Big Sur MacBook Pro crashes on start up. Battery issue?

Since I've upgraded my 15" 2019 MacBook Pro to Big Sur I've had issues with it crashing on start up.

I get the log in screen, it begins to load up my profile and then black screen.

I can't do anything with it and there is no response to any keys. I have to hold down the power button to shut it down, wait a few seconds and then start it up again.

Occasionally it will crash at the same point again the second time or it will start up and perform normally.

I have noticed this only happens when on battery power.

I have reset my smc, pram and nvram with no change.

Anyone have any idea?