Benefit to charging AGM battery to full capacity?

outback97

Adventurer
For someone with just one battery (not a dual battery setup) is there a real world benefit of having an AGM battery fully charged beyond what the vehicle's charging system can charge it to?

As I understand it, most vehicles' stock alternator / charging systems can't completely charge an AGM battery... they will only bring it to a certain state of charge. To charge it to 100% capacity, one needs to use an aftermarket battery charger that is sophisticated enough to do so.

Imagine this hypothetical situation. You have charged your AGM battery to its full capacity. You head out for a day trip (or overnight, or weekend) of wheeling and exploring. You drive an hour or two to get there, using your headlights, stereo and HVAC. You air down your tires, then spend hours driving around having a great time, still using electrical accessories, maybe even using your winch. When you're done and it's time to go home, you air back up (using your electrical compressor) and drive an hour or two back home.

Wouldn't your battery now, or more importantly, at some point during this trip, be back to where it was had you not used the charger at all... at the state of charge you'd get from your vehicle charging system alone? Or does the battery somehow hold on to the higher state of charge through all that electrical draw?
 

CaliMobber

Adventurer
Your alternator will bring the battery to 100% agm or not. Your alternator is about the best charger you can use. The alternator almost always will be makeing way more power than you electrical components than you are using so it always has extra for charging the battery.

Your battery isnt actully used beyond starting the car anyways. Say your at 100% when the car is off and you crank the starter to start the car. You only used a tiny bit of the saved energy. This is a guess but say 5-10% is used and now the car instanly begins charging it back up to fill the void.

Once your battery is full it just kinda resists charging and will no longer take a charge and just sits there waiting to be used again later.
 
Last edited:

Herbie

Rendezvous Conspirator
More likely is that the battery ends up at a LOWER state of charge than what the vehicle charges at, unless you drive for a long time, and even then long drives may not do the job.

I killed a couple of Odyssey-built AGMs by chronically under-charging them because I was relying only on the vehicle alternator. For me, a two-hour drive home after a weekend of camping was never enough to get the battery back to full. My van's alternator runs ~14.3v, but the chemistry of the Odyssey AGMs was such that they wanted to see 40+ amps during the bulk charging phase, so they weren't getting what they needed. There were times when I was getting the fridge shutting off after only about 30 hours of usage. Things got a little better when I added a small solar panel and started using an "ok" AGM charger while at home (before/after trips), but the damage was done, and I wasn't even starting a weekend of camping with a full charge.

Once I added a proper charger (IOTA DLS-45 and IQ4 dongle), things got to sustainable. I'd start the weekend at full, and get it back to full once I was home, but my little solar wasn't quite keeping up through the weekend. Now that I've got more solar, I'm pretty close to "indefinite" power.
 

Recommended books for Overlanding

outback97

Adventurer
Upon arriving home,
That hypothetical AGM battery would be at whatever voltage the hypothetical vehicle charges at.
Thanks for the reply, that was my assumption.

In my case the battery is an Odyssey 34R-PC1500 and the vehicle is an Xterra with stock charging system, but I think this would be applicable to other batteries and vehicles as well.

I have read many recommendations online that one needs a smart charger to fully utilize an AGM battery. But I have never understood the point of charging it beyond what the on vehicle charging can do.
 

Herbie

Rendezvous Conspirator
Your alternator will bring the battery to 100% agm or not. Your alternator is about the best charger you can use. The alternator almost always will be makeing way more power that you electrical components than you are using so it always has extra for charging the battery.
The first sentence just isn't true. Yes, the alternator will make more power than the loads and dump it into the battery, but depending on the chemistry, the "surplus" amps may not be sufficient to actually get the battery to 100% SoC. The alternator also doesn't do the 3- or 4-stage charge curve that's needed to get an AGM to 100% SoC, so even if you had a monster alternator with plenty of reserve amps, you're only going to get through the bulk phase, but the alternator will never properly go through the absorption phase.
 

