An emergency lighting power supply includes a rechargeable battery; ballast circuitry configured to supply power to an external light, battery charging circuitry configured to charge the battery, and a controller. The controller determines an amount of time the rechargeable battery has discharged to supply power, via the ballast circuitry, to the external light. If the amount of time is less than a threshold, the controller causes the battery charging circuitry to supply a first current to charge the battery for a first charging period that is proportional to the amount of time, and causes the battery charging circuitry to supply a second current to charge the battery subsequent to the end of the first charging period.