16 replies to this topic

[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.

Edited by TacomaAustin, 16 August 2022 - 09:33 PM.

[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.co...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.pveducat...-acid-batteries

 

https://batteryunive...t-glass-mat-agm
 

https://batteryunive...based-batteries

 

 

 

 

https://batteryuniversity.com/articles

 

I hope this is helpful


Craig

Edited by ckent323, 18 August 2022 - 01:45 AM.

[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.