Join Date: Dec 2004
Location: Portugal, West Coast
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Re: AGM Battery Upgrade and Charger
I think you're thinking of gelled batteries, not AGMs. In fact, AGMs can and should take a higher absorption voltage...14.6-14.8 VDC and, the faster they can be charged the better (i.e., the largest charging source you can fit on your boat).
As was noted above, AGMs need to regularly be fully charged or they will sulfate and suffer a shortened service lifetime.
This is hard to do on a mooring, unless you have adequate solar power and a proper charge controller.
I'd not choose AGMs for a boat on a mooring UNLESS it had a sizeable solar charging system.
Also, it's true that if the batteries are significantly discharged they may place a very heavy load on your undersized alternator and, with no protection in the way of temp sensors, burn it out. This is because AGMs can take an enormous amount of amperage, will demand it from the alternator, and alternators typically fitted on boats are not capable of putting out high power for a long time.
Battery chargers will not likely be burned out by attaching them to AGMs, but they'll take a long time to charge. This is no problem if at dockside for a long time. If your charger doesn't have an AGM setting, use the flooded setting. Flooded batteries and AGMs have very similar charging profiles.
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.