AGM batteries and chargers

[SmokyMtnGrady]

Hi Yall:
I got a few questions. This spring I replaced the dekka starting batteries that came with my boat with 2 new AGM West Marine deep cycle/starting batteries also made by dekka. One of them failed and I took it back to West Marine and they replaced it. They questioned my charger type. I told them I have a VECTOR electronic charger. My question to those folks with AGM's ...my charger is computer controlled and I can select the amperage from 2 to 30 amps. What is the appropriate charge current I should select? I was afraid to damage the battery so I charge them on the 10 amp selection. Is this correct? Do I need to buy another charger specifically designed to charge AGM batteries? I have contemplated installing an onboard charger, but a portable one is just fine with me too... Thanks in advance for any ideas here...

West Marine's customer service in this case was second to none.....

 

[SmokyMtnGrady]

Ok, I did some research on the net and found the following: AGM batteries due to their very low internal resistance can accept high current (amps) in charging. My battery charger is a 3 stage voltage regulated charger and while the research I found from 3 different websites all state standard battery chargers work fine with AGM, ideally using a regulated charger is best. One website I am sorry I did save the link, but one website and the article was dated in 1999, said AGM batteries can be hard on alternators because they will take every bit of current an alternator will deliver and then some. Makes me wonder if using these batteries will shorten the alternator life on my motor....

 

[richie rich]

I believe the AGM's require to be charged fully every time...ie, if fully charged for an AGM is 13.8 and you only charge it 13.2 the % of not charging it fully is now the peak charge....do that on a continual basis, ie 13.8, 13.2, 12.9, 12.5 and it will quickly become a low voltage battery.....so it matters less of the amperage rate, but more of the total amperage that can be developed...so charger type is critical with these batteries...I don't think it can do damage to the alternator output...thats a fixed number based on winding design and regulation....but the lack of full charge can do a number on the battery.

 

[Doc Stressor]

If you have a true 3 - stage "smart" charger with an AGM setting and either temperature compensation or a temperature set point, you can use any amp rating that you like. One of the best things about an AGM battery is that it can take a very high current flow during the bulk charge cycle. This greatly shortens the charge time vs flooded cell batteries. A smart charger will automatically sense the internal resistance of the battery as it goes through the 3 - stage charging process and reduce the charge rate accordingly.

Grady does not recommend AGM batteries because Yamaha warns against them. I don't really understand the reason for this. I've never gotten a satisfactory answer from Yamaha. There are 2 possibilities: 1) very few AGM batteries meet the 182 min reserve capacity requirement stated by Yamaha; 2) alternator life might be affected. A discharged AGM battery will take the full output of the alternator until the internal resistance comes up as the battery reaches full capacity. This may cause the alternator to operate beyond its design specifications.

I've used AGMs in boats for many years without problems. I had a pair of Optima Blue Tops in I a boat that I just sold that still passed load tests after 7 years. The engine was a 90 hp Honda with a relatively low output alternator. But I used a pair of the same batteries in a boat with an F150 Yamaha for 2 years without any problems.

I have a pair of West Marine AGMs in a skiff right now with a 40 hp Suzuki and an electric trolling motor. I use a Grp 24 for starting and a Grp 27 for the trolling motor. Again, no problems for 2 years. I use a West Marine smart charger (30 amp) to charge the trolling battery at the highest voltage setting. BTW, that is a great charger with an AGM setting. I believe that it's on sale right now.

I use the stock Deka batteries with the F250 on my Grady for warranty purposes just in case.