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.