How To Calculate A Battery Charge Time

Ever wondered how a lead-acid battery’s charge duration is calculated? This is the formula to calculate the charge duration:

Battery Charge Time = Battery Capacity / Charging Current
Time (Hours) = Amp Hours / Amps

So if you had a 100Ah battery charged with a 10A charger the duration of the charge would be calculated as in the example one below:

First Example:

Battery Amp Hour Rating / Charger Amp Rating:
100Ah / 10A
= 10 hours

In this example, for simplicity we have made 2 assumptions:

1. The battery has a zero charge which never happens unless you have a dead battery.
2. The battery charger has a charge efficiency of 100%. Usually, battery chargers have a charge efficiency of about 60 – 80%. (Our chargers have an efficiency above 80%).

Second Example:

Battery Amp Hour Rating / Charger Amp Rating
x Efficiency Loss
100Ah / 10A x (10/8)
= 10 hours x (10/8)
= 12.5 hours

In example two, we take the efficiency loss into consideration:

1. Charger with an 80% efficiency.
2. Charger Amp Rating 10A x (80/100) = 8A