A programmable uninterruptible power supply for coupling battery powered systems associated with a mobile site, such as a vehicle, to the primary electrical system of the mobile site operates to selectively couple the battery powered systems to the electrical system of the mobile site and to a secondary, or back-up battery separate from the primary battery associated with the electrical system of the mobile site. Various system control functions are performed, including reverse battery protection, over voltage and under voltage protection, battery drain protection, battery charging detection, reverse current detection and uninterruptible power switching. For detected adverse operating conditions, the load is disconnected from the electrical system of the mobile site and the back-up battery is used to operate the battery powered systems which are to be protected from such adverse operating conditions.