CaliMobber

Adventurer
I dont agree with you. I dont tons of research on just about all battery systems. I had 2 optimas in my 4runner for over 8 years and they worked great.

I understand it doesnt matter how much extra your alternator is making at he end of the charge cycle. Towards the end of the charge the battery will only take a few amps as its almost topped off and needs a slow charge to finish.

The different charging phases just speeds up chargeing time but bumping up the voltage to allow more amps to flow into the battery faster.

(People often confuse AGM and GEL.

Lead-acid batteries that have the electrolyte absorbed into a fiberglass mat (AGM) take generally the same charge voltages and charging schemes as a regular ol' sloshy flooded (FLA) battery.

HOWEVER - batteries which have the electrolyte gelled with an additive (GEL) usually DO require a special charger, because they A) require a bit lower charge voltage, and B) are much more sensitive to overcharging than FLA or AGM.)
 

CaliMobber

Adventurer
One more note.

Battery volts and amps is fully dependent on temperature. A colder battery can take much higher voltage and charge faster than when hot. Higher voltage makes a battery hot and heat is bad. on a cold morning start your car and check the volts. Ive seen 15.5 volts for the first 5-10min of driving before it lowers down to 14.4 which is the voltage for most absorbtion phase chargers including the Optima smart charcher which I love. Float voltage is around 13.5 depending. This is the voltage where your battery is happiest just sitting fully charged.
 

Herbie

Rendezvous Conspirator
Odyssey's recommended charging profiles are on page 15 here: https://www.odysseybattery.com/documents/US-ODY-TM.pdf
The bulk charge requirement is simply stated, but critical: "Note the charger current in the bulk charge mode must be 0.4C10 or more." (Page 16)
The minimum charge current for OPs 34R-1500 is 25A.
The absorption stage recommended is 14.7v for up to 8 hours.


The same page includes shows a cycle life graph which shows how the life is drastically reduced if the battery is undercharged.

My experience with replacing two Odyssey batteries after about 3-4 years each (at almost $300/pop) matches their published data. Believe me or don't. Believe Odyssey or don't. The good news is that a $4 voltmeter and proper test technique will tell you at the end of every weekend whether the battery got back to 100% SoC. Try it for a few trips and collect your own data. Or do what I did and after a couple of batteries decide that maybe $150 worth of charger for a $300 battery isn't a terrible investment.
 

CaliMobber

Adventurer
Just get a solar panel on your roof if your worried about all day charging. I have 80w panel on my 4runner roof. Will fix all your worries about fully charged battery.
 

rayra

Expedition Leader
Thanks for the reply, that was my assumption.

In my case the battery is an Odyssey 34R-PC1500 and the vehicle is an Xterra with stock charging system, but I think this would be applicable to other batteries and vehicles as well.

I have read many recommendations online that one needs a smart charger to fully utilize an AGM battery. But I have never understood the point of charging it beyond what the on vehicle charging can do.
You're looking at it backwards. The AGM needs a higher level of charging than most vehicle systems provide. Thus their early demise. Supplemental charging at home helps offset the sulfation (and whatever else) that AGM batteries suffer from inadequate factory vehicle charging systems. (which is itself backwards, I mean why would they design a battery that's incompatible with most vehicles' factory charging systems, amirite?)

Come to think of it this is probably where the DC-DC chargers idea originated. Which is now bastardizing everything else. I mean people are now designing 'house' systems which are charged at only the very low amp rates provided by DC-DC chargers, cutting off their House systems from the vehicle alternator as a source of much higher charge rates AND they are doing it by sucking down their Starter battery to feed the House side, which is utterly NOT what 'House' is supposed to be about. Because 'hey here's this DC-DC charging thingy'.
I mean it's a violation of 'first principles' to be powering your House side by siphoning power from your Starter battery. The entire point of a 'House'/Aux' side is to prevent power loss from your starter battery.
Unless these DC-DC chargers are only operated when the vehicle is running / charging, they're a terrible idea.

