Abstract: In an embodiment of the present invention, a clamp-on sensor is coupled at a single point of a battery string in an un-interruptible power supply. In embodiments of the present invention, the clamp-on sensor is a Hall effects sensor for sensing ripple current through the battery string. The clamp-on sensor is coupled with a logic circuit for detecting a battery failure of the battery string. In response to detecting a failure of the battery string, the logic circuit automatically generates a message over a communication network indicating the failure of a battery in the battery string. In embodiments of the present invention, the logic circuit can also generate a message indicating the failure of an inverter in the un-interruptible power supply.