If its the BIRD you're talking about (Bi-directional Isolator Relay Delay) its function is to charge both batteries after a short delay.
When the voltage on either side (house, or chassis) rises above a setpoint value for several minutes the BIRD tells the trombetta relay to close and connect the house and chassis batteries together in order to charge them.
Thus a few minutes after starting, or plugging in the BIRD will close the relay and both batteries will get charge voltage.
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