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Old 03-05-2016, 04:48 PM   #1
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Can I charge AGM batteries directly from the alternator?

I am thinking of getting AGM house batteries for my van. When parked they will be charged with an MPPT solar controller which (I assume...correct me if I'm wrong) will do a good job charging them properly. But when I'm driving they will be charged by the van's alternator. I have read that the alternator could overcharge the batteries since it is not a 3 stage charger etc. I have also read not to worry about it. Does anyone have experience with this? Thanks.
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Old 03-05-2016, 06:27 PM   #2
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I've had 3 AGMs charging from my alternator for four years. No problems so far.
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Old 03-05-2016, 06:58 PM   #3
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all 12 volt lead acid batteries can be charged with a 12 volt alternator, as long as the alternator isnt putting out more than 15 volts. Many people use agm's as there start batteries.

When the battery gets full, it wont accept anymore amps, even if the alternator can put out 100 amps, the battery wont take it. But if the alternator is putting out 15 or more volts then it will force the battery to take the amps and will cause it vent, damaging the agm battery.

I read where some agm's can take up to 16 volts without venting, but most agm's are rated at about 14.4 volts max. Keeping the battery at 14.4 volts all day long wont damage the battery, it might actually last longer.

On my agm battery I have connected to my solar panel, I have the float set to 14.3 volts ( the highest it will go). Its 3 years old running like new.
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Old 03-05-2016, 09:04 PM   #4
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So far so good on my optima house battery (yellow top). My red top starting battery did not fare so well. About 30 months in, it started cranking slow and I discovered the top bulging. Napa did a full exchange. About a month later the alternator stopped charging. I'm not sure which damaged which however. I'm 18 months in on the 2nd red top, so we'll so how this one holds up.
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Old 03-06-2016, 08:17 AM   #5
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...The short answer is most of us here are charging our AGM batteries with our alternators.......without issue

One case of destroying a nice pair of AGMs was my neighbor's RV....We installed a cheap Chinese MPPT solar controller and after a few months we noticed that the batteries were bloated looking with a piece of gasket blown out one one of them.

When the sun was shining we measured over 15V at the battery terminals......We replaced the controller with the Samlex 30 amp controller and temp comp sensor and got another pair of the same batteries....everything has been fine.

The bloated batteries still worked quite well....who knows for how long...
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Old 03-06-2016, 11:25 AM   #6
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Thanks for all the advice! I think I'll go with AGMs, check my input voltages, and not worry too much about it beyond that. Has anyone used these Universal AGM batteries?

Robot Check

The price and reviews are great so I am tempted but haven't actually actually heard of anyone using them in a Sportsmobile. I would rather go with 2 6 volt AGMs but the cheapest I can find them is over $500 for the pair! Is it worth it?
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Old 03-06-2016, 03:56 PM   #7
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Universal brand has been a battery used by SMB in their builds over the years but there are better quality batteries out there. It's up to the buyer to determine if the quality is worth the price. Some feel it's better to just buy cheap and replace more often.

If you search the site you'll find a thread about buying Universal battery blems on the cheap. Thought I saw some great prices on a 200Ah 4D Universal. Might be worth a look.
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Old 03-06-2016, 04:10 PM   #8
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Btw even though I did have that one problem with my optima, I'll still only
Use AGM going forward. Prior to that change, battery acid always sloshed out. Not much, but enough to warrant an under hood rinse on occasion. Some road (or lack of road) conditions just aren't compatible with wet cell batteries.
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Old 03-06-2016, 06:34 PM   #9
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I've questioned the advisability of using a lead acid charging profile to charge AGM's in the past, since the manufacturer specifys different voltages from those requred for other types. Never the less, plenty of folks seem to be using the standard alternator voltages, but I question if the life of a premium battery is not getting reduced by incorect charge profiles. I don't know what the life expectency of a properly cared for AGM is, but I had a pair of 8D AGM's in my sailboat, charged with a 3 stage regulator carefully calibrated for AGM's. They were still working fine 10 years later when I sold the boat. I have to imagine they would not have lasted nearly as long if I had charged them the same as a lead / acid battery. In addition, the standard automotive regulator can quickly reduce the charge rate once the start batterys are re-charged, causing extended recharge times on the house batterys (at least I'm seeing that now with my van, using all lead acid). If the regulator was changed to a three stage with the correct profile, I would expect a much faster recharge and a lot longer life for the batterys. Since AGM's are so expensive it might be worth considering, but yes, your alternator will charge your AGM's, but perhaps at the expense of a longer life which is one of the reasons for buying them in the first place.
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Old 03-06-2016, 06:47 PM   #10
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^^So that makes me wonder: is there any device that can be put inline from the alternator charging cable feed to the AGM house batteries that would take that feed and make it AGM friendly?
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