Keyless Go Fob Batteries Not Lasting
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Keyless Go Fob Batteries Not Lasting
2012 C 250 that is just a wonderful vehicle! Only issue after all these years are the key fob batteries that last 4 to 5 months. Just replaced them again today. The old CR 2032 battery showed only 3.05 volts with my multimeter. Replaced with another CR 2032 and checked the initial voltage and came in at 3.28 volts. After using the fob for a few miles and cycles I again checked the voltage and it came in at 3.18 volts. Seems too low too quickly. Replaced the battery with another new but different CR 2025 and it initially checked at also 3.28 volts. As previously I checked the battery after a few cycles of use and it showed 3.18 volts. These are different brand batteries showing the same results. Just seems to lose too much voltage too quickly.
I also question the activation range of the fob. Walked all the way to the other end of the house to the bedroom which is at least 100 feet from the garage. Through all the doors and house walls the fob still activated the car. Kind of shocked me but what do I know about these fobs. I have heard from other people on this forum that if the fobs are stored too close to the car they are constantly communicating with it. Could this be a problem? Someone mentioned locking the car with the fob when leaving it in the garage so there is no constant communicating.
Been living with this for years and it would be nice to come up with longer battery life.
Bill
I also question the activation range of the fob. Walked all the way to the other end of the house to the bedroom which is at least 100 feet from the garage. Through all the doors and house walls the fob still activated the car. Kind of shocked me but what do I know about these fobs. I have heard from other people on this forum that if the fobs are stored too close to the car they are constantly communicating with it. Could this be a problem? Someone mentioned locking the car with the fob when leaving it in the garage so there is no constant communicating.
Been living with this for years and it would be nice to come up with longer battery life.
Bill
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G550
Batteries of most types exhibit voltages higher that their nominal ratings when new (https://biz.maxell.com/en/primary_ba...aSheet_20e.pdf), and drop after a few hours of use. Is your keyfob failing to work at 3.0 volts?
RF signals are not impeded by non-metallic barriers such as wood framing and drywall. But try wrapping your key fob in aluminum foil and press its lock/unlock button while standing a few feet away from the car.
RF signals are not impeded by non-metallic barriers such as wood framing and drywall. But try wrapping your key fob in aluminum foil and press its lock/unlock button while standing a few feet away from the car.
Last edited by streborx; May 5, 2026 at 10:20 AM.
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Batteries of most types exhibit voltages higher that their nominal ratings when new (https://biz.maxell.com/en/primary_ba...aSheet_20e.pdf), and drop after a few hours of use. Is your keyfob failing to work at 3.0 volts?
RF signals are not impeded by non-metallic barriers such as wood framing and drywall. But try wrapping your key fob in aluminum foil and press its lock/unlock button while standing a few feet away from the car.
RF signals are not impeded by non-metallic barriers such as wood framing and drywall. But try wrapping your key fob in aluminum foil and press its lock/unlock button while standing a few feet away from the car.
My key fob failed at 3.05 volts after checking with a multimeter. I understand what you are saying about shielding in metal. Signal will be defeated. What I would like to know is an unshielded fob is always talking to the car? Some people have indicated that this may be the case. Not sure about that but if it is then the battery would be continuously draining.
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2019 C63CS
For starters you should probably use the correct battery first. CR 2025 and 2032 are different batteries. If I remember correctly, these older generation fobs need a CR 2025. I see you tried that and had similar results as far as the voltage drop goes, but does it last longer? CR 2025 batteries have a higher capacity than the CR 2032, so they should last longer. Only time will tell.
As for the communication, yes if you have KEYLESS GO, then by design the fob and car will handshake while within signal range. Doesn’t matter if the car is locked or unlocked. That’s so it knows to lock/unlock when you touch the handle. In case you are not aware, if you double-click the lock button on the fob, it turns off all communication until you press the unlock button. This has two purposes. First it saves battery and second it prevents a relay attack where somebody can steal your car using a simple relay device. This has become an all too common attack vector. Most newer key fobs such as the one in the FL W205 now have a motion sensor built-in and if it doesn’t sense any motion for 2 minutes, it stops all communication until it senses movement again. This way you don’t have to double-click the lock button twice manually to achieve the same. For older key fobs, there are aftermarket solutions. It’s a little sleeve that goes around the battery and has a motion sensor that disconnects the battery when no motion is detected. Same idea as the built-in solution.
As for the communication, yes if you have KEYLESS GO, then by design the fob and car will handshake while within signal range. Doesn’t matter if the car is locked or unlocked. That’s so it knows to lock/unlock when you touch the handle. In case you are not aware, if you double-click the lock button on the fob, it turns off all communication until you press the unlock button. This has two purposes. First it saves battery and second it prevents a relay attack where somebody can steal your car using a simple relay device. This has become an all too common attack vector. Most newer key fobs such as the one in the FL W205 now have a motion sensor built-in and if it doesn’t sense any motion for 2 minutes, it stops all communication until it senses movement again. This way you don’t have to double-click the lock button twice manually to achieve the same. For older key fobs, there are aftermarket solutions. It’s a little sleeve that goes around the battery and has a motion sensor that disconnects the battery when no motion is detected. Same idea as the built-in solution.
Last edited by superswiss; May 5, 2026 at 04:16 PM.
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Coin cell battery numbers are indicative of their dimensions. A CR2032 is 20mm in diameter and 3.2mm thick. Similarly, a 2016 is 20mm x 1.6mm and a 2025 is 20mm x 2.5mm. The battery's capacity is higher for a thicker batteries. Referencing the Maxell coin cell battery spec sheets, the CR2016 is 90 mAhr, the CR2025 is 170 mAhr and the CR2032 is 220 mAhr.
