Planet bike has a dynamo superflash , there is a bit of stand light capacitor in it but not a battery.
same case size, small as their 2AA battery powered ones..
Battery , and dynamo use the same handlebar mount,
the dynamo one has a plug in the wire to let you take it off to go in your pocket.

Red LED in tail lights draw very little power, so a couple AA or AAA batteries should last long enough..

maybe put a red lens over a headlight pointed backwards for the killer rear ender traffic problem.