If you want to charge a 12-volt battery with a 10 amp charger, it will take about 3 hours. That's because you're charging at a rate of 10 amps. If you want to charge faster, use a larger amp charger or connect many smaller chargers in parallel. A battery is charged by forcing electricity into it. The rate at which you charge the battery determines how long it takes to fully charge the battery (that is until its voltage reaches a specific level). The time required to charge a battery depends on the following factors: 1. Battery capacity (measured in amp-hours) 2. Charger capacity (measured in amps) 3. Voltage of the charger 4. Temperature of the battery 5. The state of charge (SoC) of the battery before charging begins