Key fob

[colin2x2]

Just replaced my 451 key fob with a new CR2016 small button battery, as the side lights were fast flashing with a lot of flashes, when opening or closing button pressed. Its fine now.

This is the signal to tell that you need a new battery, which i have now sorted, but my question is this....

My question is....How does the car itself know that the key fob battery needs replaced just before it runs out of battery power, and creates this fast flashing?

PS......for anyone wanting to know how to replace key fob battery, its very simple, just insert a 10 pence coin in the slot at the back of the key fob, twist the coin, and the two halves of the fob open up, then remove battery, put in the new one and click the two halves closed. sorted. Takes about a minute to do.

[Mjolinor]

I replaced mine yesterday for the same reason We will have to see which lasts longest. I use Renata batteries.

The key transmits the fob battery status every time you use it. Once it is marked as low a bit changes so the car knows it's time to change it.

It's so easy to write how to do it but it took me over 20 minutes to clip the goddam thing back together.

[reinaldo_is_spam]

The car works it out because as the battery voltage decreases , the output oscillator frequency of the key fob shifts a little ... this is how the car works out the battery is on the way out.

The key doesn’t transmit its status , other than sending the function command and the next code sequence generated by a pseudo-random number generator.
The car can work out it’s a known key because it knows the last pseudo-random number spat out from it... as it’s pseudo-random it knows what the next code sequence will be.
When you program the car , you’re just telling it the next number received will be a genuine number, after that the receiver know the next number from that fob. In a likewise way it can store another key.

[tolsen]

Interesting explanation Reinaldo.

Question for you: Why have my key fob batteries not yet expired? They are all original from 2002 and most likely made in 2001 or earlier since car was first registered in the UK in April 2002. Smart 450 Pulse Cabrio Cdi.

[reinaldo_is_spam]

Interesting explanation Reinaldo.

Question for you: Why have my key fob batteries not yet expired? They are all original from 2002 and most likely made in 2001 or earlier since car was first registered in the UK in April 2002. Smart 450 Pulse Cabrio Cdi.

You’ve been lucky , extremely so ... it’s a wonder they haven’t at least started to bulge

Are they alkaline or lithium ?

Why not just replace them

[tolsen]

You’ve been lucky , extremely so ... it’s a wonder they haven’t at least started to bulge

Are they alkaline or lithium ?

Why not just replace them

They are all Varta. My original spares are also Varta.

[Mjolinor]

The car works it out because as the battery voltage decreases , the output oscillator frequency of the key fob shifts a little ... this is how the car works out the battery is on the way out.

Do you have any links about that because I can't see why that would happen or work. I would like to read about it.

[reinaldo_is_spam]

Do you have any links about that because I can't see why that would happen or work. I would like to read about it.

Can’t find any just this minute , but i’m Busy chatting lol

It’s not that hard to believe that one could base the thing on a voltage controlled oscillator is it ?
They’re just utilising an inevitable end-game to their advantage to add a ‘feature’
The receiver can be set up to receive a range of incoming frequencies and when it gets to a certain threshold it triggers an alert

[Mjolinor]

Well I just powered a key up from a power supply and frequency does not drift at all. At around 2 volts it stops working.

[Mjolinor]

As far as I know the TX uses an NRZ protocol with an inherent clock. The RX locks to the frequency and it will not know if that frequency is correct or not.