Display not showing correct battery levels

[OzzyPT]

Shimano E7000 motor on a Voodoo Zebop E-Bike

I have been on a few strenuous rides recently (25+ miles and about 2k climbs) and I have noticed that the display and the battery are not in sync.
I found that they were about a bar out between them, with the display showing the lesser.
On my last ride, the display said that the battery was empty, and I could not get any assistance.
The battery showed 2 bars.
I have read that the display uses the last few miles to see how much battery is left, and if you turn off and turn back on, they should resync.
To be honest I am sure I tried that, but cannot guarantee.
I went to Halfords, and they said that it needs to be updated, but as the bike was 2nd hand, I cannot connect it to the app, as I don't have the log in code and the last user can't remember it!
I am going out again, this weekend and will monitor it, but if anyone has any suggestions, it would be brilliant.
There is a local bike shop, so I may pop in and see if they have any advice, Halfords aren't the best to be honest.

Cheers