Can you build a key charger?

ph001 said:
Can anybody confirm if the battery in the key fob is definitely 12V? Seems more likely it's a 3.6v lithium, in which case you would be looking at a 5v charger so the basic wireless phone chargers should work.
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It's definitely not 12V, but either way the supply voltage of the charger is irrelevant for an inductive charging system.
 
Yeh, I suppose as it's really down to the turns ratio of the coil in the charger and the pickup coil in the key. But then the coupling will be so poor it might be pretty much irrelevant as long as the induced voltage on the key is above 4V or so.
 
ph001 said:
Can anybody confirm if the battery in the key fob is definitely 12V? Seems more likely it's a 3.6v lithium, in which case you would be looking at a 5v charger so the basic wireless phone chargers should work.
Click to expand...

Just tried my key on the wireless desktop charger here - Nada..

Mike
 
Ducklakeview said:
ph001 said:
Can anybody confirm if the battery in the key fob is definitely 12V? Seems more likely it's a 3.6v lithium, in which case you would be looking at a 5v charger so the basic wireless phone chargers should work.
Click to expand...

Just tried my key on the wireless desktop charger here - Nada..

Mike
Click to expand...


I used my toothbrush charger to do mine but had to resync both keys to get it to work
 
If you can't find a spec for the inductive charging frequency then the way to find out how to drive the coil is to stick a wire loop into a spectrum analyser and measure it next to the keyhole on the steering column. I have such equipment but I would need to find time to do it :/
 
Is the frequency dictated by the the coils? Or the charge going thru them? Or both?

I plan on using a s/h coil of an e46. Basoaccly the whole barrel unit. It's just how to drive it I don't understand.

Ac or dc current and how much.
 
Dablk said:
Is the frequency dictated by the the coils? Or the charge going thru them? Or both?
Click to expand...

Neither.

The coil will pick up most frequencies but some will give a higher efficiency than others.
But as I wrote, the coil is handling a duplex signal (both the charge energy and the transponder data) so there is some active filtering.

Chances are that the coil in the key is not used for transmitting transponder data, but the coil in the lock is used for recieving it (at least, thats what BMW WDS tells us)
 
GuidoK said:
Chances are that the coil in the key is not used for transmitting transponder data, but the coil in the lock is used for receiving it (at least, thats what BMW WDS tells us)
Click to expand...

+1, when my car goes into the dealer, the key is read by the dealer to retrieve service data etc.
 
In six years of week end driving. My key has always remained charged. My wife's key did lose it's marbles once. So I went through the reboot procedure with both keys, plus a couple of trips out & soon had it working fine again. It has never done it again & that key only see's the car ever few months. :D
 
The EWS function of the key is effectively a passive RFID system. It operates even with a flat battery in the key because it is powered by the steering column EWS loop. When an electromagnetic field of the right frequency is presented to the key it modulates this carrier signal so that the EWS system can read the data back from the key. The key doesn't actually transmit anything but rather "hacks" the signal being transmitted by the EWS loop antenna.

This same carrier signal is also used to provide charging for the battery which powers the remote central locking.

So in order to charge the key using the EWS loop from a car (and without an EWS module connected to it) you would need some kind of oscillator to generate a carrier frequency similar to that created by the EWS module. It's probably quicker and easier to just try some cheap electric toothbrush chargers!
 
I'm going to have a look at this with an rf sniffer probe and portable scope. Will report back within a day or two
 
Yes, unfortunately stuck in 1965 at the mo, but as soon as I get back I'm onto this.

Actually our sniffer probe is duff so we are waiting for a new one to arrive :thumbsup:
 
ph001 said:
Can anybody confirm if the battery in the key fob is definitely 12V? Seems more likely it's a 3.6v lithium, in which case you would be looking at a 5v charger so the basic wireless phone chargers should work.
Click to expand...



This is the correct replacement, same as the original, a rechargeable Panasonic.

20170920_130357.jpg

Mike
 
OK, new sniffer probe arrived so took a look at the waveform the car emits to charge the key. Strangely it doesn't produce this continuously, it is present for around 2 seconds and then disappears for about 4 seconds - so a 25% duty cycle.

You can see the frequency is around 125Khz and is fairly sinusoidal apart from the small 'tit' at the peaks.

Charging waveform.jpg
 
ph001 said:
You can see the frequency is around 125Khz
Click to expand...

This is in the same frequency range as for example the Qi wireless charging (the low amp setting, 100-200khz).
Although I dont know if it'll be compatible. I dont know if Qi has some handshake or proximity detection, but I suspect it does.
 
GuidoK said:
ph001 said:
You can see the frequency is around 125Khz
Click to expand...

This is in the same frequency range as for example the Qi wireless charging (the low amp setting, 100-200khz).
Although I dont know if it'll be compatible. I dont know if Qi has some handshake or proximity detection, but I suspect it does.
Click to expand...

Yes it has to do the handshake to identify the device before the charging coils are energised.

Mike
 
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