*2. How is the output
power rating of the charger calculated?*

The output power rating of an industrial battery charger is based on the size of the connected batteries in amp-hours (AH) plus any standing (permanent) load current drawn by connected devices.

A battery can deliver a specified amount of current for a given period of time within a voltage band. In theory a 100AH 12V battery will deliver 10 amps for 10 hours or 100 amps for 1 hour at a voltage of 13.6 down to 10.5 volts DC. It can be safely recharged at 10% of its nominal capacity so a 10-amp charger with a 12-volt nominal output voltage is required.