Super smart USB rechargeable rear lights from our favorite bike shop, Blue Lug. Screw the mount into a braze on on the seat stays and then unthread the light when you need to charge it up.
One charge lasts 8 hours on steady mode which is the only mode to consider. Lights shouldn't even have a flashing option.
It takes about 2 hours to charge fully.
Comes with an M6 and M5 bolt for mounting, but no charger - I bet there's one in your junk drawer!