AGM requires special charger. T/F?


Expedition Leader
I've looked around the SEARCH function and see more to do with solar charging than a wall unit.

I have a group 31 sears platinum agm that occasionally sees use without the truck running. Seems like a prudent idea to ocassionally put it on a wall charger, but have been told you have to use a special unit and a regular battery charger can ruin the agm or simply not charge it sufficiently.

Any input / recommendations?


Tail-End Charlie
(Don't mind the caps. I'm not yelling. It's just for emphasis. :D )

AGM batteries do NOT require a special charger.

However, LIKE ANY OTHER BATTERY, it will benefit from the ministrations of a quality "smart" charger.

A regular ol' constant voltage charger, such as you'd buy at the local auto parts store, feeds out power at around 12.7v (some are 12.6v, and some are 12.8v). That's it. Current flows though the battery until the battery gets to whatever voltage the charger is putting out, and then the voltage is equal on both ends of the wire and no power flows.

The problem with that, is that at 12.7v, the battery is not totally, completely "full to the brim". More like 90% full. Close enough for government work, but not "as good as it gets".

Why does it matter?

All lead-acid batteries have this problem called "sulfation". Over time, sulfur crystals attach themselves to the lead plates and harden up. This reduces the overall area of lead exposed to the electrolyte, and thus reduces the overall capacity of the battery. Take a 100ah battery, and get a few spots of sulfur crystals here and there, and it's now a 99ah battery. Rinse and repeat enough times and eventually, that 100ah battery might only be a 50ah battery. It still works fine, but for only 1/2 the the time it did when new.

And of course, if you get enough buildup to deform the plates to where they actually touch...well, that's that. The battery is internally shorted and is totally useless.

Sulfation ALWAYS happens - it's just the nature of the lead-acid beast - but certain conditions can allow it to happen faster. Leaving a battery sitting around at less than full charge is one of them. Draining it too far (i.e., taking a cranking battery below 80% full, or taking a deep cycle below 50% full) is another.

Sulfation can also happen pretty damned fast. Take a battery down 50% and leave it there even a couple of days, and there can be enough sulfation to have a measurable effect. (Of course, to actually measure it, you have to go through the hassle of doing a full capacity test on the battery - which is generally only something lab rats are going to do.)

So, the absolute best thing you can do to slow sulfation and extend the battery's useful life, is to keep that battery fully charged "to the brim", and every time you drain anything out of it, immediately recharge it back to full. (And of course, never drain it "too far".)

So, how does a smart charger help?

A quality "smart" charger, such as the Battery MINDer mentioned by Tango, will do the charging in "stages". It will first push the battery voltage up to a high voltage (generally between 14.4v - 14.8v depending on the charger). This gets the battery to 90% full in a hurry. It will then drop the supply voltage to around 14.2v and hold it there for some hours (because getting that last 5%-10% charge into the battery takes a bloody long time), and then eventually, it will drop the voltage down to just "maintain" the battery; Generally around 13.2v - 13.6v (again, depending on the charger).

The two main benefits of that scheme, are that it gets the job done faster and it gets the battery full to the brim. It thereby minimizes the two conditions where sulfation tends to run wild.

(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.)
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Tail-End Charlie
While I'm on the subject of sulfation, I might as well ramble on a bit about DE-sulfation. :D

The theory behind de-sulfation is simple; Hit the lead plates with a pulse of current to vibrate the plates and cause the sulfur crystals to break up.

There are standalone units that just connect to the battery +/- and pull a little power from the battery, then dump it back in as a pulse. There are also chargers, such as the Battery MINDer, which incorporate the pulsing along with the charging.

Sounds good in theory. Reality however, brings complications.

First, there are different theories of what sort of pulse works best. Some de-sulfators use a low-frequency pulse (sort of like a bass hum), and some use a high-frequency pulse (more like a treble screech). Naturally, everyone claims that their method is best.

Second, what happens to the crystals after they break loose? Some say they get dissolved and re-absorbed into the electrolyte, others say they break loose and fall to the bottom of the battery and just lay there.

But here is the reality of de-sulfators: No one seems to know if they actually help at all.

I, and others on the solar forums, have scoured the net looking for ANY objective data about de-sulfators and there just ISN'T any.

There are lots of claims from the people who make and sell them - of course.

There are lots of anecdotal stories from Joe Schmoes about how a battery dead since 1932 and used for target practice by 6 generations of hillbillies was brought back to life by the miracle of de-sulfation (HALLELUJAH!) - of course.


Just about the only thing we know for sure about de-sulfators, is that *some* types of de-sulfator pulses WILL interfere with *some* MPPT type solar charge controllers and cause them to not charge a battery correctly. That's been tested and proven to a limited extent by a few guys with solar systems, MPPT charge controllers and de-sulfators.

