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Questions about replacing 12v battery

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405 views 16 replies 9 participants last post by  Avel Du  
#1 ·
My 12v battery is probably due for replacement, and I put it on my 2026 to-do list. I know the battery is at least 4 years old but it could be older. I'm not getting consistent start-stop when on Controlled Efficiency mode (that's the C-mode for the SLK55). I've read all that I could find about changing the 12v here and watched Wilson's video on YouTube, but I have a few questions so I won't find any surprises.

1. Given the weight of the battery, how easy is it to remove old and put in new? I know the trick is to put in at an angle. I'm not a big person and am a bit anxious about not getting a good leverage to lift it out or into place given the corner location of the battery nearest the hood.

2. There's a post here about the POS+ cable being a bit tricky to remove the bolt. I didn't see any DIY videos mentioning that.

2. Is there a breather hose that connects to the battery? I know batteries that are housed inside the car usually have one to vent out gas.

3.My car has an auxiliary battery. Will that keep the electronics charged while the 12v is disconnected? I'm reading mixed opinions; you will lose your radio/seat settings if you don't connect a power supply to the jump points and you won't lose any settings when replacing the 12v because the aux battery will provide power.

4. I'll probably get a M-B battery from the dealer, but anyone recommend the yellow-top Optima battery (which I think is pure lead) for an extra few pennies? I usually would go for Interstate (my battery of choice), but I called up a dealer near me and they surprisingly have a very good price (no more expensive than the Interstate battery)

5. Battery size. Is it H6 for the R172, but the Interstate website has it as H7 for a 2014 SLK55. Confused!

6. No coding required, right? No need to index windows when done?
 
#2 ·
I have the same car albeit 2 years older. I recently changed the main battery with an OEM MB battery and I have to say I struggled with the weight and awkwardness of removing the original and installing the replacement. You definitely need to put the bonnet/hood in the service position to give yourself enough room. When I purchased the car in 2017 I got the dealer to replace the battery and when it recently came up for replacement I thought I would give it a go. I will not be doing that again when this one needs replacing.

With regard to your points make sure you disconnect the negative terminal first. There is a breather hose to vent the battery. I didn't lose any settings but make sure you lower the windows slightly so you can open the car doors without damage when the battery is disconnected. I would recommend whatever to get an OEM MB battery from the dealer and get them to fit it. I also changed the Auxiliary Battery as this was original to the car from 2012.
 
#3 ·
Before replacing, have you fully charge (using agm setting in the charger), or had the battery condition measured?

We have a 2015 C class with s/s.
If battery is not in good condition it does not charge the reserve battery (which handles s/s function) properly. Also, short trips do not give main, nor reserve battery chance to recharge.

There are several threads on here listing ideal requirement that need to be met for s/s operation to be met.

Battery is not cheap & is blooming heavy, awkward to manoeuvre.



Final note, fully charge new battery before fitting. They lose charge gradually whilst on the shelf and many shops do not recharge before sale (some do).
 
M
#4 ·
Just change mine two weeks ago. AS Stated, raise the hood to 90 degrees. Yes, it heavy and that why the hood has to be straight up so you have room to lift. Pad the area in front of the battery. Disconnect neg cable first and then the rest of the wires as stated on the other post. Disconnect the battery hold down clamps. Looking down at battery, lift the end toward the engine up first and then with the left hand and arm lift the battery end toward the fender. You are coming up with the battery + out first. Set on the pad and then lift with handle and lift and put on floor.

Going back end is negative end first. Jut opposite from coming out. Follow all the steps post above to pre charge and hook ups. Tip, Once you start the lift come all the way out in one lift and when putting back in the same. Again, this is close to 60 lbs. plus. You do not have to worry about losing setting as I have not lost anything yet. Second time around for me. I am in my 80s. If you can manage the new battery, then you should be fine.
 
#5 ·
I had today off, so I stopped by the dealer which quoted me $278 over the phone. I had my VIN with me so gave it to the parts lady at the counter and the battery (001-982-81-08) is actually $434 total! 😳😱. When I called them the other day during my lunch, I didn't have my VIN, so the parts guy was looking up the 12v battery using year and model info. I think he quoted me for an R171 SLK55 even though I confirmed that it's for a 2014 and it's the 12v not aux battery I need. I asked if it was a size H7 and he said H6. So that's why I was confused about size in my OP. The R172 SLK55 is quite uncommon, and every time I say my car is an SLK55, they automatically assume it's the R171. The M-B dealer I used to go for my Service A said they rarely see my car when I first visited them. I might have been the first R172 SLK55. Same for the M-B indie I go to now, and they specialize working on AMG models. When they pulled data for my car, the R171 SLK55 stats came up.

