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Old 18-07-2012, 11:36   #1
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rreeves's Avatar

Join Date: Jan 2010
Location: Pacific Ocean
Boat: Far East Mariner 31, Ketch
Posts: 8
Assessing AGM house bank capacity

Question: when load testing AGM batteries, what readings indicate a healthy battery? I got a consistent 11.7v after the initial drop. Is this a good reading? I used a standard load tester, the kind one can buy at the auto parts store for $40 and it is rated up to 100amps, the rating of my batts. If I were testing wet cells and got 10.5v I'd think the batts were ok, but what reading am I looking for with AGMs? Same?


In Tahiti a year ago I was forced to replace my wet cell house bank with three 100 amphr AGM batteries.

Now we have sailed to Sitka, Alaska, and those AGMs are acting strange. I'm attempting to figure out if THEY are passing into the nether world.


During the passage here from Hawaii to Sitka, I began getting unusual voltage readings. The batteries seemed to drain more fully than I was use to, but more worrisome, they began to take higher voltage charge before they were full. One factor may be that it got cold on this trip. From an average temp of 75-80*f in Hawaii down to 40*f mid high latitudes Pacific. I was also using some different equipment...a new chartplotter and radar...and my solar panels got less sun... I was forced to run the engine to charge every other day. While in Hawaii, the top voltage reading when batteries were full (using the engine's alternatore) was about 14.3v. Now to get the same charge amount, the final voltage reading is 14.6 or higher. At night the batts may go as low as 12.5 if used heavily where as expected tropical readings were 12.8 to 12.7. To me this sounds like they've lost capacity.

The Test:

Today I performed a load test using a tester on loan from a local automotive store, the type that measures CCA and is rated up to 100amps. Beginning open cirquit voltage for each of the three batteries was 13.03. This was after charging them and letting them rest over an hour. I applied the load tester and flipped the switch and got very similar results for all three batteries. Each dropped quickly to a certain point and stayed. A) 11.8 volts; B) 11.6 volts; C) 11.7 volts. THE BIG QUESTION: Are these readings that suggest an AGM Battery is healthy? Certainly they would be for a deep cycle wet cell, but I don't know what voltage ranges to expect for AGM.

If they are healthy readings, then the only thing I can think is causing the wider range of voltage readings than what I was use to in Hawaii is the drop of 35 to 40 degrees in temperature. In your experience, is the above consistent with cold weather voltage readings for AGM batteries?

Battery Stats:

I have three 100 amphr AGM deep cycle batteries (group 31) made by YNC, Chinese and all that was available in Tahiti.
The side of the battery lists the following stats--
Valve Regulated
Constant Voltage Charge (really?--not usual for AGM)
Cycle Use: 14.5 to 14.9V
Standby Use: 13.6 to 13.8V
Initial Current: Less than 25amps

I don't know what "Cycle Use" or "Stansby Use" mean.

Thanks in advance,


Aboard Murre
Sitka Harbor
rreeves is offline   Reply With Quote


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