Electrical connector to car battery terminal can develop issues in many ways causing problems ranging from broken connector, corrosion, loose contact & dirt.
The battery that enables starting of a car dies after some period of use but faults from other areas can shorten its lifespan. Examples are faulty alternator and bad battery terminals.
When fan of hybrid battery becomes continously noisy & fast, it points to the fact that there is something wrong with hybrid battery or its ventilation system.
The true health of a battery cannot be checked by simply measuring the terminal voltage. A resistive load is placed and the rate of discharge is monitored by watching the battery terminal voltage.
Batteries setup is important to longevity of inverter setups. Depending on capacity, there is choice between having single battery or battery string(s) backup.
When a hybrid battery is dying, there are symptoms that accompany it like increased fuel consumption, erratic charge state, frequent ICU operation and so on.