Hi Ian,
I agree the Sterling unit is working correctly.
When we are drawing over 15 amps from our
battery for long periods (for our navigation computers, fridge etc), and the
battery temperature exceeds 40º (which can be due to the ambient
temperature, not just the high charge rate), the Sterling unit shuts down and reverts to the built-in in regulator. The built-in regulator will also reduce the charging rate after a time to a trickle charge. This might maintain the
battery voltage level, but does not replenish the charge.
So effectively we can actually loose
battery charge, which is not good and I don't know the answer, except resetting the Sterling unit (by switching it off and on again - not recommended), or driving only when cooler (not practical).
As far as I know the charging voltage of the Sterling DAR12 (AR12VD) is not adjustable.
Dandj