A current-sense amplifier with a “sign” output can be used to protect a charging circuit against unintended discharge of the battery if current is drawn in the wrong direction (i.e., if the charger cord shorts to ground or an incorrect charger attempts to draw current from the battery). Alternatively, you may already be using these useful current-sense amplifiers to control the current from your charger-now just add a complementary MOSFET and resistor to add the reverse-current protection feature.