I recall other posts in which owners have said Energizer coin cells don't perform well -- even new out-of-the-package Energizers still provide low battery warnings. Perhaps there are subtle voltage differences among battery brands, but a key fob shouldn't be that sensitive. A 3.0 volt rated CR20xx battery exhibits a very flat discharge curve over its usable life, and then drops off rather rapidly at end of life. And most batteries exhibit an accentuated voltage differential open circuit versus under load as they age. Maybe MB key fobs aggressively anticipate low battery prematurely.
You can try other battery brands such as Duracell or Panasonic and see if they perform better.
I recall other posts in which owners have said Energizer coin cells don't perform well -- even new out-of-the-package Energizers still provide low battery warnings. Perhaps there are subtle voltage differences among battery brands, but a key fob shouldn't be that sensitive. A 3.0 volt rated CR20xx battery exhibits a very flat discharge curve over its usable life, and then drops off rather rapidly at end of life. And most batteries exhibit an accentuated voltage differential open circuit versus under load as they age. Maybe MB key fobs aggressively anticipate low battery prematurely.
You can try other battery brands such as Duracell or Panasonic and see if they perform better.
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From: San Francisco Bay Area
2019 C63CS
Coin cell battery numbers are indicative of their dimensions. A CR2032 is 20mm in diameter and 3.2mm thick. Similarly, a 2016 is 20mm x 1.6mm and a 2025 is 20mm x 2.5mm. The battery's capacity is higher for a thicker batteries. Referencing the Maxell coin cell battery spec sheets, the CR2016 is 90 mAhr, the CR2025 is 170 mAhr and the CR2032 is 220 mAhr.
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For starters you should probably use the correct battery first. CR 2025 and 2032 are different batteries. If I remember correctly, these older generation fobs need a CR 2025. I see you tried that and had similar results as far as the voltage drop goes, but does it last longer? CR 2025 batteries have a higher capacity than the CR 2032, so they should last longer. Only time will tell.
As for the communication, yes if you have KEYLESS GO, then by design the fob and car will handshake while within signal range. Doesn’t matter if the car is locked or unlocked. That’s so it knows to lock/unlock when you touch the handle. In case you are not aware, if you double-click the lock button on the fob, it turns off all communication until you press the unlock button. This has two purposes. First it saves battery and second it prevents a relay attack where somebody can steal your car using a simple relay device. This has become an all too common attack vector. Most newer key fobs such as the one in the FL W205 now have a motion sensor built-in and if it doesn’t sense any motion for 2 minutes, it stops all communication until it senses movement again. This way you don’t have to double-click the lock button twice manually to achieve the same. For older key fobs, there are aftermarket solutions. It’s a little sleeve that goes around the battery and has a motion sensor that disconnects the battery when no motion is detected. Same idea as the built-in solution.
As for the communication, yes if you have KEYLESS GO, then by design the fob and car will handshake while within signal range. Doesn’t matter if the car is locked or unlocked. That’s so it knows to lock/unlock when you touch the handle. In case you are not aware, if you double-click the lock button on the fob, it turns off all communication until you press the unlock button. This has two purposes. First it saves battery and second it prevents a relay attack where somebody can steal your car using a simple relay device. This has become an all too common attack vector. Most newer key fobs such as the one in the FL W205 now have a motion sensor built-in and if it doesn’t sense any motion for 2 minutes, it stops all communication until it senses movement again. This way you don’t have to double-click the lock button twice manually to achieve the same. For older key fobs, there are aftermarket solutions. It’s a little sleeve that goes around the battery and has a motion sensor that disconnects the battery when no motion is detected. Same idea as the built-in solution.
Very good information!
I have installed a Duracell CR2025 battery. I did not know about the double click procedure so will be trying that method from here on out.
The range of this fob still amazes me. I have tested it at approximately one hundred feet of distance and it unlocks the car. My operator's manual says something different. With Keyless Go the manual states that you have to be at no more than 3 feet from the door handle to lock or unlock. Starting the car you do have to be that close. With the engine running the message "key not detected " alert will display as soon as I am more than a few feet with key in hand from the car. That seems to be correct. For now I will be trying the double click method and see how the battery lasts. Interesting that the manual says nothing about double clicking the fob.
Bill
Last edited by Pepperschatz; May 5, 2026 at 10:02 PM.
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2013 c63 Magnuson SCed
My c63 has a regular key, but the wife's Lexus has a BT fob and mine would go through batteries like yours. The wife's battery would last years. Turns out, my key to her car sits too close to the car all the time and is constantly communicating. Her key is just far enough across the kitchen to not be constantly talking to the car. I bought a cheap Faraday key case to keep it in when I'm not using it and haven't had to replace the battery yet. Less than $10 on Amazon and probably a year later. Never got more than a few months before.
Last edited by Accidental L8 apex; May 5, 2026 at 10:32 PM.
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My c63 has a regular key, but the wife's Lexus has a BT fob and mine would go through batteries like yours. The wife's battery would last years. Turns out, my key to her car sits too close to the car all the time and is constantly communicating. Her key is just far enough across the kitchen to not be constantly talking to the car. I bought a cheap Faraday key case to keep it in when I'm not using it and haven't had to replace the battery yet. Less than $10 on Amazon and probably a year later. Never got more than a few months before.
Your C63 has a regular key? I am not familiar with a C63 key. So yours still needs a battery. Could you elaborate on that? A Faraday key case sounds interesting and maybe that will be my solution if my double clicking test does not pan out. Thanks for your suggestion.
Bill
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G550
If you normally set your key fob down in your house somewhere that's close to your garage, you don't need a fancy Faraday shield for storage. Any metal container will suffice, such as cookie tin.
