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Replace Key Fob Battery

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E-Class (W212) 2010 - 2016: E 350, E 550

Replace Key Fob Battery

 
Old 07-30-2015, 10:07 PM
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Replace Key Fob Battery

Our E250 is now telling us to replace the key fob batteries. Does anyone know how it determines whether the batteries need replacing? Is the fob sending a message to the car?

I'm a little suspicious because both my wife's key fob and mine began sending the message on the same day, and when I connected the 3V battery to a multimeter, it read 2.96 volts.

Has anyone left their fob for some time after receiving this message? How much longer can I expect it to work without changing the battery?
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Old 07-30-2015, 10:38 PM
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These are nominal 3.0 V batteries and, when new, measure 3.29 V. I have found that the key stops working properly @ 2.99 V. (KG requires higher voltage than "in the ignition".)
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Old 07-31-2015, 07:29 AM
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This topic come up time and again and we all have had different experiences depending on a whole number of variables.
Rather than debate yes/no, why not just change the batteries? They are cheap and shuts down the aggravation of the warning and the debate.
Don't get it from the dealer. They will charge you $15 for a $5 battery. Size is either a CR2025 or CR2032 I can't remember but you can pop the back off the fob in 2 secs and find out.
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Old 07-31-2015, 10:01 AM
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^what he said^
You've already taken the battery out once. LOL
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Old 07-31-2015, 05:37 PM
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There's a section in Operator's Manual on how to check battery. Just press a button and a red light lights on the fob. It it lights, it's good.
I replaced mine because it was over five years old and just keep old ones in glove box.
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Old 07-31-2015, 11:08 PM
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Originally Posted by KEY08 View Post
You've already taken the battery out once. LOL
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Old 08-01-2015, 03:23 PM
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I buy my CR2025 batteries on AMAZON. 20 SONY batteries for $8--or .40 ea.
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Old 08-01-2015, 05:44 PM
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Originally Posted by Alex.currie44 View Post
This topic come up time and again and we all have had different experiences depending on a whole number of variables.
Rather than debate yes/no, why not just change the batteries? They are cheap and shuts down the aggravation of the warning and the debate.
Don't get it from the dealer. They will charge you $15 for a $5 battery. Size is either a CR2025 or CR2032 I can't remember but you can pop the back off the fob in 2 secs and find out.
Was curious if something had gone wrong with whatever the voltage sensing mechanism was as both started reading low on the same day - which is an odd coincidence.

I don't know why anyone would take this to a dealer. It's just a battery swap. I ordered 10 from eBay for $3.
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Old 08-02-2015, 04:48 PM
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Looney, I think there is a problem with the FOBs. My 2014 250BT has been through three batteries in one year. I brought it to the dealers a week ago and they said everything checked out fine.

My 2000 Corvette needed a new battery after seven years, and is now on its second CR2032. My Dodge pickup has been through three batteries in 15 years.

The dealer asked me how far I keep the key from the car and I told him about 20 feet (inside the house with two walls between where it hangs and the inside of garage). He said that was too close, and the problem is the FOB and car will continue to talk to each other if they are within range. I don't know if it is true, but regardless, there is some bad engineering here with three batteries in one year.

Mine is a Keyless GO. Your mileage may vary.

Last edited by JohnAJohnson; 08-02-2015 at 08:07 PM.
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Old 08-02-2015, 10:05 PM
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Looney, FOB batteries work up to 1.85 Volts, you should get a CR 2025 it is the exact MB replacement size to ensure the FOB's rubber gasket keeps out foreign objects and debris (FOD). As mentioned in many posts 7 Meters or 22 feet +/- a few inches is normal full WiFi handshake range, and as mentioned above your FOB's are actually pinging away to maintain contact with the vehicle just like any BT or Wireless device. A good test of this is to leave the FOB's in the house and go out and try to enter the vehicle-if it unlocks-you know it is maintaining the Wireless handshake and draining your battery. This is not rocket science and so when washing your vehicle remove K-GO button, and insert FOB unless you want your vehicle to lock-unlock and beep every time you ground the system while washing the vehicle-rinsing the vehicle, the water grounds the door handles creating resistance on Keyless GO systems just like your hands.
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Old 08-02-2015, 10:28 PM
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i wouldnt even bother to check the voltage, just replace it straight away no dramas. if it gives the warning again after 5 mos or so, change it again. it the fob works as intended, its all good. for .40 each, ill change the battery just for the heck of it.

Last edited by beejAMG; 08-03-2015 at 08:16 PM.
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Old 08-03-2015, 10:35 AM
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My batteries last a minimum of 12 months in my 2010 Keyless Go. No drama.
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Old 08-05-2015, 02:40 PM
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Originally Posted by greasykid View Post
I buy my CR2025 batteries on AMAZON. 20 SONY batteries for $8--or .40 ea.
Exactly.

After buying two-pack blisters at the local pharmacy or supermarket and having to replace them every two-three months to avoid the dash low battery warning, I did exactly what greasykid did. Amazon > same batteries > same quantity > same price. A lot of batteries I thought, but figured I'd go through them in no time anyway. Well, after my keys eating batteries ferociously for months, I'm still on the very first Amazon Sony batteries placed in service nearly a year ago in both keys. And they have a 2024 freshness date, about the best you'll find anywhere.

For anyone afraid to tackle replacing their own key batteries:


It's easy.
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Old 08-05-2015, 02:52 PM
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Same here. Almost 12 months & still going.
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