If you are noticing the volt gauge fluctuating on a cold start, that is normal. The intake heater grid is doing its normal cycle. It pulls enough voltage to cause the gauge and the headlights to dim. If you are having a dead battery you need to check to see if both batteries are getting the charging voltage from the alternator. You could have a connection problem where only the passenger side battery is actually being charged.
The battery will attempt to charge the charger since the battery is at a higher voltage than the charger. Thus, the battery will simply discharge and generate heat. If you leave it plugged in long enough, eventually the battery will discharge to a point where it can maintain only 6V, where it will stay until you actually charge the battery.
If you use the radio without the engine running you will eventually discharge the power from the battery. It should be fine for a couple of hours, If not, your battery may need replacing.
Use a battery maintainer.
Either there is a dead cell in the battery or something is drawing power from the battery. Even a good battery will slowly discharge over time. Normally it will take months.
The engine computer is behind the battery.
To get the rated capacity, limit the discharge rate to 0.25C
If you keep your laptop all the time on charge, eventually it damages battery. It's recommended to discharge batteries lower than 10% at least once per weak.
chemical reactions take place in battery which reduce the charge.
It depends on the type of battery, and the capacity of the battery, and the application you are using the battery for.
recycle it
6-12 hours. The older your battery, the quicker it will discharge.
Automotive battery is used for cranking purpose of a vehicle,it needs high current discharge at the time of ignition.Where as an Inverter battery used as a power bank and its discharge is need based at the time of power failure.