A latching automatic dual battery switch employs a first comparator to determine that a condition requiring engagement of a secondary battery exists. For example, a primary battery voltage is below a predetermined threshold, current drawn from the primary battery exceeds a predetermined threshold or ambient temperature is below a predetermined threshold. Engagement of an ignition switch is detected and used to clock the output of the comparator to a trigger circuit. The trigger circuit activates the triggerable switch on the leading edge of a signal generated by the detector circuit to connect a secondary battery or power source in parallel with the primary power source. The trigger circuit is leading edge activated so that conditions immediately prior to engagement of the ignition switch determine if the secondary battery is engaged. This avoids engaging the secondary battery as a result of a load surge dragging down the voltage after activation of an ignition switch. In addition, when the ignition switch is released, a timing circuit is engaged to limit the additional time during which the secondary battery is switched in the circuit. The automatic engagement of the secondary battery eliminates the need for manually switching to an emergency backup and manually disengaging the emergency backup after a vehicle is started.