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?

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".)

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.

^what he said^
You've already taken the battery out once. LOL

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.

:D:D:D:rolf:

I buy my CR2025 batteries on AMAZON. 20 SONY batteries for $8--or .40 ea.

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.

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.

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.

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.

My batteries last a minimum of 12 months in my 2010 Keyless Go. No drama.

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.

Same here. Almost 12 months & still going.

I am having the same problem, but my car is over 50 feet away. Changed batteries, then within a month had to do it again, so I then ordered some Duracell's from Amazon, I noted that Amazon stated that over 10,000 had been purchased this month, I figure that is about as fresh as you can get. Well, changed both FOBs with NEW Duracell's, 1 month later, one is working good, and one just said replace your battery. I'm going to chalk it up to one bad battery. That is unless you hear back from me again.
If this keeps up I'll try jamming a 2032 in there, I just test fitted one and it would not fit, too thick, but I'm sure with a little carving on the FOB I could solve that problem.

Before you start carving on the fob, consider that a replacement is about $500 on the cheap end...
that's a lot of batteries.

FOB CR-2032 is it 👍
 

The CR-2032 fits great. I've been using that for almost a decade. Replace every other year or more. FOB batteries are No longer an issue.

https://cimg0.ibsrv.net/gimg/www.mbw...e68606dd8a.jpg
2032 go the distance

If your FOB batteries don't last years and Keyless functions are also unstable at times, you may direct your attention to the amazing Keyless module in the trunk righ side pocket.

Interesting article. Thieves sit is parking lots and wait for a target car to pull in. Then as driver walks away and presses fob, they duplicate the signal with a cheap device they bought on internet. Then send the signal to another person who uses it to get into vehicle and starts the car and drives away. Probably works the other way as well. As for batteries, haven't had any problems in years.

Interesting, the 2032 fits in my non-keyless go remotes, but makes the covers on my keyless go fobs bulge out.

CR2032 will NOT fit in my keyless go fobs at all.

2032 👍
 

make sure the rubber is inserted to match well the opposite side. I did not even spend 5mn the first time I super-sized my 3V cell.

https://cimg4.ibsrv.net/gimg/www.mbw...48581155d5.jpg
matching shape

> Batteries pay for external troubles:
The solderless KeylessGo is a piece of solderless work that keeps poling the keyfob draining tiny coin cell.
The fix is to solder module at once!
Reboot the car to refresh soft-crashed modules

Even today my own car performance still clearly benefits from reboots in addition to the generous bug extermination I have completed experimentally.

This has me motivated to fix 100% of solderless modules: ESP and MFK. Each one I've fixed rewarded me with latencies improvements.

Bosch F-SAM-CGW is one unstable flaky module that likes his reboots (no fuse for its own brain!).

> Quick fix:
So for your keyfob: free reboots are in order!
:zoom:

Yes, 2032s fit greatly in my key fobs . It should also have a longer lifespan over 2025s too I also use it in my bascule as well but that's a different topic :rolf:

Yup, most definitely! IMO the warning for low battery is far too early, as in grossly premature. After having changed batts in mine and my wife's fobs several times, I figured, shoot, I'm gonna test this theory. I think I went over 5 months after the dang light said change batt! I easily could have gone longer, but we were going on a week trip and didn't want it to crap out at an inconvenient time. Eh, I should have left the old one in! So after that, no more premature changes for me. I cancel the idiotic warning message and just keep going, So far, so good! :)

Edward

Question: how does one reboots the car modules to recover from this soft-crashes? The batteries on my KeylessGO really don't last more them a month before I get a low battery warning. They are very cheap (the batteries) but it did stroke me as odd that they would last so little.

first thing first
 

To Reboot car computers...
You simply need to disconnect both batteries: the MAIN and AUX for 15mn.

Don't jump car side ( + ) side to ( - )... let is safely self-discharge.

Following that reboot, scan the car to collect fault codes, note codes then reset faults with scanner.

At this point your car SAM/CGW will work as good as possible without fixing the KeylessGo solderless circuit board. So try the above procedure as a first step knowing expectations - I personally reboot once or twice a month despite my keyless now working perfectly.