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Go Back   SailNet Community > On Board > Gear & Maintenance > Electrical Systems
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  #11  
Old 07-15-2013
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Re: m25 alternator question

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Originally Posted by MarkSF View Post
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...
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Last edited by Maine Sail; 07-15-2013 at 08:21 PM.
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  #12  
Old 07-15-2013
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Re: m25 alternator question

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Originally Posted by btrayfors View Post
Point is, absorption voltage maximum varies directly with the charging capacity. Most boats don't have more than 10-25% of rated AH charging capacity.

Bill

Precisely! And as your available current goes down, compared to the banks size, the point in the SOC at which you hit absorption voltage also rises. I have solar panels on boats that won't hit absorption until 96% + SOC or so because the panel is merely a trickle charger and can't raise the voltage until the bank needs very little current..

One problem with a charger or alt too big on deep cycle batts is that you can tend to build up a surface charge on the plates and hit voltage before the battery bank is actually all that charged. If you don't greatly extend the absorption time you can actually wind up less charged than one might assume.. TPPL and some AGM's do better with this than deep cycle FLA's but they can still be forced into early absorption by large charge sources.

Fortunately on boats we rarely if ever need to worry about our charging sources being too large....

I actually have four 6V Lifeline's in my shop right now to see if I can save them. I just did a 100% discharge on one pair of them and am recharging them at just 4A constant current or (2.5% of "C") with no voltage limit. They have been on since about 2:00 and I just went out to the barn and they were still only at 12.2V.... I have had this deep discharge / constant current re-charge work fairly well in the past so we'll see how it works this time..
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Last edited by Maine Sail; 07-15-2013 at 08:30 PM.
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Re: m25 alternator question

Quote:
Originally Posted by Maine Sail View Post
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...
Possibly those alternators are automotive-sourced, and on a car the assumption that the alternator is at a similar temperature to the battery (unless you have the battery in the trunk) is a reasonable one. Probably not so much on a boat....
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Re: m25 alternator question

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Originally Posted by MarkSF View Post
Possibly those alternators are automotive-sourced, and on a car the assumption that the alternator is at a similar temperature to the battery (unless you have the battery in the trunk) is a reasonable one. Probably not so much on a boat....

Yep.. I replace more Yanmar/Hitachi alts than anything else but the Valeo's come close.
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Old 11-18-2013
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Re: m25 alternator question

An update.
Well after emails and a bit of testing the battery supplier suggested a place with a high end tester to check the battery's. The batteries failed the test and were only outputting 70% of what they should be. So my battery monitor was correct. They replaced my two 6 volts for new ones.
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