What amp extension lead do I need for in car cigarette lighter.

What are you trying to connect the other end to?

 

IIRC the lighter socket is protected by a 15 A fuse, so that's the maximum current you can draw at 12 V making 180 W the highest rated device you can plug into it.

Just charge phone or sat nav.

Then any you can find will suffice.

Is the myth about charging phone in car true will it flatten the battery.

If the car's battery is in good shape it shouldn't.

Good thanks

1 hour ago, bowrob said:

Is the myth about charging phone in car true will it flatten the battery.

 

That is a very vague myth.

I would have thought that the modern cars battery is stressed enough as it is - battery protection kicks in pretty quickly in my experience.

14 hours ago, bowrob said:

Is the myth about charging phone in car true will it flatten the battery.

Highly unlikely at worst; if the engine's running it's an extra load on the alternator not the battery, and even engine off the cell will only take about 1% of battery charge at worst from flat til it's battery oversharge protection kicks in.

So a 5amp extension lead will b ok.

The fuse in my Yeti for the front and rear 12v outlets is a 20 amp. Fuse 30 IIRC.