Battery doorbell powered, how quick does yours charge?

Mine seems to drop to 94% and then charged within minutes back to I assume full as I get the plug symbol.

This is from a Hager ST301 8v 4VA transformer so very small power input.
What’s it supposed to charge from and to anyone have an official answer? Surely topping up from 90% is worse than keeping it between 60 and 80%?

Can this be addressed in a firmware update so the battery doorbells stop pulling power at 80% for example?

See my install thread here