key fob batteries

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I was wondering how the battery in my Mini Cooper key fob has lasted more than 10 years.  I found a uTuber video showing how to replace the battery, which is said to be rechargeable.  I noticed when the video guy opened the key fob there was a pretty hefty looking coil on the board.  I am assuming there is an RF antenna somewhere in the car near the key fob slot to charge the battery as there are no contacts on the fob.  I found doing a Goggle search that the car will still start if the fob battery is dead.  Is the fob basically an RFID device?  I am tempted to roll my giant old scope out to the car with a loop on the probe and see if I can detect an RF signal near the fob.  Does anyone know how these things work?  I suspect the two Jims know.

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MarkThomas wrote:
I was wondering how the battery in my Mini Cooper key fob has lasted more than 10 years.

Did you get a "Key Fob Battery Low" message on the dash display ?

Are you sure BMW didn't change the battery at any of its previous services ? That would be a reason to charge you extra, so I bet they did.

 

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In the past, keyfobs and things like garage door openers transmit around 300MHz. I assume that is still correct as frequency assignments have not changed. However, really short range inside a car might be different.

 

 But, if it uses wireless recharging, that will be driven at a few hundred KHz, at most. Analog Devices makes LTC4126 specifically designed for the same function in hearing aids. You could probably learn something, there.

 

Jim

Jim Wagner Oregon Research Electronics, Consulting Div. Tangent, OR, USA http://www.orelectronics.net

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N.Winterbottom wrote:

Did you get a "Key Fob Battery Low" message on the dash display ?

Are you sure BMW didn't change the battery at any of its previous services ? That would be a reason to charge you extra, so I bet they did.

 

No, never got a message about the key fob, and the battery has never been changed during service.  I would have noticed the charges (pun unintended).  The key fob is the same old one as always, and from the video the glue joint has to be broken to get at the board and battery.  It is not something done easily, like some of the Honda key fobs have a little screw you take out to open it up to replace the battery.  This key fob it totally sealed.

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Our Jeep Key is chipped; it must be electronically present to run the engine more than 5 seconds.   The connection is wireless; it looks just like a plain old key.  So perhaps the fob is chipped & its presence can be read even if the battery is dead, but you won't be able to unlock the door or remote start--the radio portion will be kaput.   Batteries can last.  We have some doll that fell into the couch that plays some kids tune for about 5 seconds at around 9 PM everyday.  Though it is now rather weak, it has been doing that for about 15 years!

 

 

When in the dark remember-the future looks brighter than ever.   I look forward to being able to predict the future!

Last Edited: Fri. May 22, 2020 - 11:18 PM
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The car manufacturers have used a number of techniques over the years.

The early ones were RFID, most likely down around the 125kHz area.

Many use UHF with a variety of encoding techniques - some more secure than others.

Whilst I have no experience with Minis, it would not be unreasonable for them to use a RF charging method much like the wireless chargers used on mobile devices these days. Just make a small coil of wire of a few turns and hook up to the 'scope. I'd expect something in the range of 100kHz.

 

Last Edited: Fri. May 22, 2020 - 11:20 PM
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sounds like they may have wised up and added charging:

 

04 and prior keys are sometimes called the "land rover" key, because its the same design and heritage. '04 and prior model do have a battery, which eventually requires replacement.
The 2005 key recharges in the ignition. It was part of the redesign effort, and that's also why it resembles the BMW key.

 

===

As a last resort I thought I would read up on induction charging as this is how the battery is meant to recharge whilst the key is in the ignition. My thoughts were that it could not get enough time to charge the battery if it was completely drained (common when the keys aren’t used for long periods). So I needed to find a way to boost the charge for an extended period of time.
So last night I left my key fob lying on top of my electric toothbrush induction charger in the hope that it would charge the key. :rolleyes:
HEY PRESTO! :biggrin::biggrin::cool: This morning I followed the re-programming instruction below and it worked (also re-programmed the working one to make sure they both worked!).

