There are many battery shapes that, unfortunately, allow users to incorrectly insert the battery. The simple circuit in Figure 1 gives a single-cell Li-ion battery charger added protection from damage due to backward battery insertion. In the circuit, the MAX1551 linear-mode single-cell Li-ion battery charger delivers the charging current to the cell from either an AC adapter or USB power supply. The MAX9001 is also powered from one of these supplies.


Figure 1. This circuit protects the MAX1551 single-cell Li-ion battery charger from surge current caused by backward battery insertion.

During normal operation, the MAX9001's comparator output is high and the FET is on. When the battery is inserted backwards, the MAX9001's output goes low, a condition that turns the FET off. This blocks the flow of potential battery surge current out of the BAT terminal. The rapid switching of the FET keeps the current surge to less than 50mA; that surge lasts for less than 200ns before dampening, as shown in Figure 2. Connecting the battery in reverse without this external protection would destroy the MAX1551.


Figure 2. The top curve of this oscilloscope capture represents the backward insertion of the battery in the Figure 1 circuit. The bottom curve is the battery current.