Battery alarm when charging current drops

[sailor_leo]

Hi everyone,

On my D2-40 Volvo-Penta, everytime when I turn down the revs of the motor the battery alarm starts ringing because the charging current drops. It clearly still charging, I have a battery monitor and see that there are still at least 10A coming in. This 10A are just much less than the charging current in higher revs and this probably confuses the alternator.

Does someone has an idea what can cause this? And It could be solved?

[Lepke]

Some alternators don't charge at slow speeds. Also at higher charge rates you have higher voltage. So when charging 20 amps you may have 14.5v and at 10 amps maybe 13v. Your alarm maybe reading voltage as an indication of charge.
Turning the alternator faster may solve the issue. Smaller pulley on the alternator or larger pulley on the engine. Or a low speed alternator.