Alternator questions

[Ken13559]

I've Yanmar 2GM20F fitted with a 3rd party 80A alternator LR180-03B. I've never seen the charging current to my 270AH battery bank going above 25A.
I might need to replace the alternator for other reason so should I go with a 35A unit instead?

 

[whatfiero]

Just have the existing one rebuilt by a local reputable repair shop

 

[Gunni]

Short answer: You want external regulation of your alternator based on battery state. Long answer: Study at the feet or an expert.
https://forums.sailboatowners.com/index.php?threads/musings-regarding-external-regulation.125392/

 

[Ken13559]

Whatfiero, will check it but thinking if needed replacement would a 35A unit be suffice given the charing current never seems to exceed 25A?
Gunni, thanks. I'm keeping my simple for now.

 

[Rich Stidger]

IMHO, no. Get your alternator tested and repaired as necessary. If you have never seen more than 25A charging rate, either you have never discharged your batteries very much or you have other problems in your electrical system. Inadequate wiring, bad connections, just to name a couple. If your batteries are healthy and are staying charged, why do you think you need to replace the alternator? As far as a 35A alternator, I have not seen one in 30 years. And the 35A rating is for peak current, not average, not RMS current, and it is only rated at a cold alternator. A typical automotive internal regulated alternator rated at 35A will actually only put out about 24A RMS when cold and probably 30% less when hot. You have asked an apparent simple question but the answer is complex.

 

[Ken13559]

Rich, thanks. You reminded me on alternator performance vis-vis temperature. Hmmm....

 

[Gunni]

My stock Yanmar Hitachi alternator NEVER sent more than 25 Amps into my battery. Ever. It really is nothing more than a charging system for the starter battery.

 

[JohnShannon]

You could try thicker and or shorter cables.

 

[Bill Roosa]

Battery resistance (aka State of charge) determines the charge rate not the alternator. The alternators max charge rate is only valid for very (<50%) discharged batteries and cold alternators. BTW most OEM alternators do not have a temp sensor so they don't automagically cut the amps as the alternator coils get hot. The rely on the battery internal resistance to limit their output amps. 5 minutes after starting charge the surface of the battery plates will have absorbed all the charge they can and the battery internal resistance will rise. An internal regulator will see this and reduce the voltage. An external regulator will stop charging the batteries for a minute or two and measure the "resting" (no flaming please I know it is not really the resting voltage) voltage and determine if it should reduce the voltage from 14.4.
If your alternator is charging and not giving you the amps you want the first thing you need to do is measure the alternators output voltage while it is trying to charge the batteries. If it is below 14.4 volts and the batteries are deeply discharged you have a regulator issue not an alternator one.

 

[Ken13559]

Bill, Thanks. I'll get on with voltage checks coming weekend.