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Old 13-07-2009, 18:27   #1
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Can Stock Yanmar Alternator Handle AGM Batteries?

Hello everyone,

I need to replace my aging deep-cycle flooded-cell batteries and everything I've been reading says that AGM batteries are the way to go. It concerns me however that everywhere I look I encounter warnings about how easily they can be destroyed by over-charging, how they can burn out alternators because they're able to absorb so much power, etc. Unfortunately none of the warnings have been very specific about when and how this can happen.

I've got a 1983 Yanmar 2GMF with what I assume is the original alternator. I think it puts out 35 amps. My power requirements are fairly modest (bilge pump, small GPS, occasionally RADAR, very occasionally an autopilot and simple cabin lights.) My boat stays on a mooring all summer though so the only charging the batteries get between launch and haul is from the engine. Are there likely to be problems if I use AGM batteries with my stock alternator?

Any help would be greatly appreciated.

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Old 13-07-2009, 18:31   #2
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Your power requirements are so low that the stock alternator might be ok. I wouldn't even bother with AGM's. Just get the regular flooded batteries.

Rick I
Toronto in summer, Bahamas in winter.
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Old 13-07-2009, 18:47   #3
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AGMs have their purpose but it seems like a total waste in your application. You don't have a charging method to use the advantage AGMs inherently have in terms of acceptance rate, don't mention any particular need for maintenance-free alternatives and don't need to discharge yourt bank significantly so why incur an unnecessary expense?
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Old 14-07-2009, 00:16   #4
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The problem you will have regardless of flooded or AGM batteries, is that your standard Yanmar (usually Hitachi) alternator will have an inbuilt automotive regulator which is fine if all you want to do is charge your start battery. The minute you start using your batteries for deep cycle type use, that alternator / reg system will never fully charge them...and they will have a short life.

IF you are going top get AGM batteries, I would advise that you have your alternator modified to remove the internal regulator, get an external regulator from Xantrex or Balmar or .... and set it up properly with your alternator size and battery bank size programmed in to deliver the right charge curve for your system.

Your batteries will be well charged, last longer and your alternator won't burn out.

Good luck

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agm, alternator, yanmar

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