I recently installed a new AGM battery; I keep it on the charger on the weekends. So far, everything looks great! It has been very cold here this winter and I wonder if I should get a battery heater for it?

How cold and for how long?

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"Rudeness is a weak man's imitation of strength" - Eric Hoffer

It is about seventeen degrees tonight in an unheated garage. So far keeping it on the battery charger seems to be working.....

Unless the extra power of a warm battery is needed I wouldn't put a heater on it.

__________________
"Rudeness is a weak man's imitation of strength" - Eric Hoffer

Are AGM batteries immune to cold?

Lead acid batteries, of which AGM is a variety, can have electrolyte freeze when their state of charge drops too low. A fully charged lead acid battery will require extremely cold temperatures (I want to say -65 or colder) to freeze, while a fully depleted battery may freeze around 30F.

AGM still has the liquid electrolyte, still has the acid, its just absorbed in a matt. That doesnt change the freezing point. The good news is that AGM tend to self-discharge less...

If youre keeping it on a charger, youre well ahead of the game. The small amount of dissipated energy in a float charge can create some heat, and it keeps the electrolyte in a concentration that is freeze resistant.

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Current Diesels:
1981 240D (73K)
1982 300CD (169k)
1985 190D (169k)
1991 350SD (116k)
1991 350SD (206k)
1991 300D (228k)
1996 Dodge Ram CTD (442k)
1996 Dodge Ram CTD (267k)

Past Diesels:
1983 300D (228K), 1985 300D (233K), 1993 300D 2.5T (338k), 1993 300SD (291k)

Most modern 'smart' chargers perform temperature compensation. Which charger are you using?

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Current fleet:
1998 E300 turbodiesel
2008 E320 bluetec - leaky oil cooler seals replaced
2012 S350 bluetec
2014 GL350 bluetec

Sold:
1996 Passat TDI
1997 E300 diesel

I am using a CTEK multi 7002 12 volt battery charger. It has a setting for AGM and seems to be working well.

The vendor's literature states "The MULTI US 7002 is certified for use between -4°F and +122°F" with no mention of any heating requirements. Your 17 F falls well within the specified range.

__________________
Current fleet:
1998 E300 turbodiesel
2008 E320 bluetec - leaky oil cooler seals replaced
2012 S350 bluetec
2014 GL350 bluetec

Sold:
1996 Passat TDI
1997 E300 diesel

I also purchased the 7002 to use on my AGM. So far I think it is a great product.

Personally I wouldn't connect the charger more than once a week in weather above zero fahrenheit. But that is my thinking, not based on anything I've read so far.

I believe the battery will be more than adequate to handle the normal starting load if the glow plugs are working properly. I prefer to use the block heater that was installed either by the dealer or the factory. It costs about 5 cents an hour ( ten cents per kilowatt ) and in 15 degree weather two hours is typically the time I use the heater before attempting to start the car.

If the charger does not switch to ' maintain ' mode there is a chance the battery can be damaged ' from what I've read '.

For a battery that has too little to start automatically charging, I will set the 7002 to the setting ( SUPPLY ), that supplies a constant voltage. After a few hours I'll either select the ( NORMAL ) or the ' snowflake ' setting again for an AGM battery.

After the battery is charged and the 7002 switches to maintain mode. I will then do a reconditioning cycle by selecting the ( RECOND ) cycle.

This website link goes to the manual for the CTEK 7002:

https://smartercharger.com/products/ctek-multi-us-7002

It describes the 7002, has a video on how the charging cycle works, and a link to look at the manual or download a copy.

I had a 12 volt battery blow up once when removing jumper cables. Luckily I was wearing glasses and had a shower close at hand. So I try to be pretty careful to not ignite any hydrogen buildup.

Also I would not do the RECOND cycle with both battery cables connected. It pulses at close to 16 volts.

Yes, I always wear full coverage protective goggles when connecting or disconnecting battery cables. I need to start wearing chemical resistant gloves as well.

I have two E320 Bluetec diesel cars. They start easily at -20F without a battery charger or battery heater

Many of us could say that.

Doesn’t mean much. Batteries still self discharge. Cars naturally have a parasitic draw. Kinetics of batteries are still slower in the cold.

__________________
Current Diesels:
1981 240D (73K)
1982 300CD (169k)
1985 190D (169k)
1991 350SD (116k)
1991 350SD (206k)
1991 300D (228k)
1996 Dodge Ram CTD (442k)
1996 Dodge Ram CTD (267k)

Past Diesels:
1983 300D (228K), 1985 300D (233K), 1993 300D 2.5T (338k), 1993 300SD (291k)