an inverter for a client and in less than 1 year the client started to complain.
The client claimed the inverter was no longer working and on getting to his house we found out that his battery was bad. The battery voltage dropped below 10.5 volts.
Sometimes our battery becomes so deeply discharged and we think the battery is bad.
The truth is that most battery charging equipment are built to start charging your battery from 10.5volts and above.
Therefore if for any reason your battery voltage drops below 10.5 volts, your battery charger will not be able to recharge it.
To revive his battery, we simply took it to our support center and connected his battery in parallel with a good battery with 12.5 volts.
We then connected the battery charger to the terminals of the good battery and allowed the battery to charge. We frequently touched the battery with low voltage to make sure that it was not over heating.
After about one and a half hours, we turned the charger off and checked the voltage of the bad battery. It was now 10.7 volts.
We then hooked up the battery charger directly to the bad battery and revived it.
The battery is still work up till now.
The truth is this. Charges will not charge batteries with very low voltage. But if you connect the battery with low voltage in parallel with a good battery, the battery charger will assume it is charging a good battery and in the process also charge the bad battery, thus increasing the voltageof the bad battery.
Note that this is a simple thing to do but if you are afraid, kindly take the battery to a specialist.