To calculate the estimated charging time of a fully discharged battery, divide its ampere-hour rating by the amperage of the charging current. To account for efficiency lost to heat and battery inefficiency, the estimated charging time is multiplied by 1 + (percent of energy lost/100).
- Identify the ampere-hour rating of the battery
Start by identifying the ampere-hour rating of the rechargeable battery. This value is typically printed somewhere on its outer casing.
- Find the amperage of the charging current
For consumer battery chargers, the rating of the charging current is typically printed on the body of the charger. It appears as an amperage range, like "Min 100mA - Max 180mA." The charging current for lead acid car batteries is often pegged at 10 percent of the ampere-hour rating of the battery.
- Divide the two values
Divide the ampere-hour rating of the battery by the amperage of the battery charger to obtain the approximate charging time in a perfectly efficient system. For example, charging a 1000mAh battery with a 145mA charger from fully discharged to fully charged takes 6.9 hours.
- Account for efficiency loss
To account for the efficiency loss that occurs in real life, which is often pegged 20 percent, multiply the calculated charging time by 1 + (percent of energy lost/100). Using the example charge time of 6.9 hours coupled with 20 percent energy loss, calculate 6.9 * (1 + (20/100). This shows that it takes approximately 8.28 hours to charge the battery.