 

https://www.mini2.com/threads/resurrecting-a-r53-cooper-s-remote-key-fob.193147/ ​​​​​​​

When in the dark remember-the future looks brighter than ever.   I look forward to being able to predict the future!

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The fob for my jalopy is multipurpose.  It has a transponder that I assume doesn't need a battery.  There are also 4 buttons that unlock doors, etc.  These use a coin cell.  There is a slot that allows me to pop the plastic housing apart to change the battery. 

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I have never had any trouble with the fobs.  I was just curious how the batteries lasted so long.  The videos I watched made me squirm.  The makers were really bad with the soldering irons they were using to unsolder the old batteries and solder in the new ones.  The batteries have pins on them that are soldered to the circuit board inside the fob, so they are really not meant to be easily replaced.  Plus you have to cut a glue joint to open the fob.  The videos clearly showed a big coil on the back of the board that must be for induction charging.  I am impressed that the fobs have continued to take a charge for more than 10 years, and still work to unlock the doors from 5 meters away, or more. 

 

I still wonder if the car will start with a dead fob battery.  I read somewhere that it will.  There is a hard key that slides out of the fob that will unlock the driver's door the old fashioned way.  Then I assume if the fob is pushed in place the car will start when I push the starter button, with the fob acting like an RFID tag.  I wish I had a modern tiny scope I could easily get to the car and put a little coil near the fob slot, but getting my scope out to the car is a major operation.  I think theory will have to suffice here.

 

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avrcandies wrote:

Our Jeep Key is chipped; it must be electronically present to run the engine more than 5 seconds. 

My Toyota won't start without a transponder being present.  Once started, I can remove the transponder, which is on another key, and the engine continues to run.

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The Mini fob/transponder has 3 buttons.  One unlocks the hatchback door.  One unlocks the drivers door only, and if pushed twice unlocks everything.  One locks everything.  The fob plugs into the dashboard and wont come out unless the car is in Park and the engine off.  Start/Stop is a button on the dash that works when the fob is plugged in.

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No panic button?  I wish mine was like that.  I was at the grocery store this morning waiting for curbside pickup when one of those went off.  The horn blared for 5 minutes.  

 

I guess it was the panic button.  I think some cars will blare their horns if the car is jostled a bit.

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Depending on the fob, some coil can be included for:

 

1) charging a radio batt using inductive coupling

2) acting as a no-battery power source to interrogate the fob (like some ID tags). 

3) both of the above.

 I wish I had a modern tiny scope

Yeah but you've got a triple redundancy set up! blush 

When in the dark remember-the future looks brighter than ever.   I look forward to being able to predict the future!

Last Edited: Sat. May 23, 2020 - 08:38 PM
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Our Jeep Key is chipped; it must be electronically present to run the engine more than 5 seconds. 

 

My Toyota won't start without a transponder being present.  Once started, I can remove the transponder, which is on another key, and the engine continues to run.

 

Our jeep will always start, if it doesn't see a chip within 5 seconds the engine halts.  I don't know if it only checks once & is forever happy, or keeps rechecking later on as well.  Might be interesting to try.  I have a spare key that is non-chipped, that is mostly backup to unlock the doors if needed (or to drive the jeep for 5 seconds cheeky )...we only have one chipped key.   To program a fresh key you need to have two already working chipped keys to make a third or see the $D$E$A$L$E$R

 

When in the dark remember-the future looks brighter than ever.   I look forward to being able to predict the future!

Last Edited: Sat. May 23, 2020 - 08:52 PM
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My car is better than yours.  I just need 1 good key and some $$$.

 

It appears that Ford vehicles, and maybe a few others, use a simpler scheme than my Toyota.  The transponders come from the factory unprogrammed.  To make a second key, it's only necessary to program the new key's code to be the same as the original.  I think Home Depot has a machine that can copy the code in a few seconds.  I know it can cut a key to match the original in a few seconds.

 

My Toyota,and I think many cars, do it the opposite way.  Each key transponder comes from the factory programmed.  Then you have to program the friggin' car to accept the new key's secret code.

