CL with flat battery now Chip Card not Recognized

[WCA]

I have stuck the car on charge after the battery has been flat for about a week. The ignition key goes in, turns but instead of starting the engine it clicks a bit and then displays 'Key Card not Recognised'

I have searched this forum and the resynch method is to leave the key in the ignition, right? Should the key be left turned on, with the dashboard all lit up, or just left in the off position?

Is this the right method?
Is there anything else I should worry about?

 

[WCA]

How long do I have to leave the key in for?

 

[Cole@MBS]

By the sounds of it the battery has very low voltage, have you another battery to swap over rom another car, chip card warning due to a undervoltage at the EIS i suspect!

 

[WCA]

Cheers - I will leave in on charge for a few hours before retrying then

 

[WCA]

You were right, it has just started fine.

Thanks

 

[Cole@MBS]

You were right, it has just started fine.

Thanks

Result, Glad its runnung!!

 

[barose]

Replacing key card battery

Can anyone help with which battery to use in the key card for an S class

 

[television]

Should be CR2025 I think