Charging BMW Remote Key(Fob) With Watch/Toothbrush Wireless Chargers  - Page 1
Forum section: electronics / projects

author: Trurl
author_rank: Regular Contributor
timestamp: February 11, 2025, 05:21:55 am
content: I recently replaced the rechargeable battery (Panasonic VL2020) in my BMW E90 remote key fob.As I drive less these days(great subway access), the fob's battery doesn't get much charging time in the car's key socket. After a while, the remote key's effective transmitting distance can drop significantly or drop dead. There are many folks that seek a wireless charger that would charge the remote key at home.There are many online claiming that the Qi type wireless chargers for phones don't work for the remote key fobs, and indeed this seems correct as placing the fob on the charger doesn't trigger the charger to ON state, it just remains in its standby mode. My understanding is that in standby mode(charger is not in charging mode), the Qi charger only checks for a compatible receiver(device) by transmitting a particular type of pulse at certain intervals, which a Qi compatible receiver would recognize and respond to by sending back to the transmitter(charger) a certain signal representing the receiver's presence / coil alignment etc.. Since the fob/transponder is not a Qi device, the Qi charger doesn't respond to the fob's presence at all, and just stays in the standby mode only sending out brief pulse(s) to check for a compatible(Qi compliant) device within the charger's range.But there are many claims of watch chargers and toothbrush chargers working for charging the remote key fob, so I decided to check for myself.First, I found a nice TI(Texas Instruments) article from 2011 which demystifies how wireless charging transmitter & receivers work, and attached below.Wireless Charging one-way communication via loading modulation-backscatter (slyt401).pdf(114.84 kB - downloaded 79 times.)So before purchasing any type of watch chargers or electric toothbrushes, I attached some wire leads to the VL2020 battery soldered on the fob PCB, and took the fob with a multimeter and a USB powerbank to the local electronics shop to test various chargers and got some interesting results.1. None of the chargers resulted in the charging voltage at the battery's leads to "jump" to say 3.4V +/- 0.15V as I'd expected(according to Panasonic's datasheet for the VL2020 battery), instead the multimeter showed a very slow increase in voltage from 2.987V up to 2.991 over several minutes of charging with 3 types of chargers (generic Samsung Galaxy Watch charger, Oral-B/Braun electric toothbrush charger, Apple Watch display stand/charger). Note, the VL2020 battery takes about 25 hours to fully charge from minimal state to about 3.4V.2. The multimeter's numeric voltage seemed to not change at short term glances, but the graphic display of voltage showed a steady ripple suggesting that the transponder's voltage regulation either pulses very small voltage changes for the charging, or the ripple is just a result of the rectification at the transponder's side.Therefore, unlike how a car battery is charged(say at 14.6V for a 12V battery), the fob battery seems to be charged at relatively very small voltage difference from the present voltage of the VL2020 battery increasing gradually in mV increments. As the receiver(in this case the fob/transponder) controls the charging(around 125kHz according to ISTA - the BMW diagnostic software) not the transmitter(fob socket/other chargers), it seems that the fob would behave similarly in the car's socket, unless the transponder chip includes BMW's proprietary programming with various communication messages for charging like the Qi chargers.I suspect that the fob/transponders and watch & toothbrush chargers don't have this relatively complex and strict communication protocol between the charger & device as with Qi specification devices, and this allows some charging(albeit inefficiently) of the fob.If anyone has more insightful info, I'd be grateful to be enlightened.

author: Jeroen3
author_rank: Super Contributor
timestamp: February 11, 2025, 08:03:14 am
content: Uhm, get a key socket from your local dump?