(I'll also mention that theoretically, if high-frequency pulsing does actually help break loose sulfation - then a PWM solar charge controller might actually be performing that function to some extent. But again, no one actually seems to know if that's true or not.)

Now, don't get me wrong - even if de-sulfation turned out to be BS, the Battery MINDer is still a SWEET little multi-stage charger (WITH remote battery temperature sensing and temperature compensation - that's a BIG plus).

They also make FAA approved Battery MINDers for all sorts of special aviation batteries. Those guys are definitely clued up.

Personally, I tend to believe that their de-sulfation scheme actually does work - but I can't PROVE it.

My only gripe with the Battery MINDer, is that the biggest one only does 8a charge current, which is great for a single battery, but is too small for a typical RV type battery "bank" - unless you don't mind waiting a long while for the job to get done.


Expedition Leader
AGM batteries do require a "special charger" in that the charger must not charge in excess of 15.3 volts. Many less expensive chargers can and will over charge a battery delivering in excess of 17 volts. AGM batteries use a self closing valve to deal with expansion. When you overcharge the battery hydrogen gas is generated and vented. Once vented there is no way for the gas to be reabsorbed and the battery is diminished. Keep this up and the battery will not live long.
Battery tenders are not usually a problem. Larger chargers capable of 10 amps or more can be. Look to see if they have a setting for AGM that will limit the voltage and not boil your AGM dry.


Tail-End Charlie
Well, no; "15.3v" is not some special hard and fast number for "AGMs". Different battery manufacturers specify different charge voltages. Even different AGMs.

That statement also over simplifies, because the actual charge voltages are temperature dependent.

Interstate Battery charge specs for AGM batteries:

Specifically, note the absorb voltage of 15.5v.

So any charger that won't do a 15.5v absorb stage won't comply with the specs for an Interstate AGM - and none of the non-programmable multi-stage chargers will do that (unless there is one specifically made just for Interstate that I haven't seen).

Optima AGM charge specs:

Specifically, note the max voltage allowed is 15.6v, not 15.3v.

Lifeline AGM charging specs:

Bulk 14.2v - 14.6v ... no max voltage mentioned.

Concorde Sun Xtender AGM manual:

The .pdf is secured and won't let me copy and paste from it, but section 5.4 (page 19) covers the temperature compensation formulas.

Odyssey AGM manual:

(page 16)

"It is extremely important to ensure the charge voltage does not exceed 15V."

Not that it much matters, since in general, non-programmable non-temperature compensating multi-stage chargers, don't go over 14.4v.

Iota chargers can. They default to two-stage - 14.4v bulk, 13.6v float - but the bulk stage can be bumped to 14.6v by inserting a dongle plug. With their IQ/4 three-stage module, they'll bulk up to 14.8v, absorb at 14.2v and float at 13.6v. (In fact, the Iota with IQ/4 is pretty much EXACTLY what the Odyssey manual specs as the best charge profile for their AGMs.) The Iota with IQ/4 will automatically re-initiate bulk stage and run through the charge profile after 7 days at float.

Progressive Dynamics, with their Charge Wizard does what they call 14.4v boost mode (bulk), 13.6v normal mode (absorb) and (after 30 hours) 13.2v storage mode (float) - slightly lower than Iota with IQ/4. The PD will automatically re-initiate bulk mode and run it for 15 minutes every 21 hours, and their dongle has a button you can push if you want to initiate it manually.

The Samlex SEC-1215a charger that I'm planning to put in my camper, does a constant current bulk up to 14.4v, then switches to constant voltage and absorbs at 14.4v until the current drops below 1.5a and then drops to a 13.5v float. (If set to GEL, it tops out at 14.0v instead of 14.4v.)

The Battery MINDer is of course temperature compensating, so the voltages of the three stages vary depending on the battery temperature, but in general, it does bulk in constant current mode at 14.6v for AGM, 14.4v for FLA and 14.1v for GEL and then adjusts those numbers up or down depending on the temperature. It'll do that for up to 20 hours, and then transition into constant voltage absorb mode for up to 5 hours before dropping to 13.4v float (and again, that number is before it gets adjusted for temperature).

The Battery MINDer is probably capable of exceeding 15v - but it won't unless the battery temp is way down - like maybe 40 degrees F. In which case, 15v isn't going to hurt the battery anyway.

I can look at the antique Schauer 10a constant voltage charger that's in my camper right now. Looks exactly like this:

The fine print on the bottom of the face plate says, "DC OUTPUT 10 AMPS AT 12.6 VOLTS".

If you zoom in on this image of a Schauer 6a 6v/12v charger, you can see basically the same statement, except it says "AT 6.3/12.6 VOLTS":

Maybe mine's broken, but in two years of regular use - I've never seen it do 17v.

I remember looking at an early 70's Winnebago that was being parted out. I checked the "converter/charger" in it - 12.6v constant voltage.

