Just purchased a new an AGM (absorbed glass mat) battery for the 1984 300DT. Can I charge it up using a regular battery charger?

You shouldn't but if you do, use a low amp setting on your charger, preferably 2 amps or so.

I use the CTek MXS 5.0 charger. Yeah, it's pricey but with the cost of AGM batteries anymore, well worth the money.

The BMK 12V 5A Smart Battery Charger Portable Battery Maintainer isn't a bad unit for half the cost.

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“Whatever story you're telling, it will be more interesting if, at the end you add, "and then everything burst into flames.”
― Brian P. Cleary, You Oughta Know By Now

the charging profile on AGM batteries is different from flooded acid batteries. same with the old gel batteries. You need to follow the manufacturers recommendations. The charging and float voltages are different and it is possible to ruin an AGM in short order, depending upon the battery .

If the charging voltages and the profile for AGM batteries are indeed different than those for flooded batteries, what should we know, and what changes, if any, are required in a vehicle's charging system when a flooded battery is replaced with an AGM battery?
Conversely, if the vehicle's charging system is OK for an AGM, then why is an external charger required to be different?

Difference between a rectified (standard old battery chargers) and a modulated current.

The old chargers put out a set amount of voltages per setting. Try this, install a voltmeter in-line with the standard charger. Connect the charger to a battery, read the different voltages being supplied at each setting. You'll be surprised at how high the voltage reaches. The old chargers tend to "shock" the batteries into storing a charge. I have seen some chargers reach up to 21+ volts on the high settings.

AGM batteries are not designed to receive this amount of current and have no provision to dispel the off-gassing/heat which occurs unlike the standard "wet plate" batteries which have vents and need water added periodically.

Automotive systems use regulators to limit the current/voltage being fed to the systems.

AGM specific chargers use a computer chip to mimic the action of the regulator by a modulated amount. It ensures the voltage/amperage being fed to the storage device to be a safe level.

We could be a bit more technical but that is the gist of the differences.

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“Whatever story you're telling, it will be more interesting if, at the end you add, "and then everything burst into flames.”
― Brian P. Cleary, You Oughta Know By Now

Good stuff, Mike D. In-car charging systems generally operate in the range of 13.8 - 14.5V; what is the upper limit for AGM batteries? Did your C-Tek charger come with any literature that would shed more light in the matter of voltage and current limits, and/or ramp-up & ramp-down voltage profiles?

Tough to nail down an exact number. It obviously depends on the amp hours of the battery but a reliable rule of thumb is a charge rate of 14.2- 14.8V. The float rate is 13.2 - 13.8V.

The manufacturer doesn't list the voltage at each stage of the charge procedure. The maximum amperage is (surprise!) 5.0 amps at about 15.1V which is adjusted/modulated by the charger's internal circuity.

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“Whatever story you're telling, it will be more interesting if, at the end you add, "and then everything burst into flames.”
― Brian P. Cleary, You Oughta Know By Now

I've had two DieHard Gold AGM batteries die in my 300. I even got an adjustable regulator to charge at the needed 14.5v in cold weather. Both times the AGM batteries had a cell die. They would charge to ~12.8v then fall down to ~10.6v within an hour resting and both failed load test at ~8.6V with 450amps load.
I've since switched to an ACDelco Advantage flooded cell battery last month and its been resting to 12.8V overnight consistently. I wanted a genuine Mercedes battery, but they only have AGM now.

I have a Harbor Freight Batter Charger and it has a AGM setting on it (I just took a look to be sure). That can be verified by going to the Harbor Fright sight and looking at the picture.

So you don't need to get an expensive charger to do an AGM.

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84 300D, 82 Volvo 244Gl Diesel

CTEK (56-353) MULTI US 7002 12-Volt Battery Charger

This is the charger I chose after installing my first AGM. Wish I had bought both when first available. Car starts so much quicker.

AGM batteries have worked fine in my car. I ran a made in USA Optima AGM battery in my '82 300D-Turbo - it lasted over 9 years. This was with the original alternator and original voltage regulator.

