Formula:charge time = battery capacity ÷ charge current Accuracy:Lowest Complexity:Lowest The easiest but least accurate way to estimate charge time is to divide battery capacity by charge current. Most often, your battery's capacity will be given in amp hours (Ah), and your charger's charge current will be. Formula:charge time = battery capacity ÷ (charge current × charge efficiency) Accuracy:Medium Complexity:Medium No battery charges and. Formula:charge time = (battery capacity × depth of discharge) ÷ (charge current × charge efficiency) Accuracy:Highest Complexity:Highest The 2. None of these battery charge time formulas captures the real-life complexity of battery charging. Here are some more factors that affect charging.