How to Calculate the Charge Time of a Battery
Every battery will eventually lose the ability to provide its rated voltage. Fortunately, many batteries, including those for vehicles, are rechargeable. Battery charging can take hours, so it's important to determine approximately how long the process will take in order to plan for later use.
To roughly determine how long it takes to charge a typical 12 volt battery you need to know the battery's capacity in Amp-hours (Ah) and the capacity of the battery charger in Amps (A).
Determine what the battery capacity is in Amp-hours (Ah). Examine the battery for this value, which is typically 45 Ah. Note that the following calculation assumes that the battery is completely depleted and needs a full charge.
Determine the capacity of the battery charger. Examine the label or other wording on the battery charger to find the capacity. If it's not clearly listed on the device, check the user's manual to find the charger's capacity. For this example, assume the battery charger has a 15 amp capacity.
Enter the battery capacity and the charger capacity into the following formula: Charging time (hours) equals Battery Capacity (Ah) divided by Charger Capacity (A). Therefore, the charging time for this example is 45 Ah divided by 15 A which equals 3 hours.