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I have just replaced my 55amp alt. on my 3GM30F with an 80amp yanmar unit. Before I replaced the 55amp I checked the voltage and it was at 14.3 the 80amp is only puting out 13.7 at all rpm''s.
Is there a problem? There was only 20 mins. between the voltage test.

Thanks,
Chris
 

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Chris,

I am somewhat unfamiliar with that alternator. If you could answer the following questions, I might be able to form a better picture of the problem:
1) Is the 80amp unit off a boat with gel-cells?
2) What is the size of the bank(s), its/their state of charge, and what type of battery (Gel, AGM, flooded)?
3) I assume that the OEM alternators have internal regulation (i.e. no external box that attaches to alternator via a set of wires), please advise.
4) Do you use any kind of diodes, combiners, etc. to distribute charge among several battery banks?

For example, if you use diodes to split your alternator output among battery banks and the alternator has no external voltage sense wire, then the 0.6V mismatch would correspond perfectly with the voltage drop associated with diode systems. In this case, you''d have to adjust the voltage setpoints in the alternator to accomodate the diodes or get rid of the diodes.

Personally, I would have dropped the OEM altogether and gone third party like Ample Power, Balmar, etc. These systems have external regulators with battery voltage sense wires, so no matter how your electrical system is configured, they''ll adjust the voltage output from the alternator. Furthermore, there is nothing to change or adjust if you reconfigure.
 
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