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Can you build a key charger?

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Mangozac
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Can you build a key charger?

Post by Mangozac » Fri Aug 18, 2017 1:37 am

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!
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ph001
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Can you build a key charger?

Post by ph001 » Fri Aug 18, 2017 7:46 am

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
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Can you build a key charger?

Post by RedUn » Tue Sep 19, 2017 10:42 pm

One month later... Did you go back to the future or something? 8)
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Can you build a key charger?

Post by ph001 » Wed Sep 20, 2017 10:31 am

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:
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Can you build a key charger?

Post by Ducklakeview » Wed Sep 20, 2017 1:10 pm

ph001 wrote: Thu Aug 17, 2017 9:44 am 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.


This is the correct replacement, same as the original, a rechargeable Panasonic.
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Can you build a key charger?

Post by ph001 » Wed Sep 20, 2017 1:13 pm

3V lithium as suspected. Thanks Mike.
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Can you build a key charger?

Post by Ducklakeview » Wed Sep 20, 2017 1:15 pm

ph001 wrote: Wed Sep 20, 2017 1:13 pm 3V lithium as suspected. Thanks Mike.
It's tagged top and bottom btw..
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ph001
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Can you build a key charger?

Post by ph001 » Fri Sep 29, 2017 2:32 pm

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.
Z4 key charge waveform
Z4 key charge waveform
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Last edited by ph001 on Fri Sep 29, 2017 3:14 pm, edited 1 time in total.
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Can you build a key charger?

Post by GuidoK » Fri Sep 29, 2017 3:00 pm

ph001 wrote: Fri Sep 29, 2017 2:32 pm You can see the frequency is around 125Khz
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.
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Can you build a key charger?

Post by Ducklakeview » Fri Sep 29, 2017 3:03 pm

GuidoK wrote: Fri Sep 29, 2017 3:00 pm
ph001 wrote: Fri Sep 29, 2017 2:32 pm You can see the frequency is around 125Khz
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.
Yes it has to do the handshake to identify the device before the charging coils are energised.

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Can you build a key charger?

Post by GuidoK » Fri Sep 29, 2017 3:04 pm

Maybe you can put the key next to the phone while it charges and the key can steal some of that induction fields away 8)
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Can you build a key charger?

Post by ph001 » Fri Sep 29, 2017 3:13 pm

What I need now is for some kind soul to send me the innards of a key (pickup coil and battery are the only real important bits). I'll then wind a small air-cored choke and run it with a 125KHz oscillator and monitor the charge into the 3V lithium cell.
2007 E85 Z4 3.0Si manual :driving:
19" CSL's | Eibach Springs | Aeroskirts | Z4M front bumper | Clear brake light | ZHP | RCH+| Stubby

2004 E46 M3 coupe manual - gone but not forgotten.

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