/then again I think having grossly different batteries in a dual setup is a terrible idea too.
 

Buddha.

Lurker
For someone with just one battery (not a dual battery setup) is there a real world benefit of having an AGM battery fully charged beyond what the vehicle's charging system can charge it to?

As I understand it, most vehicles' stock alternator / charging systems can't completely charge an AGM battery... they will only bring it to a certain state of charge. To charge it to 100% capacity, one needs to use an aftermarket battery charger that is sophisticated enough to do so.

Imagine this hypothetical situation. You have charged your AGM battery to its full capacity. You head out for a day trip (or overnight, or weekend) of wheeling and exploring. You drive an hour or two to get there, using your headlights, stereo and HVAC. You air down your tires, then spend hours driving around having a great time, still using electrical accessories, maybe even using your winch. When you're done and it's time to go home, you air back up (using your electrical compressor) and drive an hour or two back home.

Wouldn't your battery now, or more importantly, at some point during this trip, be back to where it was had you not used the charger at all... at the state of charge you'd get from your vehicle charging system alone? Or does the battery somehow hold on to the higher state of charge through all that electrical draw?
The benefit of charging to manufacturers specification is longer life.
 

DiploStrat

Expedition Leader
Thanks for the reply, that was my assumption.

In my case the battery is an Odyssey 34R-PC1500 and the vehicle is an Xterra with stock charging system, but I think this would be applicable to other batteries and vehicles as well.

I have read many recommendations online that one needs a smart charger to fully utilize an AGM battery. But I have never understood the point of charging it beyond what the on vehicle charging can do.


The real question is whether your vehicle's system is designed for an Odyssey battery. Is that the OEM battery? If so, it probably is. If not, based on all of the anecdotal reports I have read on this forum about the Odyssey battery being next to impossible to fully charge, I would suspect that you need some form of shore charger. But here is the trick - to do any good, that shore charger as to meet the needs of your Odyssey, otherwise it will be no better than your Xterra.

With the infamous example of Mercedes Benz setting a bunch of alternators 13.9v and then mounting batteries that wanted 14.4v, most manufacturers will match their charging systems to their batteries - they don't want a lot of warranty costs.

So the issue is not AGM's but rather Odyssey's very specific requirements.
 

luthj

Engineer In Residence
Lead batteries need higher voltage when cold, but they actually charge slower, and deliver less power when cold.

Typical alternator voltages are not high enough to fully charge a lead acid battery. Most require ~14.4V. This plus a lot of time. A full 100% charge for a deeply cycled battery can take 8 hours often up to 12 with the correct absorb voltage.

Failing to regularly charge a lead acid battery to 100% causes sulfation. Essentially the lead sulfate that forms during discharge crystallizes, and cannot be returned via charging. Thus lost capacity. The more time a battery spends at partial state of charge, especially if you do deficit cycles (multiple cycles without a full charge), the more sulfation occurs.
 

Recommended books for Overlanding

outback97

Adventurer
Thanks for the replies.

It seems like the consensus is: proper charging does not help the battery perform better in day to day use. But it may extend the service life of the battery. "Ask not what your battery can do for you - ask what you can do for your battery".

The Odyssey is around 6 years old; it was installed by the previous owner when he added a winch. I've owned the vehicle around 18 months. It's still going strong and appears to be healthy despite not ever having been charged with an approved Odyssey charger. I recognize at some point I'll need to replace it, and I'm wondering if investing in a charger is worth it. Since I've gotten this many years out of it, I'm not sure it's needed, but I have seen many instances of people (including in this thread) saying that they won't last long without proper charging.
 

luthj

Engineer In Residence
If the battery isn't being deeply cycled often, then the alternator can get it back to full on most vehicles. (13.8V or higher). So you are probably okay with your current setup. If you start running it down a bunch with a fridge etc, then consider a shore power charger to get it full after a deep cycle event.
 
Top