W221 S350 main battery change proecdure

[mbmb123]

I picked up a 2012 S350 in June. Back then, when warm, it would go into ECO mode and turn off the engine when stationary. It's stopped doing that. One suggestion is that it wont go full eco warrior if it thinks the battery is in poor shape. It's probably right.

Using a iCarsoft I can graph the battery voltage during startup. There is a momentary drop to 5.9V which I'm going to ignore, then during the cranking phase the voltage hovers in the low 8s. eg 8.3V.

Googling around suggests different values for a minimum cranking voltage. I've seen 10.5, 9.6, 8.8.
So my 8.3V seems well low.

The current battery is a VARTA 95A 850A AGM cold cranker. Eurocarparts have an equivalent Bosch S5A13 for £200.

The questions are
- can the dealer do a better price?
- when changing the battery, do I first have to disconnect the smaller auxiliary battery in the boot?

Thanks

 

[Harrythedog]

I’d certainly get a quote off the dealer first. I changed the battery on my old 212 e class, waited for the car to go into sleep mode and swapped them over no problem

 

[AMGeed]

Tayna offer the same Varta battery for £185 inc next day delivery, but as Harry above, get a quote from the dealer. They will often beat the best price you already have.

G14 Varta Start-Stop Plus AGM Car Battery 12V 95Ah (595901085)

No need to disconnect the aux battery but you may have to reset the windows afterwards.
(Ign to Pos#2, window all the way down on the button, then fully close and hold button down for a few seconds. Repeat for each window switch.)

 

[Borys]

Shouldn’t be the new battery coded to the car via Star?



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[AMGeed]

^ Never heard of that before?

 

[wongl]

Shouldn’t be the new battery coded to the car via Star?



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No coding should be required.

 

[mbmb123]

I've ditched Eurocarparts (Bosch £200) and bought a £172 Varta from the main dealer. Result!
Maybe I shouldn't be surprised, they're selling 20 litres 229.51 oil for not much more than £60.

I was wondering about the various google "wisdom" I've seen over the last few days, in particular the idea that
a good battery shows 12.6V and a very poor battery 12V. Well, the new battery reads 12.6V and since standing a day the
old battery reads 11.85V.

I haven't swapped them yet. I'm still not sure if I need to disconnect the auxiliary in the rear first, or leave it connected so the
various electronic systems continue to see 12V during the swap. The google advice is contradictory.

Oh, earlier, I'd been looking for a date code on the battery label, didn't find it. Just noticed two numbers stamped on
the negative terminal: 32 19. I think this is week number and Year of manufacture. My old battery shows 12 12 which
I had assumed was voltage.

 

[AMGeed]

I haven't swapped them yet. I'm still not sure if I need to disconnect the auxiliary in the rear first, or leave it connected so the
various electronic systems continue to see 12V during the swap. The google advice is contradictory.

You can swap the main battery over without disconnecting the aux battery. The windows may need resetting afterwards.
Just follow the rule of disconnecting the Neg terminal first, and reconnecting it last.

Oh, earlier, I'd been looking for a date code on the battery label, didn't find it. Just noticed two numbers stamped on
the negative terminal: 32 19. I think this is week number and Year of manufacture. My old battery shows 12 12 which
I had assumed was voltage.

You are correct in your assumption. Your old battery is 7 years old and on the way out at 11.85V after a day.

 

[Riva811]

I just bought a Varta G14 from tyana myself. The procedure is disconnect negative, disconnect positive, replace battery, connect positive, connect negative
You will also need to recalibrate windows, sunroof and steering wheel, simply open and close iwndows and sunroof a couple of times all the way and turn the steering wheel left and right all the way a few times. Coding is not required.

 

[mbmb123]

I was hoping to get this done in the fast failing light, but after removing the top clamp, the battery was still standing firm.

 

[AMGeed]

There is usually a bolt at the bottom clamping the battery from sliding side to side

 

[Chrishazle]

If it's like my S204 there's a bolt/clamp at the bottom rear of the battery. 1/4" drive socket and extension bar on ratchet handle - not sure if it's 10mm or 12mm - does the job!

Oops - just seen your post on an old thread, different problem so as usual I'm talking through my fundamental orofice!!

 

[mbmb123]

Got it done. There was a bracket holding down the bottom short side.
But this was covered by a black plastic electronic box which was itself attached to the battery bracket.
Once that was released by gently pulling on it's cables while pressing two plastic tabs, the bolt was exposed.

With the new battery in place, once the car warmed up the eco light went green and stop/start started functioning.
The cranking voltage is now somewhere in the 10's rather than 8.3V

 

[mbmb123]

Several days later realised I had to reset the windows, that's done. But the radio is misbehaving and the date is set to a far future date, and won't let me change it :-(

 

[tode]

You shouldn't have to reset anything - I've had my battery out several times and never had to do a reset.

I think you have a problem elsewhere - is the aux battery OK ?