Anyway, I'm digressing...thank you so much @Mountainaire for the tip on taking out/putting in the battery! That's what I was looking for but didn't know how to ask it.

@Avel Du, My battery seems to be fine with starting and everything. I'm mostly wanting to replace it based on age and sort of a preventative maintenance. With winter coming soon, I may have to use airscarf and bum warmers and other accessories that will put a load on the 12v battery, so I want to have a good strong battery in place. I'm aware of the criteria that need to be met for start/stop to work. The green led on the ECO button is usually lit, but the 8/4 rectangle on the MFD is usually yellow. I get excited when it turns green, like it did today while I was driving. I do notice that when I have climate control on, such as when A/C is on, s/s is likely to activate (green rectangle) and that may have something to do with creating an optimal cabin temperature. But I agree with many people's opinion that the s/s criteria is very sensitive and will not activate if you so much as breathe too hard.

@MartinatMersea, thanks for the info about the breather hose and the tip for having the window down. I'll need to write that down as a preliminary step to do! While I was at the dealer today, I asked service how much for a 12v battery replacement and I was quoted around $600!😱😨This is one of those jobs I know is easy in principle, and the cost just motivates me to do it myself. (I was actually leaning toward taking to dealer at first.) I know owning any AMG isn't cheap, and I took that into account when I got my SLK55, but I know I'm gonna feel like a sucker for paying that much for a job that should be straight forward. But one reason for this post is just getting as much info as I can and getting mentally prepared! Speaking of cost, I'm leaning toward getting an Interstate AGM battery, which is $280

MTX‑94R/H7 AGM Battery – 850 CCA, 80 Ah | Interstate Batteries

Power modern vehicles with the MTX‑94R/H7 AGM battery: 850 CCA, 80 Ah, 140-min reserve capacity, flexible mounting and backed by a 36‑month free-replacement warranty. Engineered for reliability and high accessory use.
www.interstatebatteries.com www.interstatebatteries.com
 
M
#11 ·
dirty hippy said:
I had today off, so I stopped by the dealer which quoted me $278 over the phone. I had my VIN with me so gave it to the parts lady at the counter and the battery (001-982-81-08) is actually $434 total! 😳😱. When I called them the other day during my lunch, I didn't have my VIN, so the parts guy was looking up the 12v battery using year and model info. I think he quoted me for an R171 SLK55 even though I confirmed that it's for a 2014 and it's the 12v not aux battery I need. I asked if it was a size H7 and he said H6. So that's why I was confused about size in my OP. The R172 SLK55 is quite uncommon, and every time I say my car is an SLK55, they automatically assume it's the R171. The M-B dealer I used to go for my Service A said they rarely see my car when I first visited them. I might have been the first R172 SLK55. Same for the M-B indie I go to now, and they specialize working on AMG models. When they pulled data for my car, the R171 SLK55 stats came up.

Anyway, I'm digressing...thank you so much @Mountainaire for the tip on taking out/putting in the battery! That's what I was looking for but didn't know how to ask it.

@Avel Du, My battery seems to be fine with starting and everything. I'm mostly wanting to replace it based on age and sort of a preventative maintenance. With winter coming soon, I may have to use airscarf and bum warmers and other accessories that will put a load on the 12v battery, so I want to have a good strong battery in place. I'm aware of the criteria that need to be met for start/stop to work. The green led on the ECO button is usually lit, but the 8/4 rectangle on the MFD is usually yellow. I get excited when it turns green, like it did today while I was driving. I do notice that when I have climate control on, such as when A/C is on, s/s is likely to activate (green rectangle) and that may have something to do with creating an optimal cabin temperature. But I agree with many people's opinion that the s/s criteria is very sensitive and will not activate if you so much as breathe too hard.

