Steam Deck won't power on unless it's plugged in.

I noticed earlier today that my Steam Deck wasn't charging above 67%, even though it's been plugged in most of the day (via the official power cable).

If plugged in the white power light is illuminated and SD will power on. If unplugged, the power light is off and the Steam Deck will either power off straight away or not power on.

Once the SD is powered on through the power cable the battery icon on the screen sometimes has the 'lightning' symbol displayed.

Hope someone can help.