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Alternator Battery Charging Project Issues/Questions


Jul 17, 2020
Hunter 376 Washburn, WI on Lake Superior
Hi all,

This past weekend I was helping my dad, who owns a 2000 Hunter 340, and who recently updated his electrical charging configuration. He ran into some issues with his charging system and is looking for some input.

He recently installed the following:

1. A 40 amp shore power battery charger
2. A Sterling 12V Alternator to Battery charger (I'm not sure on the exact model number, but I think it's the 80 Amp)(this is connected to the 55 Amp Hitachi alternator on his 3GM30F motor)
3. 2 new 160W solar panels, each connected to independent Victron 75/15 MPPT solar chargers
4. An AGM 4D battery and an AGM group 27 start battery
5. A pre-existing wired battery monitor (Balmer) and a Victron wireless battery monitor

He ran all the wiring out to a buss bar, added appropriate fusing, etc. Each of the devices also have the appropriate battery temperature sensor wiring, including temperature sensing on the alternator. It looks really nice!

This past weekend he took his boat out for the first time with the new set up. As he was motoring the Alternator to Battery charger (henceforth referred to as "The Sterling") began flashing the "High Volts In" warning. He shut off the motor, then shut off the The Sterling. He said that when installing The Sterling the instructions do NOT have you disconnect the normal alternator to battery connection. He started the motor again and watched his battery monitors, with The Sterling disabled (alternator charging directly) he said he saw "normal" alternator current (about 14.3 volts right after starting) which then tapered down to mid-13's.

He said that the batteries never felt hot, the temperature sensors never spiked, he never saw voltage above ~14 Volt range. Right now his theory is that the solar panel inputs were holding the voltage high confusing The Sterling resulting in the high volt LED. However, this is just a theory. Which leads to some questions for folks who have more experience in this area:

  • If the alternator IS showing the proper charging profile with The Sterling powered off is the warning light on The Sterling a false indicator, or is the alternator charging profile irrelevant here?
  • Is running multiple charging devices concurrently an issue (I.E. both the solar panels and The Sterling)? He's considering adding disconnects to the solar panels, but is wondering if this is necessary.
  • Finally, the assumption is that the 80 Amp sterling will work with the 55 Amp alternator (I.E. it will reduce the call for current if the alternator gets too hot)--is this a safe assumption, or is there a strong risk of burning up the alternator here?

Thoughts/ideas are appreciated. Thanks!
Oct 29, 2016
Hunter 41 DS Port Huron
Sounds to me that the alternator regulator is not working when connected to the Sterling, was the cable from the alternator output to the starter removed and separate cable run to the starter directly from the start battery?

It make little sense to go through all that refit without adding an external regulator and new alternator, the alternator is weak at best and if the goal is to improve battery charging and battery longevity then one must start with the heart of the engine charging system, the alternator/regulator.

Here is some great reading