Diesel Benz

The AUX battery is isolated with a relay if the car is let to go to sleep. Disconnecting the rear battery is not a bad idea but I would always disconnect the ground cable first. You have to be careful with the rear battery and disconnect it when the car is fully in sleep (definitely no key in ignition etc.), the rear SAM appears to be sensitive and is expensive to replace.

sdifox

Humm, this morning the red light came on again. But not in the afternoon on the way back. Time to call stealership.

vettdvr

IMO your main battery needs replacing. Check Carquest for AGM exact replacements.

sdifox

I live in Canada...wish I had access to the wonderful online stores in the US

vettdvr

Actually there are carquest stores in canada but you will have to go online and search for them. These will be walkin stores because most places wont ship batterys due to the hazards with them.

your profile does not show any location so it is hard to help you with locations, but I have confidence you will find a battery.

mbuskuhl

Thought I'd update to avoid others getting confused like I have been, the GTX14-BS is now replaced by the CTX14-BS. Says so on my new battery I just picked up. Still $79.99 less $10 when you return the core.

sdifox

Excellent, found quite a few near me (well, closer than Buffalo) on their website... Let's just hope their prices are not the standard US Price * 1.5

sdifox

No battery warning light this morning and last night was milder than the previous nights. I am still going to take the car to my mechanic to have the charge system/ battery checked. But I suspect it could be a combination of frigid temperature and low mileage over the weekend that did it.

JimPurdy

I would disconnect the ground wire from the aux battery and check the voltage with the engine off. When my aux battery failed it was only producing 11.2 volts.

dieselophile

I just replaced my aux battery with a Walmart (Everstart Maxx) AGM 14 BS, did not isolate the positive from the main during the change. The red message is no longer coming on but I have had some erratic wiper switch issues that I posted elsewhere, and felt it might be related to the battery change and hence this post. The wipers just stop responding to the stalk switch - no push in wash/wipe, rain sensor wipe, slow or fast wipe. If I restart the car the functions often returns, then mysteriously malfunctions again. Any thoughts???

Diesel Benz

The multifunction lever issues are quite common and this would have nothing to do with your battery work. Sounds odd though if restarting the car makes a difference.

007_e350

Folks, lately whenever the car is off and I have park lamps on or radio running, it gives me "Convenient Features disabled" when engine started. Today I got ESP, ABS & Convenient Features Disabled alongwith stuff like Steering became hard, ambient light shut off, radio off etc ... but Headlights were fine ---- Yesterday during driving I also got the Red Check Battery/Alternator sign, but it went away when I drove at higher RPMs ... Also my driver passenger parklamp front has went out thrice in 2 months --- I dont know if its getting short circuited or what..

Should I replace the Aux Battery ? Or is it more than that ?

vettdvr

You just listed about 3 different problems and one possible solution .

If you had the load on the battery with then engine off and after a while started you might get a low battery warning from the main battery if the main battery was old and losing reserve capacity.

Steering hard. wow,, it is hydraulic and not electrical so no electrical reason for this but hydraulic problem.

Red Check Alternator,, possibly very low main battery putting too much drain on alternator where the voltage drops excessively triggering the alarm. Or alternator failing from having to continually boost battery from pulling power while engine off. or just failing.

Pass park light.

My suggestion given all the issues you have listed is take it to the dealer to do a detail check.

You need some data.

What is the voltage of the aux batt engine off.

What is the voltage of the main batt engine off,, then on.

Is the alternator charging?

These are just the beginning of problem solving.

How old is the main and aux battery?

So to answer your question on replacing the aux battery given the info you have listed is.

007_e350

Hey vettdvr, I got the ES 14-BS from Walmart ($65) // Came home, opened the hood, but cant open that A/C box cover where it says "Battery in the Trunk". I did see a red cable running towards the underside of it so it has to be the battery in there // how did you guys reach to the batt ?

vettdvr

It is under the cabin air filter back by the firewall passenger side.

007_e350

Great thanks!

Here's a pdf I found on the web, detailing functions of each battery - a little hard to comprehend thou
W211 Two Battery Vehicles Function.pdf

this one on removal of AC cover
w211065 Auxiliary Battery R&R.pdf

007_e350

Holy *hit - I broke the left tab of the cabin air filter's housing .trying to juggle it out...

here's whats happening with my car:

Aux Batt: 12.48 V (Orig Made in Italy one)
Main Batt: 11.7 V (black battery) this is the culprit then

From NTB I'm getting one for $188 with 7 yrs warranty - Can someone give aH rating for OEM battery, just to make sure its the right one

konigstiger

Main (trunk) 12V/95AH
Aux (engine) 12V/12AH

vettdvr

FYI.. Carquest Battery BAT 49AGM-70 12V Europe OE AGM $189.99


Check on ebay for a replacement cover.

Make sure it is AGM correct ahr rating and vented properly outside.

You are right 11.7 V main is probably a dead cell in the battery assuming the voltage comes up with engine running.

007_e350

Just replaced both the batteries - I think I got a great deal on both:

- Aux Batt: I got from Walmart Everstart Maxx EH14BS - $65
- Main Batt: I got from Advance Autoparts: $99.98 w/ 2 yrs replacement warranty, it was $132 w/ 3 yrs warranty but I might not keep the car that long (its a 12v 850CCS 95aH)

Stealership prices were:
Aux: $188
Main: $306 w/ tax

vettdvr

My guess is this battery is conventional lead acid not AGM. You might want to check if it is correct for the car. Acid venting in the trunk would not be a good thing.

Clinton Horn

I replaced the aux battery in our 2003 about 13 months ago. The OEM battery lasted about 75 months. The replacement came from Batteries Plus as they are convenient and I had good luck with the same size from there in my bike.
I started getting the red battery warning about a week ago. Checked the main battery voltage at 12.4 and alternator output at 14.2 v idle. Put chargers on both the front and back as we don't drive the car for long distances. Initially, warning disappeared and every thing goes back to normal. After a few days off, start the car and red warning is back. Very disappointing. I begin researching BCM problems etc. I decided to check the aux battery with leads off and got 10.4V
Moral: You cannot trust these Chinese batteries. I asked for and was granted an adjustment but was warned that no further warranty would apply. I asked about the manufacturer. Answer "Somewhere in China".
So if you get stranded by a battery problem and you don't know where to turn, just face east and scream for somewhere in China.
Next time, I am going to be very inquizitive about what I am buying.

007_e350

You were right Jim - I drove with this bat for abt a week and the signs started coming up again ... Finally I went to the stealership's parts & bought original Main Batt for $288 --- No price for peace of mind.. Enjoyin the car everyday!

vettdvr

I'm glad it worked out for you.

If you picked it up you might have noticed how VERY heavy the battery is. When I tried to remove mine from the holder I thought it was still bolted in. My wife was laughing because it wasn't. Guess I am getting a bit older and really noticed the heavy weight of the battery.

fortlaudsummer

When the message comes up again so soon are you checking the BCM before throwing another battery in?