The present invention relates to battery chargers. A first controller can be configured to set, when a first power supply is coupled to a first port, a current produced by a variable current source at a safe rate to charge a battery. An ammeter can be configured to measure, when the battery is coupled to a second port, the current flowing into the battery. The first controller can be configured to increase, after a passing of a quantifiable amount of time, the current produced by the variable current source by a quantifiable amount of current. The first controller can be configured to continue iteratively to increase, after the passing of the quantifiable amount of time, the current produced by the variable current source by the quantifiable amount of current until the safe rate is near or at a highest safe rate to charge the battery.