Hi I purchased a Qashqai  2017 a year ago. 

It is still under warranty. 

The red airbag light comes on every time there is a frost.

The garage have suggested its because the battery dips in power when there is a frost.

Although 4 months ago I had to have a new battery when it was flat.

It now needs a mot, obviously it will fail with the light on again. 

It costs £100 to clear the fault.

Any advice greatly appreciated. 

Sounds more like a connector to the seat belt pretensioners under the seat.

Try moving the seat to a different position with the ignition off and then switch the ignition on to see if it has cleared.
If so the its the connectors under the seat