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Charging AGM Batteries to 100% after depleting?


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#11 TacomaAustin

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Posted 18 August 2022 - 03:25 PM

Solar is going in next month, along with a new charge controller and a 100 Ah lithium battery.  My current AGM, which is less than 100 Ah , is going to be 7 years old in September.  It's time for an upgrade. 


Andrew, I'm guessing you don't have solar? Mine keeps my batteries at 100% pretty easily. 


Edited by TacomaAustin, 18 August 2022 - 05:09 PM.

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Andrew in Austin, TX


#12 Dirt Rider

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Posted 18 August 2022 - 04:51 PM

Most older vehicles cannot fully charge an AGM battery as they need 14 volts which they are not given. I know on some newer trucks you can put in a diode to trick them into doing 14. When I plug my FWC into shore power, I'm not Shure how much it is given, but it has lasted years now. Assuming my short trips and recharge from shore power will do the trick. 


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#13 Krist003

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Posted 18 August 2022 - 07:54 PM

Great stuff everyone, thank you! Took me a while to get through all of it, but worth it (even though some of it is still over my head).

 

I am hoping that this (2) 100AH battery setup allows me to stay above 50% DOD for my short trips, then I can just plug it in to shore power when I get home to give it all the time it needs to get back to 100%. From what I gather, it seems like I would actually be doing the batteries a disservice by charging them back up here and there (but never getting back to 100% SOC) with the trucks alternator. No reason to recharge if the batteries are not at or below 50% DOD. But when hooked up to the truck, the truck would be charging every time you drive around regardless of what % DOD the batteries are.

 

How long does charging (2) 100AH batteries take compared to charging (1) 100AH battery? The same?...twice as long?...somewhere in between?

 

 

 


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#14 Vic Harder

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Posted 20 August 2022 - 11:43 PM

Your vehicle is constantly recharging the starting battery after only a partial discharge.  I've had my starting batteries last for 10 years, so I'm not sure I would worry about partially recharging the house battery.  As for the alternator not being enough to charge with, in practice that is true for a few reasons: 1) Too much voltage drop over too thin wiring between the truck and camper battery and 2) No way for the   alternator to "sense" the presence of more than one battery connected to it.

 

To resolve this, get a good DCDC charger and insert it just before the house (aka camper) battery. Those can be programmed as well as an MPPT can, and thus fully and properly charge your house battery.


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#15 Krist003

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Posted 25 August 2022 - 03:47 PM

That makes sense to me as well Vic, I won't worry about it too much.

 

If I was to take those suggestions above and make a connection to the truck, what kind of charging speed could I expect from the alternator? I am just not sure I would do enough driving during a weekend trip to really charge much.

 

Truck: 2016 Ford F250

Camper: (2) 100AH 12V AGM batteries:

from 75% SOC to 100% SOC = hour many hours?

from 50% SOC to 100% SOC = hour many hours?


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#16 Vic Harder

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Posted 25 August 2022 - 07:06 PM

Lots of unknowns, but let's assume you get a Victron 30A DCDC charger.  It could provide 30AH of charging in one hour of driving.  Your 200AH of battery at 75% SOC is down 50AH.  Two hours of driving would top off your batteries.  At 50% SOC they are down 100AH, so 3 hours of driving should do it.

 

Without a DCDC charger?  Who knows.  Many folks with the stock FWC/ATC wiring report little or no charging.  On the other hand, I had 2g wire from my 2006 alternator to 225AH of AGM batteries and could see 90A going into them at times.  So YMMV.


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#17 Krist003

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Posted 26 August 2022 - 03:46 PM

Thank you very much, that really helped me wrap my head around it. I know its a best guess, but that is exactly what I was looking for. Much appreciated.


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