OK, What is a "BIRD" system in reference to charging.

It's a logic board and 2 or 3 solenoid relays. If it senses charging voltage on the house or chassis, it closes the relay that connects the two sides so that both get charged. That operation is subject to certain requirements being met, so it won't happen if the charging side is deeply discharged. The other 1 or 2 relays are latching relays which serve as disconnects for the house and optionally the chassis.

I see YC 1 said about the same.