Programmable chargers - generally speaking, either good quality solar charge controllers or higher dollar inverter/chargers - are of course...programmable. You can input the exact charge profile specified by the battery manufacturer.

Non-programmable chargers are just "in the ballpark", which is good enough most of the time.
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Expedition Leader
true or false...

your vehicle charging system has a special AGM setting?
It is factory set ;) The alternator is not a battery charger. It recovers the battery from a starting load. It is not designed to charge a battery in a low state of charge. They are generally regulated well below the voltage that would boil a battery , unless the regulator fails.

Voltage is the force. Amperage the flow. If your charger will not exceed 15 volts, you have a "special AGM charger". If it will trickle charge below 13.8 volts it may be possible to run it indefinitely without harm.

The thing to avoid is creating conditions to cause the AGM battery to vent.

Some useful information:
Voltmeter Reading State of Charge

12.84 Volts or higher 100%
12.50 Volts 75%
12.18 Volts 50%
11.88 Volts 25%

Warning: Gel and AGM (Absorbed Glass Mat) batteries
require a voltage-limited charger. Charging a Gel or AGM
battery on a typical shop charger – even one time – may
greatly shorten its life. It is imperative not to exceed 15.0V as this will cause the pressure valves to open and out-gas hydrogen, oxygen and water from inside the battery. This will shorten the life of the battery and cause premature failure.

Recommended charging information:

Alternator: 13.65-15.0 volts
Battery Charger: 13.8-15.0 volts; 10 amps maximum; 6-12 hours
Float charge: 13.2-13.8 volts; 1 amp maximum (indefinite time at lower voltages)
Rapid Recharge (Constant voltage charger): Maximum voltage15.6 volts. No current limit as long as battery temperature remains below 125°F (51.7° C). When current falls below 1 amp, finish with 2 amp constant current for one hour.

All Limits Must Be Strictly Adhered To

Recharge time (assuming 100% discharge - 10.5 volts):

100 amps - 35 minutes
50 amps - 75 minutes
25 amps - 140 minutes

Recharge time will vary according to temperature and charger characteristics. When using constant voltage chargers, amperage will taper down as the battery becomes recharged. When amperage drops below 1 amp, the battery will be close to a full charge. (all charge recommendations assume an average temperature of 77°F , 25°C)


Maximum Rate at

12 V ---------- 50 Amps 30 Amps 20 Amps 10 Amps
12.6 100% – F U L L C H A R G E –
12.4 75% ---- 20 min. 135 min. 148 min. 190 min.
12.2 50% ---- 45 min. 175 min. 195 min. 180 min.
12.0 25% ---- 65 min. 115 min. 145 min. 280 min.
11.8 0% ------ 85 min. 150 min. 195 min. 370 min


Tail-End Charlie
Yea, that info looks like what's on the Optima page.


If you go to their Tech Tips page:

And look at Tech Tip see one of the chargers on the bench, is a regular old constant voltage bench top charger. Which is the charger he actually uses in the demonstration.

Then, look at Tech Tip #4, go to the end, and listen to his very last statement. The one that starts with, "The bottom line is..."
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Expedition Leader
Sorry, I don't have any confidence left in Optima batteries since Johnson Control took over. They can talk circles round and round. I have not been impressed.


American Adventurist
Wow, I've learned so much from the replies to the OPs original question that my brain hurts.

dwh, I am glad you're on our side. ;)


I've looked around the SEARCH function and see more to do with solar charging than a wall unit.

I have a group 31 sears platinum agm that occasionally sees use without the truck running. Seems like a prudent idea to ocassionally put it on a wall charger, but have been told you have to use a special unit and a regular battery charger can ruin the agm or simply not charge it sufficiently.

Any input / recommendations?
Ideally you would get a plug in charger with an AGM setting. From a pratical standpoint I wouldn't worry about charging it at all. If you use an older more generic charger don't use it on a high wattage setting. But don't trickle charge for an extended period a battery like the sears Platinum. With an old charger, your more likely to damage the sears platinum than get an optimal charge.

I have both Odyssey starting and deep cycle batteries. In addition to the vehicle alternator, both sets of batteries are charged by programmable charge controllers (line voltage and solar) that are set specifically set to the specs for Odessey batteries. The charging specs are certainly not the same between flooded and AGM battery types. Per Oddesey specs you do not normally want to desulfate the batteries.

Optima deeper cycle type batteries don't give performance specs comparable to the rest of the industry and IMO aren't even worth discussing.


Tail-End Charlie
Sorry, I don't have any confidence left in Optima batteries since Johnson Control took over. They can talk circles round and round. I have not been impressed.

I wasn't promoting Optima batteries.

I was pointing out that an official rep of a company that makes ONLY AGM batteries, stated that "The bottom line is....[redacted]...ANY battery charger will work fine". (And that emphasis on "ANY" was his - it's how he says it in the vid.)

