Battery guard

We are using Li-Po batteries for our rig, and in order to keep them in good condition for a long time it is very important that you dont discharge them too much.
So in order to prevent over-discharge we created a circuit that would let the rig fully disconnect from the battery when the voltage get too low.

We had already made a power management module that lets the Arduino control the power to the other components of the rig. What we now needed was a circuit that would enable the Arduino to switch off it's own power supply.

This turned out to be a bit tricky when designing a transistor based circuit. When the voltage supplied to the Arduino goes too low you loose control over the state of the digital pins. This resulted in the Arduino leaking current to the digital pins, leaving the Arduino in a state of limbo between fully on and fully off.

There are ways of solving this using pull-down resistors and transistors with lower gain, however since it was absolutely imperative that the circuit would not leak any current in the off-state we opted to go with a relay in place of the transistor.

The above picture shows the full circuit for the battery guard.

Some comments to the circuit:
Arduino_A2 - Reads the battery voltage
Arduino_A1 - Reads the state of the ON/OFF button
Arduino_D1 - Outputs 5V, holding the relay and keeping the Arduino alive