Join Date: Jan 2003
Location: Maine Coast
Thanked 296 Times in 229 Posts
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Re: m25 alternator question
I think I was actually trying to say pretty much what you just did, except that you explain it better. I am fully aware that an alternator simply outputs a voltage, but my point is that a shore charger may output a higher absorption voltage BECAUSE it will switch to trickle when absorption is done. An alternator will not switch, so there has to be a compromise in the absorption voltage, so as to not cause excessive electrolyte consumption over the long term.
You didn't mention the thermal compensation in most alternators. Mine will start out at 14.4V, but once the engine has run for a while and the engine compartment has warmed up, the output will drop to 13.6V or so due to the alternator's internal compensation reacting to the higher temperature. So I'm never going to reach 100%, and 85% is a good assumption. Suits me, as I hate topping off the battery electrolyte.
Yes the internal temp compensation is a factor as well and a poorly suited alt for charging a deeply cycled bank.. Those alts, usually Valeo on Volvo's or Hitachi's on Yanmar's were never meant to charge large banks. As a way to keep them from burning up during the warranty period some genius decided they could drop the voltage as the alt temp increased to reduce the output... Yep, it works but is not at all optimal for a cruisng sailboat but neither are the jokes of charging circuits on many sailboat aux engines.. There are piles of non-temp compensating alts out there that put out 14.2V - 14.6V and of course you can always go with a smart regulator too...
The OP's alt does not have internal temp compensation though and many of them were factory set for 14.2V - 14.4V though some with the ammeter circuit were actually set for 15.0V...
-Maine Sail / CS-36T
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Last edited by Maine Sail; 07-15-2013 at 08:21 PM.