I replaced it with a Group 49 AutoZone Duralast Platinum AGM battery about 6 years ago. This battery has been great as well. The alternator was replaced with a new one about 2 years ago.

I use a CTEK MUS 4.3 Test & Charge to maintain it. It has a choice of 3 charging voltages. I recondition the battery about once a year as recommended. The last reconditioning cycle I ran took about 5 hours.

The CTEK user manual lists the charging voltages as 14.4 V for standard charging, 14.7 V for cold weather charging, which is what I would use if I needed to charge the battery with the charger; and maximum of 15.8 V for the reconditioning mode. The charger varies the voltage during various charging/reconditioning stages.

The user manuals are available here: https://smartercharger.com/pages/manuals

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Marshall Welch
Seattle, WA
1982 300D-T

I also have a CTEC MUS 4.3, and it is a great unit. However, unless I am mistaken, one should NOT use the "reconditioning" cycle on an AGM battery. IIRC, the reconditioning cycle is used to remove sulphite deposits from the plates on conventional (liquid electrolyte) lead-acid batteries. This is done by shocking the dielectric layer off of the plates by using the higher voltage, and de-stratifying the electrolyte by (lightly) boiling it.

While the bubbles will help stir up a conventional battery, if bubbles form adjacent to plates on an AGM battery, they go nowhere... and you've now got less electrolyte contact area on the plates. NOT good.

If I am wrong, someone please correct me so that I am not disseminating incorrect information!

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Current rolling stock:
2001 E55 183,000+ Newest member of the fleet.
2002 E320 83,000 - The "cream-puff"!
1992 500E 217,000+
1995 E300D 412,000+
1998 E300D 155,000+
2001 E320 227,000+
2001 E320 Wagon, 177,000+

Prior MBZ’s:
1952 220 Cab A
1966 300SE
1971 280SE
1973 350SLC (euro)
1980 450SLC
1980 450SLC (#2)
1978 450SLC 5.0
1984 300D ~243,000 & fondly remembered
1993 500E - sorely missed.
1975 VW Scirocco w/ slightly de-tuned Super-Vee engine - Sold after 30+ years.

According to my tech support contact at CTEK, while reconditioning an AGM doesn't do any good, neither does it do any harm. Here's what he said:

"Actually, reconditioning is only required with flooded lead-acid batteries. It is not necessary to recondition AGM batteries however there is no harm in reconditioning them either. AGM batteries are not flooded therefore do not suffer from stratification like normal lead-acid batteries do. Reconditioning attempts to fix a stratified battery."

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14 E250 Bluetec 4Matic "Sinclair", Palladium Silver on Black, 157k miles
06 E320 CDI "Rutherford", Black on Tan, 175k mi, Stage 1 tune, tuned TCU
91 300D "Otis", Smoke Silver on Tan, 144k mi, wastegate conversion, ALDA delete

19 Honda CR-V EX 70k mi
Fourteen other MB's owned and sold
1961 Very Tolerant Wife

Interesting and useful .

I usually only get 5 ~ 6 years out of my Diesel Merc. batteries, they work and test fine until they don't, usually far from home in the Desert during a protracted 100 + degree road trip .

What brand and where to buy an AGM W123 battery please ? .

Maybe I'll loose one this Summer and need to try this out .

Great knowing I'm supposed to slow / gentle charge these, I always wondered .

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-Nate
1982 240D 408,XXX miles
Ignorance is the mother of suspicion and fear is the father

I did then what I knew how to do ~ now that I know better I do better

AutoZone Duralast Platinum AGM was recommended in these forums, in one, maybe more than one thread(s). They were said to be made by East Penn. Mfg. (Deka), at the time I was looking. I haven't checked lately. I've been very pleased with mine, at least so far- bought it in April, 2015. I'm in a fairly mild climate zone, though, so take my experience for what it's worth.

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Marshall Welch
Seattle, WA
1982 300D-T