12V Battery undervoltage monitor

Description

The 12V battery drain is known for causing issues, even in production electric vehicles. This seems counter intuitive as we have a high voltage battery onboard often with plenty of energy. So the trick is to prevent the 12V battery to be drained, even if it is a small load and recharge in time from the main high voltage battery.

We have found and tested a solution that works!

This 12V battery under voltage monitor continuously measures the 12V battery voltage. As soon as the voltage (on pin 15) drops below 11,9V it triggers. In most cases this is still enough to be able to close the main HV battery contactors.

We use the trigger to wakeup the battery management system (BMS) and get the car into a state where there is high voltage available and the DC/DC is enabled. This will charge the 12V battery. Normally the undervoltage monitor would cancel the trigger as soon as the battery voltage is above 12,5V again. However in our case the DC/DC will probably cause this to happen immediately and charging the 12V battery does take some time. Therefore this undervoltage monitor delays cancelling the trigger for 10 minutes from the point in time where the voltage became more than 12,5V.

Obviously this process fails when the BMS decided the state of charge (SOC) of the high voltage battery is too low or if there is another error or reason why it does not close the HV contactors.

Max. allowed load for the relay is 5A. If you’d feed the “under voltage triggered” active high (11,9V) signal into a VCU you can even expand the possibilities. For example you can check if a charge plug inserted and in that case trigger charge mode or you can take high voltage battery SOC in consideration, send messages to remote devices, etcetera.

This is how I implemented it in the ‘dash-board’ PCB that will be released soon (that will make your life easier regarding handling ignition, ‘starting and started’, OK to Drive and a door open buzzer when started, etcetera).

You can use the 12V guarded to cut a load (up to 5A) when the voltage monitor triggers.

There is one glitch with this monitor: at power on it triggers even if the voltage is above 11,9V. But after that (after 10 minutes) it operates as described above as long as the 12V is present.