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avrcandies wrote:
Yeah but you've got a triple redundancy set up! blush

 

The top one is a digital scope that I only use if I want to capture single events.  Which is rare.  I dont like the digitizer between the source and the display, although it is nice to get FFTs and such.  The middle one is my go to scope.  It is analog and has no annoying fan.  The bottom one is from the 60's and has the little screen with the green trace.  If I use two probes they both trigger at the same time, which is no good if I am trying to dial in a TXO with a known good source.  I trigger on the known good source, and then adjust the TXO until the traces stop moving relative to each other.  That's the best way.  I cant seem to do that with the ancient scope on the bottom.  I had a young EE over here and he laughed at my scopes.  He did like the transistor tester I have that is about as big as 10 scopes and has glowing tubes inside, and weighs about 100 pounds.

 

steve17 wrote:
No panic button?

 

Nope.  And the car has no alarm either.  People around here ignore them until they get really annoying, then they bust a window, pop the hood, and clip the battery cable so the stupid thing shuts off.  Those car alarms were one of the worst ideas ever for city dwellers.

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Maybe this will help:

 

https://www.youtube.com/watch?v=...

 

 

Jim

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jgmdesign wrote:

Maybe this will help:

 

https://www.youtube.com/watch?v=...

 

 

Jim

 

Yeah, that is the one I watched.  You can see the big coil on the back side of the board when he turns it over.

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In lieu of a 'scope you could build a simple circuit consisting of a few loops of wire, a diode and a capacitor. You should be able to harvest some voltage if there is a healthy field in the area. Use a multimeter to measure the volts.

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Couldn't finish the vid - soldering was too painful. Even I can do better. Probably would have cut the leads and used a solder sucker or just tapped them out.

 

Don't know about the coild, but I doubt that it's rechargeable. Probably just has an RFID tag included so that the car can know that the key is in the area. Then the battery is only needed when you push a button.

 

My Dodge key has a removable batt -- 2032 just like the one in your PC sso its not rechargeable.  I think that my Kia key is the same deal.

 

Not sure why BMW went for a solder in battery -- possibly more reliable, but what a PITA.

 

hj

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My push-button-start car will start with the key almost anywhere inside the vehicle.  Passengers purse, back seat, in the boot (trunk).

 

It has a CR16xx user replaceable battery.  However if the battery is flat (or actually removed) you can still start the car by holding the fob right next to the start button for a few seconds. So it must be able to get some charge from there.

 

It also has a mechanical key the can be pulled out of the fob so you can open a locked door from the outside even if the battery is flat.  So they seem to have though of everything.

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My 25-year-old Fiat uses a fairly sophisticated system for its time: The key is interrogated by the immobiliser box, and the immobiliser box is interrogated by the ECU. If the ECU doesn't get an OK from the immobiliser, it won't drive fuel pump, injector power, or starter relay. The immobiliser recognises one factory 'red' master key; with that key one can tell it to recognise a number of other keys (I think four) courtesy some dancing on the ignition switch and accelerator pedal. It's only asked at startup so once it's running it'll continue to run.

 

The door open plibber is separate from the immobiliser and is in fact an IR signal from a fob.

 

Mine has behaved itself for 25 years, but if it ever becomes a problem I have the ECU code version available which does not include the immobiliser test.

 

Neil

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ford2go wrote:
Couldn't finish the vid - soldering was too painful.

 

yeah.  It made me squirm.

 

ford2go wrote:
Don't know about the coild, but I doubt that it's rechargeable

 

What I found searching with Goggle is the 2009 fob battery is indeed inductively rechargeable.  Why else would they solder it in and make it impossible to get at.  Even BMW isn't that evil, I dont think.

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Yes, but maybe they are like smart phone manufacturers who glue the phone shut and overcharge the battery.  I buy phones with replaceable batteries and I use a timer with the USB charge supply.

Last Edited: Mon. May 25, 2020 - 06:06 PM