Charging AGM Batteries to 100% after depleting?

[Krist003]

I have heard that it is best to FULLY recharge AGM batteries once they have been depleted (don't recharge unless you have the time to recharge to 100%). So if you don't routinely recharge your AGM's up to 100% after you have depleted them, it shortens the life of the batteries.

This begs the question: If you are plugged into the trucks battery system for alternator charging, wouldn't that mean you are rarely ever recharging your batteries up to 100% after depleting (unless of course every time you start the truck up you are driving for hours at a time to allow the alternator to recharge your batteries to 100%).

The reason I ask is that I have yet to wire my camper to my truck. Still debating on it. I camp like a nomad, often moving every night, say 30min-1hr away. This would not give the alternator enough time to recharge the batteries to 100% after I have depleted them some.

Another question: About how long does it take for an alternator to recharge (2) 100AH 12V batteries from 50% to 100%. Just a rough guess please.

Thanks all!

 

[TacomaAustin]

if you have a newer vehicle equipped with a smart alternator, I would hazard to say that without a DC to DC charge controller in the circuit the house batteries will never reach a 100% charge. Be aware that a DC to DC charge controller has the added benefit of providing a regulated output voltage that can be greater than the alternator output voltage.

With a DC to DC charge controller, the bulk charging cycle may take 2 hours for the batteries to reach 90% for two 100 Ah batteries depleted to 50% when using a charge controller that outputs 50 amps at its peak.

Battery charging is a three stage affair. The initial bulk cycle is high voltage / high amps. At that stage, as I mentioned they'll be near 90%. That's when the high volts / low amps boost/accumulation cycle kicks in, which takes about 4 hours to finish. The final cycle is a float cycle.

 

[TacomaAustin]

By high volts, I mean the charge control unit has an output of 14.1 to 14.2 volts. The float cycle is usually about 13.1 / 13.2 volts with a low milli-amp output.

 

[Krist003]

That is how I understood it as well. So isn't there a concerned about AGM's needing to be fully recharged after depleting? The trucks alternator would never really get them back to 100%. And wouldn't a solar system cause the same concern? You'd keep charging to somewhere under 100% and then depleting again, over and over.

Is the AGM charging rule not as critical as some make it seem?

 

[TacomaAustin]

To me, the only number that is critical with an AGM is when it has discharged to 12.2 volts - at which point I'm looking for a charging solution. The secret sauce to AGM longevity in my book is to have it on a proper aftermarket maintainer when not in use between trips.

I am assuming that for most RV'ers, if they see 12.7 volts - with the AGM having rested at lest 6 hours - they will call it fully charged - and - for the most part get by just fine for years of use - even though it is technically possible for an AGM to sit for hours unused at 13.1 volts.

With that said, on my now almost 7 year old AGM - I am a happy camper when my AGM is at 12.9 volts after resting - (but that is an ideal situation). 12.8 volts is acceptable as well.

In practice however and without solar, after only hour of driving or slightly more each day, (running my a small DC fridge 24/7), I am 100% happy, if the battery only gets back to 12.7 volt state of charge. Keep in mind this on a single AGM that has been in service for a number of years.

 

[TacomaAustin]

If you need fast bulk charging of 2 AGM batteries from the truck alternator, I do recommend adding a DC to DC charge controller - which requires an upgrade from that skinny #10 gauge wire used by RV manufacturers.

To me bulk charging gets an AGM to near 90%.

This article below mentions three options - but omits the CTEK unit - which I have.

https://nerdtechy.com/best-12v-dc-battery-charger

 

[ckent323]

AGM batteries achieve the longest life when 1) They are not discharged more than 50% and 2) When they are fully recharged every time they are discharged.

It is best not to fast charge a lead acid battery.

The reason most lead acid (FLA, AGM, Gel) batteries fail prematurely is from sulfation due to chronic undercharging or from chronic overcharging. Secondarily they fail prematurely from more frequently than seldom (or infrequently) discharging below 50% of full charge. That written look carefully at the plots of life vs discharge.


Ref:

https://www.pveducation.org/pvcdrom/lead-acid-batteries/characteristics-of-lead-acid-batteries

https://batteryuniversity.com/article/bu-201a-absorbent-glass-mat-agm

https://batteryuniversity.com/article/bu-403-charging-lead-acid

https://batteryuniversity.com/article/bu-415-how-to-charge-and-when-to-charge

https://batteryuniversity.com/article/bu-706-summary-of-dos-and-donts

https://batteryuniversity.com/article/bu-214-summary-table-of-lead-based-batteries




https://batteryuniversity.com/articles

I hope this is helpful


Craig

 

[TacomaAustin]

Definitely a good read - but when on the trail with a 12 volt fridge in use, it isn't always possible to top off an AGM battery back to a 100% state of charge.

When on the road for short stretches, I find myself playing the game of only offsetting what was consumed by the fridge and other devices while at camp. This is what I believe the original poster of thread was referring to.

Bottom line: after the bulk charging phase to get an AGM up to 100% requires multiple hours on a milli-ampere trickle charge at 14.1 ro 14.4 volts. On a new 12 volt AGM - the battery will read 13 to 13.1 volts at rest when it is fully charged. It is easily doable from the truck's alternator and the use of an onboard DC to DC controller, if on a drive across Texas or Interstate travel, but the battery needs a fair amount of time to get to 100%.

 

[Old Crow]

Though it's not an easy read, I believe the article below makes important points on AGM battery charging.

It's a test-bench report on how long it takes to recharge a depleted (i.e., 50% State of Charge) AGM battery to 100% SOC. The test battery in this case is a five-month-old Lifeline AGM battery with an advertised capacity of 105 amp-hours.

Charging is done at .2C and .4C (and those terms are explained). According to the chart, that was 21 amps and 42 amps.

But take note of how many amps the battery actually charges at and for how long.

And, surprisingly, there was only a few minutes difference in total charging time to 100 per cent SOC between charging at 21 amps versus 42 amps.

The article also provides useful info on how far the battery is charged after an hour and after two hours.

How Fast Can an AGM Battery Be Charged?

PS-- the author throws a bit of a curve at us for the last diagram. Most of the article has been talking .2C and .4C charging of the 105ah battery and all of a sudden we have a diagram of charging a 125ah battery at .15C. He does say it's not the same battery but it's a little confusing. Still, there are important points in this section... namely voltage rise and voltage/current flip-flop.

 

[craig333]

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

 

[TacomaAustin]

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.

 

[Dirt Rider]

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.

 

[Krist003]

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?

 

[Vic Harder]

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.

 

[Krist003]

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?

 

[Vic Harder]

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.

 

[Krist003]

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.