@MartinatMersea, thanks for the info about the breather hose and the tip for having the window down. I'll need to write that down as a preliminary step to do! While I was at the dealer today, I asked service how much for a 12v battery replacement and I was quoted around $600!😱😨This is one of those jobs I know is easy in principle, and the cost just motivates me to do it myself. (I was actually leaning toward taking to dealer at first.) I know owning any AMG isn't cheap, and I took that into account when I got my SLK55, but I know I'm gonna feel like a sucker for paying that much for a job that should be straight forward. But one reason for this post is just getting as much info as I can and getting mentally prepared! Speaking of cost, I'm leaning toward getting an Interstate AGM battery, which is $280

MTX‑94R/H7 AGM Battery – 850 CCA, 80 Ah | Interstate Batteries

Power modern vehicles with the MTX‑94R/H7 AGM battery: 850 CCA, 80 Ah, 140-min reserve capacity, flexible mounting and backed by a 36‑month free-replacement warranty. Engineered for reliability and high accessory use.
www.interstatebatteries.com www.interstatebatteries.com
Click to expand...
As the above post, I put in the H7 /94r and have plenty of room Want the biggest one that will fit. Most stores want to give you a H6.
 
#9 ·
171 but same idea.

I open the bonnet as normal and then hold as central as I can and walk a couple of steps along the wing.

Post a photo of both hinges (not struts) as I think that some have a locking mechanism to hold the bonnet at 90 degrees and you’d need to release that to lower again.
Then the 172 owners can advise what to do.
 
W
#12 ·
dirty hippy said:
My 12v battery is probably due for replacement, and I put it on my 2026 to-do list. I know the battery is at least 4 years old but it could be older. I'm not getting consistent start-stop when on Controlled Efficiency mode (that's the C-mode for the SLK55). I've read all that I could find about changing the 12v here and watched Wilson's video on YouTube, but I have a few questions so I won't find any surprises.

1. Given the weight of the battery, how easy is it to remove old and put in new? I know the trick is to put in at an angle. I'm not a big person and am a bit anxious about not getting a good leverage to lift it out or into place given the corner location of the battery nearest the hood.

2. There's a post here about the POS+ cable being a bit tricky to remove the bolt. I didn't see any DIY videos mentioning that.

2. Is there a breather hose that connects to the battery? I know batteries that are housed inside the car usually have one to vent out gas.

3.My car has an auxiliary battery. Will that keep the electronics charged while the 12v is disconnected? I'm reading mixed opinions; you will lose your radio/seat settings if you don't connect a power supply to the jump points and you won't lose any settings when replacing the 12v because the aux battery will provide power.

4. I'll probably get a M-B battery from the dealer, but anyone recommend the yellow-top Optima battery (which I think is pure lead) for an extra few pennies? I usually would go for Interstate (my battery of choice), but I called up a dealer near me and they surprisingly have a very good price (no more expensive than the Interstate battery)

5. Battery size. Is it H6 for the R172, but the Interstate website has it as H7 for a 2014 SLK55. Confused!

6. No coding required, right? No need to index windows when done?
Click to expand...
Mountainaire said:
As the above post, I put in the H7 /94r and have plenty of room Want the biggest one that will fit. Most stores want to give you a H6.
Click to expand...
 
W
#13 ·
I just replaced the battery in my 2012 slk 250 with an AGM Group 48 H6 Anlibatt battery for $135 at Walmart online. (made by Weize I believe, chinese). It's only been a few weeks, but so far so good.
This is the 3rd battery, and yes, it is a bit tricky to maneuver it in and out. Takes a little muscle. I remove the negative cable first as others have said. I've never lost anything, clock, radio etc., and no need to reset anything. Just replace and go. That's been my experience 3x.
 
R
#14 ·
If you can buy a battery with a top handle it makes things a lot easier. Getting a battery that doesn't have a handle out of the car will test your patience...and your muscles.
 
#16 ·
If no handle work a couple of straps under the battery and, making sure straps are strong enough and secure, use them to lift the battery. If a second person is available, use a bar through the straps or handle and lift out that way.
 
W
#15 ·
Rick_In_Tas said:
If you can buy a battery with a top handle it makes things a lot easier. Getting a battery that doesn't have a handle out of the car will test your patience...and your muscles.
Click to expand...
I
Rick_In_Tas said:
If you can buy a battery with a top handle it makes things a lot easier. Getting a battery that doesn't have a handle out of the car will test your patience...and your muscles.
Click to expand...
It does,mine had it. Still not what I'd call easy however.
 
#17 ·
Wrt above
Very large zip ties can be useful.
Strong, some flexibility and can be made longer bye fixing an extra tie.

Any hardware store, or online.
 
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