author: Trurl
author_rank: Regular Contributor
timestamp: February 11, 2025, 08:44:00 am
content: I certainly considered that option*, but I soon realized that the "socket" itself is simply an assembly consisting of a coil(possibly 2) with hall sensors, but no control function itself. The electronic control of the fob socket is done by the CAS (Car Access System) ECU via a 14pin cable. The socket and CAS could be had used for say under $100(?), but that's way more cost and complexity for such a simple task compared with getting a $5 generic watch charger.*Note the following thread on 'E90 Post':https://www.e90post.com/forums/showthread.php?t=1564904I've commented on the 2nd page of the thread above, wherein I mention details I found in the diagnostic software ISTA regarding this matter, before I realized it would be way overkill to pursue the real fob socket & CAS option.Of course, I could drive more often, but even so the fob battery would likely not get a "full" charge ever unless I entered a 24hour race or something like that. Ha!Just hoping someone can fill me in on any specific details of how the fob/transponder chip is enabling these chargers for watches & toothbrushes to charge the fob battery, despite being unrelated devices. As indicated in the article I attached above regarding general workings of wireless charging, I'm aware of the general concepts of wireless charging. I'm seeking more specific info on how the hack charging is possible from a technical perspective(i.e. more info regarding the programmed functions within the fob's transponder chip, which is regulating the charging of the fob battery). Thanks...

author: Jeroen3
author_rank: Super Contributor
timestamp: February 13, 2025, 06:37:00 am
content: Does communication happen using the same coil?

author: Trurl
author_rank: Regular Contributor
timestamp: February 14, 2025, 08:17:59 am
content: Quote from: Jeroen3 on February 13, 2025, 06:37:00 amIt seems so according to ISTA(the BMW diagnostic software);[Electronic immobiliser]"Remote key with integrated transponder chipA transponder chip is integrated in each remote key, which can both transmit and receive. Data transfer takes place like in a transformer between the ring aerial in the insertion slot or ignition lock (only E83) and the transponder chip. The remote key transmits data to the EWS or CAS control unit and receives data from this."As I've mentioned in the other forum's post #38, I'm not sure if there is 1 coil(for both charging & communication) or 2 coils(1 for charging and 1 for communication) in the fob socket. According to ISTA, the 14pin cable connecting the CAS and socket consists of 2 pins assigned to transponder aerial(coil) 1 and 2;Pin 1 (Output) Transponder aerial 2Pin 2 (Output) Transponder aerial 1If there is actually only 1 coil and pins 1 & 2 represent this coil's ends, it would make sense that the AC voltage/current applied to this coil would produce the magnetic flux for charging the transponder, and at the same time there would be a sensor in the CAS that is able to decipher signals sent from the transponder(e.g. for access to the car's ignition and perhaps charging as well) to the socket's coil. I've yet to find any such info in ISTA or elsewhere online.It would be a real treat if someone that knows more details regarding any protocol or specifications used in these BMW fob remotes would share the details here. Cheers~

author: Trurl
author_rank: Regular Contributor
timestamp: February 24, 2025, 02:22:19 pm
content: So I made some progress finding the attached Philips presentation of its car security solutions. Page 11 (screen capture below) shows a simplified circuit for the charger(socket) on the left and the transponder(fob) on the rightState of the Art Car Access Security Systems.pdf(2121.21 kB - downloaded 33 times.)It seems to suggest that the transponder(fob) regulates the voltage at around 6V. I'd appreciate some explanation of what type of electronic component the circuit diagram(what looks like 2 zener diodes facing each other) represents and what is being implied about the transponder circuit as a whole, as shown in the slide above.It just occurred to me that the right side circuit above for the transponder(fob) may just be showing only the input AC voltage regulated at about 6 volts, and the rectification circuit to charge the battery in DC(around 3.4V +/- 0.15V) has been omitted?I'd appreciate any insights... Cheers~

author: Jeroen3
author_rank: Super Contributor
timestamp: February 25, 2025, 06:37:06 am
content: That's a TVS diode. It is not a regulating component. It's a surge supression component..

author: Trurl
author_rank: Regular Contributor
timestamp: February 26, 2025, 05:09:34 am
content: Ah~... thanks!Now that I understand that the 6V is the trigger point for the surge protection, then it seems feasible that the operating voltage is 5V(or lower) DC(I realized there is that diode up top that may be simply suggesting rectification)? I'll dig some more to see if I can find specific info on the actual regulation of charging the 3V battery(VL2020). Cheers~