[Joshe93]

Hi,

This has plagued me several times and found very interesting post on how to leave fob in socket in position 1 for 24 hrs. But are fobs and batteries are getting older and there is a better solution!

Use a watch charger or almost any wireless charger for mobile devices. Even if the mah or whatever is a little low it will still push charge via conduction.

Leave on charger for good 10 hrs and check. All my keys went from doing nothing to working 50 yards and thru walls.

I've tried on phone chargers, watch chargers (specifically galaxy s chargers and even off brand stuff.

It works. This is so cheap just try and enjoy. Even if the battery light comes on in e series e90 thru e93 then charge via those chargers because we normally drive 20 miles per day. Especially comfort ones but I think comfort had a built in when u put the key in tray below radio. But non comfort we are at the mercy of soldering. Dont,.
U can buy a generic galaxy watch charger at Walmart for 10 bucks. Get the s3 or s4.buy they all work fine. Those r cheaper.

Hope this helps. Could fine much on this so after practical use her you go.

[tinkerman]

Glad to have run into your post. I was just wondering if even a 3rd party Galaxy Watch wireless charger (dirt cheap/about $5 in my area!, 5V 1A) would work. I'm tempted to just get one, plop the naked transponder PCB with new battery(VL2020 recently replaced) on the charger (without the fob casing) and check with a multimeter if the voltage on the battery's leads rise from around 3V to say 3.4V+/-0.15V. If so this would suggest that the transponder chip on the fob PCB is regulating the charge voltage(as it is supposed to) within the VL2020 battery's specified charging voltage.
*Note: I've tried using a regular Qi charger, but the voltage on the FOB battery's leads did not budge from around 3V, and this was not surprising as Qi chargers for phones(unlike watch chargers apparently) are supposedly turned "on" via some sort of loading-sensing communication between the Qi charger and phone, and the FOB transponder doesn't trigger such sensing in the Qi charger.

I've been looking into how the actual key fob socket works and found info on ISTA that the frequency is 125kHz(within the range of Qi and watch chargers - probably why this works despite not being optimized for distance between transmitter & receiver / associated resonant frequency etc.), and some other interesting info, which I uploaded in another thread:
https://www.e90post.com/forums/showp...3&postcount=38
*note: actual full thread:
https://www.e90post.com/forums/showthread.php?t=1564904
with interest in perhaps setting up a fob socket based charger if the power supply could be simulated as in the car.

But until more info such as the voltage applied on the socket side etc. is revealed, this watch charger option sounds like a good alternative (so long as the charging voltage on the battery is within VL2020 specifications). Cheers~!

[tinkerman]

I found a nice TI(Texas Instruments) article from 2011 which demystifies how wireless charging transmitter & receivers work, and attached below.

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, 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).

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 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.

Of course, despite my observations above, if anyone has more insightful info, I'd be grateful to be enlightened.

[krhodes1]

Fobs for Comfort Access cars do not have rechargeable batteries, plain old CR2025 coin cells.

Why BMW felt the need to re-invent the wheel on this for the non-CA cars is a mystery for the ages. A $2 battery every 5 years is not exactly a hardship, and they already make the fob with the removable battery for the CA cars. Completely baffling. And annoying given how expensive the things are to replace when the battery does finally die.