Originally Posted by windmist
I should have mentioned that the battery is dedicated to the bow thruster and windlass so will never put out any current when the autopilot is in operation.
That helps' but it will be receiving a charging current. Lead acid batteries take a long time to charge so there will be some current going into the batteries for some considerable time after the bow thruster/windlass is shut down.
This current will be much smaller than during operation, but the earths magnetic field is only weak.
It also sounds like it will be close to the bow thruster and windlass so this also needs to be taken into account.
Everything on a boat is a compromise, but it does not sound the ideal location