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post #39 of Old 10-08-2012
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Re: AGM Battery Upgrade and Charger

Find the specs from your battery charger, confirm that the "wet" settings are within the optimum voltage settings for your AGM batteries. AGM are very similar to wet in their requirements, but some chargers may be too crude or too hot to use with them. Rashly assuming that you will find your charger is suitable...

Then using a small solar panel to top them up on the mooring is also a good idea in any case and should extend battery life. A 5W panel is peanuts, it won't need a controller but won't do much. Basically it will keep a charged battery charged but not much more.

You've got maybe 200AH of batteries, let's say you run 'em down and moor the boat with them 30% down and expect that to be recharged in 5 days by your solar. Call it four days because of bad weather. Average, five or six hours of full rated power on sunny days. So you'd looking at maybe 25 hours of a panel's rated output, over the course of one week, to put in the missing 30% of 200AH. That would be roughly 60AH of panel output in 25 hours, or 2-3 amps of rated panel output. They're typically rated around 17 volts, pick your own min/max values and soething between a 30-60W panel should keep the batteries fat and happy. And at that power, yes, you'd need a controller. (Again, one that matches the optimal voltage charging points for your batteries.) For light use, a 25-30W panel might very well be all you want or need.

And coming out to the boat to see the batteries always fully topped up, even if there have been some gray days, can be a real pleasure. Having them FULLY topped up, really should extend their life, too. A good investment all around, considering the way battery prices keep going up.
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