Looking for info on the key battery charge function

[AceEngineer]

Does anybody out there have knowledge about the ring antenna in the ignition switch? I’m of a mind to build a standalone charger for the battery in my keys. My spare key goes dead and then I don’t drive enough to get it sufficiently charged and my other key barely maintains a charge because I drive my X3 so little. I plan to 3D print a holder for the standalone charger and buy another ring antenna. I know the ring antenna is dual purposed so it also reads the ignition interlock chip in the key, but I think the data signal isn’t required for the battery charge function. The charge function is probably a sine wave for efficient power transfer. I’m hoping somebody has some info about the signal so I don’t have to tear into my car to measure the signal on a scope. How about it, any experts have some useful knowledge?

AceEngineer

[getty150]

I'm surely not an expert...however, to achieve your concern as you describe it, and I interpret it.
All you would have to do is simply leave the key in the ignition. You see the car charges the key while the key is inserted in the ignition...even when the car(engine) is off. Of course make sure the car is in a secure place...(thieves). This works well for you seeing you don't drive the car much!
No need to build a better mouse trap!

[AceEngineer]

I have no place to leave the car secure with the key in the ignition. That is why I want to build a standalone charger, thus my request for that specific info. I want to build a better mouse trap, that is what I do.

AceEngineer

[workingonit]

Originally Posted by AceEngineer

I have no place to leave the car secure with the key in the ignition. That is why I want to build a standalone charger, thus my request for that specific info. I want to build a better mouse trap, that is what I do.

AceEngineer

Not an EE, but I (think) I know a cheap charging solution for us non-technical guys (not you). I couldn't find specs on the EWS charging ring, but did on the charger I am using.

I followed the advice of several sources, on YouTube and several BMW forums, and bought a Philips Sonicare (toothbrush) inductive charger for my only diamond key, that had gone dead shortly after I got it.

Though some people insist that it doesn't work on these keys, I tried charging the original dead battery on it and it worked again (for a day or so), but eventually died (a 20 years-old battery). After replacing the rechargeable battery myself, I've used the Sonicare charger everyday in the two years since then, as I sometimes don't use the key for two months at a time.

My reasoning to try the toothbrush charger, was that I found out that the Sonicare toothbrush battery is a Sanyo LI-ion 3.7v (but the charger puts out 3.7v-4.0v), and the VL2020 battery used in the X5 key is nominally 3v (but actually charges at 3.4v +/- 1.5v), so the Sonicare charger is in the ballpark (for a non-electrical guy like me).

I have several Samsung Qi charging pads at home, but they have an output of 5-9v (per Samsung), so I didn't try them on my key. Those pads have caused three of our Samsung phones' (a S7, S8, and S8+, all Galaxy models) batteries to overheat while charging, and swell up or lose functionality. So far, my S9+ hasn't (yet). My sole BMW diamond key is too valuable to risk.

If you build a better mousetrap, I'd be interested in the specs, but I'd not be able to make one myself, so I'm stuck using what I've got.

[AceEngineer]

I have used a toothbrush charger, but the coupling I got between them did not charge the battery very fast (left it on the charger 2 days). The coupling efficiency depends on the proper alignment of the electric/magnet field lines so 3D alignment and spacing are critical for efficiency. This lead me to duplicating the arrangement that is used in the car. In other words use the actual antenna in exactly the same physical layout. Being an engineer, it is easy for me to design and 3D print a nice setup to hold the key and antenna. It is also easy to gin up a circuit to supply the correct signal. My spare key will then reside fulltime in a optimum charger and I won't tie up my toothbrush base. Plus it gives me an interesting project to do.

AceEngineer