The following are the inspection activities to be carried out for a battery system:
Topping up of water inside the battery. Nowadays most of the inverter batteries are coming with inbuilt electrolyte level indicators. Mainly people use tap water for topping up. Some local technicians suggest filling of boiled and cooled tap water. Remember these things will not substitute distilled water. Tap water generally contains some impurities, minerals, and other chemicals that react with the battery electrodes and causes life reduction. Some people use the water drained out from air conditioners, which is comparatively a better alternative but not the best. During maintenance, one must check the electrolyte level regularly.
Inspect all corrosion and lose connections. Clean and tighten as necessary. If you find that the nuts and bolts are rusted, replace it, or clean mechanically by iron brush, sandpaper. Later on, eventually, employ rust inhibitor to protect the part against corrosion (but not the electrical connectors). Connectors should clean, free of oil and rust.
Observer Battery State of Charge (SOC) using a hydrometer. In the case of VRLA battery use voltmeter to measure voltage and SOC.
(Ref: Best Practices in Operation and Maintenance of Rooftop Solar PV Systems in India – GERMI)