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post #12 of Old 07-15-2013
Maine Sail
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Re: m25 alternator question

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.


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..

-Maine Sail / CS-36T

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Last edited by Maine Sail; 07-15-2013 at 07:30 PM.
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