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post #7 of Old 09-30-2012
Maine Sail
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Re: AGM Battery Upgrade and Charger

Originally Posted by PCP View Post

No, I am no wrong about the max charge or at least that was explained to me by the technician that recently installed a complete upgrade of the batteries and electrical system in my boat. On the instructions that came with the batteries (AGM Mastervolt) they also say that the max charge should be no more than 14.2V and less than what is optimal for lead batteries.

Their dedicated regulator (Mastervolt) as well as the port charger (220V) also have specific regulations for AGM batteries, with less voltage than for lead batteries.

However you are right about the rest: They like to be fully topped from time to time and they can "absorb" much more electrical energy much faster than lead ones (they can be charged much faster by a big alternator) but that has nothing to do with the max voltage allowed, that is smaller than the one that is optimal for lead batteries.


Most AGM batteries can be charged at much higher voltages than the Mastervolt line. Mastervolt suggests 14.25V absorption voltage at 77F. They also suggest temp compensated charging and not exceeding 5 hours at absorption voltage and also limit charge capability to .3C.

On the other hand Lifeline, Odyssey, Optima and the large private labelers like Deka/East Penn and Full River all suggest significantly higher absorption charging voltages for their AGM's.

I've done one bank of Mastervolt and found them expensive for what they are. I found it frustrating that they limit the charge to .3C. This means they really can't be charged that much faster than wet cells, which is one of the main benefits of owning an AGM bank..

Bottom line is we don't know what brand AGM the OP has so can't really recommend a charging voltage.

What we can recommend are:

*Alternator capability equivalent to the manufacturers MINIMUM standards. Lifeline for example wants a minimum of 20% of bank capacity and sets no limit. Odyssey also sets no limit and wants a minimum of 20% of "C".

*Solar or wind is a MUST for a mooring sailed boat with AGM batteries. If you do not get them full, and the alternator will not do this unless you motor for 10+ hours, they WILL die an early death. With only an alternator expect two maybe three years. With solar you can push 5-6+ years.

*Do not discharge below 50% of capacity very often. The shallower the discharge the longer the bank life.

*Temp compensated charging can be very important with AGM batteries. You'll need a alternator regulator that can compensate for battery temp.

*Charge to full as often as you can. At the least once weekly is much preferred and will yield longer life.

*If you own Lifeline AGM's then you should equalize/condition charge when they begin to show signs of capacity loss.

*Do NOT equalize other brands of AGM batteries without checking with the manufatuerer.

*If you convert to external regulation and a bigger alternator invest the extra $40.00 on an alternator temperature sensor.

*Be sure your pulleys are aligned and CLEAN. With a stock 3/8" or 1/2" belt AGM batteries can eat belts for lunch. Carry a spare.

*Charge the batteries at the manufacturers recommendations or as close as you can get.

There is mounting evidence out there that suggests insufficient charge current can have a negative impact on AGM battery bank life. 20% of a 200Ah bank is and alt that can produce at least 40A when hot. These banks seem to like to be hit with some decent current. Bigger is better in the case of most AGM batteries and 20% of capacity with Lifeline for example is a BARE MINIMUM...

-Maine Sail / CS-36T

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