I replaced the aux battery on my 06 CDI 2.5 years ago. About a week ago, I got the red battery visit workshop message. Pulled the code, said aux battery missing or defective. But I hadn't driven the car in about 2 weeks...so thought maybe it wasn't adequately charged. Then drove it consistently for a week or without any more messages. Then got the message again this morning.

2.5 years doesn't seem like a very long time. Anyone know anything about the longevity of aux batteries? I guess the only thing to do is replace it again.

FYI it's an Autocraft AGM that I got at Advance Auto.

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14 E250 Bluetec 4Matic "Sinclair", Palladium Silver on Black, 154k miles
06 E320 CDI "Rutherford", Black on Tan, 172k mi, Stage 1 tune, tuned TCU
91 300D "Otis", Smoke Silver on Tan, 142k mi, wastegate conversion, ALDA delete

19 Honda CR-V EX 61k mi
Fourteen other MB's owned and sold
1961 Very Tolerant Wife

Deep discharge for a couple of weeks of disuse can kill some small batteries. If my 211 is sitting for more than 2 days I hook it up to the trickle charger.

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CENSORED due to not family friendly words

I often do the same with the main battery. But I don’t think that helps the aux battery..

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14 E250 Bluetec 4Matic "Sinclair", Palladium Silver on Black, 154k miles
06 E320 CDI "Rutherford", Black on Tan, 172k mi, Stage 1 tune, tuned TCU
91 300D "Otis", Smoke Silver on Tan, 142k mi, wastegate conversion, ALDA delete

19 Honda CR-V EX 61k mi
Fourteen other MB's owned and sold
1961 Very Tolerant Wife

I'm charging it now. It pretty quickly came 80% charge, which seems odd. I'll test it with my Solar tester once it's charged.

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14 E250 Bluetec 4Matic "Sinclair", Palladium Silver on Black, 154k miles
06 E320 CDI "Rutherford", Black on Tan, 172k mi, Stage 1 tune, tuned TCU
91 300D "Otis", Smoke Silver on Tan, 142k mi, wastegate conversion, ALDA delete

19 Honda CR-V EX 61k mi
Fourteen other MB's owned and sold
1961 Very Tolerant Wife

Shertx, my 06 still has factory aux battery, I say that because nowhere in the Vehicle Maintenance History it mentions that it has been replaced ( dealer maintained car), before I purchased it.

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06 MB E320 CDI 123k miles
14 Sienna (wife's DD) 36k miles
11 GL350 Bluetec 136k (Sold)
96 E300; 292k+ miles (Donated)
11 Sprinter 22K+ miles (Sold)

Wow, that's impressive. Mine is definitely shot...tried to charge but then only putting out 9.5v. Found on on Amazon for $45 so I'll try that.

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14 E250 Bluetec 4Matic "Sinclair", Palladium Silver on Black, 154k miles
06 E320 CDI "Rutherford", Black on Tan, 172k mi, Stage 1 tune, tuned TCU
91 300D "Otis", Smoke Silver on Tan, 142k mi, wastegate conversion, ALDA delete

19 Honda CR-V EX 61k mi
Fourteen other MB's owned and sold
1961 Very Tolerant Wife

Shertex-I have 05 with 327000. I haven't had battery problems but I'm overcautious. I drive from Fl out west (where I am now) and go through desolate areas(montana, az, wy,) and go to remote trailheads. I replace both batteries at about 4 years with MB batteries. I know that sometimes the big battery will just die and won't even crank the engine. Some small cars can't jump start it at times. It gives me peace of mind and at my mature age, I dread being stuck someplace and I'm out of phone contact. But as of now, no problems. steve

I have had to replace the Aux battery in my '05 e320 CDI. Storing the car over the winter puts stress on it. I have the main battery on a Ctek trickle charger. I do not know if the relay in the trunk feeds juice to the Aux batterty or not.

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-- Chris

'95 E300, 216k miles, Silver Surfer
'05 E320 CDI, 138k miles
'07 S550 4matic, 69k miles

Gone but not forgotten:

'76 300D, 350k miles?, SOLD in 1995
'75 240D, 300k miles, SOLD in 1991

Batteries are hit or miss. I installed a brand new expensive main AGM battery in my bro's W211 and about 8 months later a cell failed and it would barely charge or start the car. Replaced it under warranty, was fine from there on out.

The aux batteries are essentially glorified (same) as a computer battery backup battery. I've seen those last anywhere from 1.5-7 years depending on usage and operating environment. If the car is in very hot weather a lot it will decrease the life significantly.

Autocraft are middle of the road batteries from what I've seen....some of the best would be East Penn (often branded as duracell), or a factory MB battery/Varta Germany.

The factory Varta MB battery lasted 18 years in my 2001 E320....and it would still start the car, was just getting weak in the middle of winter. Replaced it with an East Penn AGM.

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-diesel is not just a fuel, its a way of life-
'15 GLK250 Bluetec 118k - mine - (OC-123,800)
'17 Metris(VITO!) - 37k - wifes (OC-41k)
'09 Sprinter 3500 Winnebago View - 62k (OC - 67k)
'13 ML350 Bluetec - 95k - dad's (OC-98k)
'01 SL500 - 103k(km) - dad's (OC-110,000km)
'16 E400 4matic Sedan - 148k - Brothers (OC-155k)

It does but only if the aux battery is low and the engine is running.

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92 e300d2.5t
01 e320
05 cdi
85 chev c10

FYI the one I ordered is a Mighty Max YTX14-BS for $45. Figure it's worth trying. If that proves unsatisfactory I know that the one Napa sells is made by Deka so I'd probably go with that going forward. It's $107 less whatever discount I can get. Dealer wants $165 but acknowledges that they last only 2-3 years typically.

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14 E250 Bluetec 4Matic "Sinclair", Palladium Silver on Black, 154k miles
06 E320 CDI "Rutherford", Black on Tan, 172k mi, Stage 1 tune, tuned TCU
91 300D "Otis", Smoke Silver on Tan, 142k mi, wastegate conversion, ALDA delete

19 Honda CR-V EX 61k mi
Fourteen other MB's owned and sold
1961 Very Tolerant Wife

How much $ was this one @ Advance Auto Parts?

I've been 2X buying the auxiliary 06 E320 CDI from Napa. So, I've replaced it every 3.5 yrs.

Advance Auto Parts is now the Sears battery retailer. They've got a -20% on the battery I use from Sears in my CDI, which is the Diehard. Sears Auto Center is NOT discounted. I checked over the 4th wknd.

I've had it about 5-years, so, I should probably replace it as a good measure in not being broken down. Remember that these W211s don't like to be jump started. Can cause permanent systems malfunctions in the instrument cluster.

Right.... Napa was about $110.00. I'll be interested in how you like the Amazon battery resource?

FYI, you guys using trickle chargers as maintaining your batteries are burning them up if you set it and forget it.

You should graduate and step up to battery tenders... . I keep my '06 CDI/ '17 C7/ '02 7.3 PSD, all on tenders.

FYI the aux batt in my other CDI is also an Autocraft and is going on 3.5 years.

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14 E250 Bluetec 4Matic "Sinclair", Palladium Silver on Black, 154k miles
06 E320 CDI "Rutherford", Black on Tan, 172k mi, Stage 1 tune, tuned TCU
91 300D "Otis", Smoke Silver on Tan, 142k mi, wastegate conversion, ALDA delete

19 Honda CR-V EX 61k mi
Fourteen other MB's owned and sold
1961 Very Tolerant Wife