Weren't those Tech Tips vids made before Johnson took over?

Doesn't matter - the POINT is that AGMs DON'T require a special charger.

(Except maybe the ones from Interstate. Well, some of them anyway...Interstate also has other AGM batteries which have different charging specs than those I linked to.)

But if you do have a good smart charger, it can extend the useful life of the battery. But that's true for all batteries, not just AGMs.


Tail-End Charlie
Per Oddesey specs you do not normally want to desulfate the batteries.
Did you mean "desulfate", or were you referring to "equalize"?

AGMs do require a special routine to do an EQ, which is VERY different than the routine to EQ an FLA and DOES require a special charger.

(Actually, not even a "charger" - a "power supply" - because most "chargers" can't do what is required. Something like this: )

For the most part, you don't EQ AGMs. It can be done but it's usually some oddball process along the lines of, "Set the power supply to provide a precisely controlled voltage of X and a precisely controlled amperage of Y for Z hours."

It's not something an end user would ever do, unless some engineer from the company told them to. And that's not going to happen unless the end user happens to have a lab spec power supply laying around.

So most AGM manuals just say, "Don't equalize".


Tail-End Charlie
Well...since this thread has turned into a general treatise on batteries anyway...AND since it's making Tango's head hurt...

I was thinking I need to expand on this statement, because it is EXTREMELY important as regards AGM batteries:

So most AGM manuals just say, "Don't equalize".
Translation: Never, ever, EVER use the "equalize" function of a normal multi-stage battery charger on any sealed battery (FLA, AGM or GEL).

To do so, will end up causing exactly what Henry described - you'll overcharge the battery and pop the vent and the battery will lose some of its chemistry. As I've said before, "It's all downhill from there".

And so, off we go...

What is "equalize" (which I'll also refer to as EQ)?

Well, in flooded batteries, the liquid can settle and separate and occasionally, it needs to be stirred up a bit more than it gets under a normal charging. This is particularly true in tall case batteries, such as L-16.

Also, in any multi-cell battery, the cells can get a bit out of balance - maybe one of the six cells in a 12v battery has a slightly higher resistance than all the others.

You want them to all be equal, so when that happens, you need to "equalize" the battery.

(And this can happen to any battery. Even AGMs. Which is why most AGM manufacturers have their Top Secret, burn before reading, eyes only (what, no tongues?) AGM equalizing routine. Which they won't tell you about except on a need to know basis. And you don't need to know. Unless you happen to have a lab with the right bit of kit. In which case, they might tell you, but probably not.)

Technically, an EQ is, "a controlled timed overcharge".

The charger will first charge the battery "to the brim" and then it will kick up the voltage - usually to around 15v or higher - and then hold it there for a set amount of time.

This works great for equalizing the cells in flooded batteries, although it will use up a little extra water, so you need to keep an eye on the water levels.

But it's a BAD BAD BAD thing to do to ANY SEALED BATTERY - FLA, AGM or GEL - because it will almost certainly pop the blow-off valve. (And, it's all downhill....well, you get the picture.)

This is not really a problem with a regular ol' charger, since generally, only multi-stage chargers even HAVE an EQ function.

Now the next problem, is that "equalize" doesn't always mean, "equalize".

For example, an Iota charger (if it has the IQ/4 control module) does a standard 3-stage charge. Then, after 7 days at float, if nothing else happens, a timer trips and starts a quick run-through of the three stages again. They call this the "equalize" stage. It isn't really, but that's what they call it.

Progressive Dynamics pulls the same stunt. Every 21 hours, it fires up the bulk stage for 15 minutes. They call this the equalize stage. Again - it isn't, but that's what they call it.

So...if they call it EQ, but it actually ISN'T - no worries. Iotas and PDs are perfectly fine to use on sealed batteries - including AGMs (but they are NOT okay for GELs).

There are quite a few 3-stage chargers which ALSO have a real, no BS equalize function. Solar charge controllers in particular. MOST, but not all, will not run the EQ unless the user manually pushes a button to start it.

If you hook one of those up to a sealed battery - Whatever you do, don't hit that button...

(3 Old Timer points hereby awarded to anyone who can tell me where the line, "Whatever you do, don't hit those trash cans..." came from. :D (And NO Googling for it, dammit.))

There are also some "3-stage +EQ" chargers which will automatically fire up the EQ on a timer. Might be every two weeks, might be every 30 days, might be every 90 days. Depends on who made it.

With those, there is usually a way to disable the EQ function. If you hook up a sealed battery to it - you'd BETTER disable the EQ.

Some marketing genius had a brain fart one day, and now, some chargers which are actually "3-stage +EQ", are sold as "4-stage".

But yea m'kay, don't EQ sealed batteries, m'kay? Cause it's bad, m'kay?
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