I had a B&M ? brand (german make) rear bike 'standlight', removed the capacitor as I found it annoying when switching to a battery. Used a 6x cell AA 1,2v pack for awhile, with no issue. Upgraded all the wiring to silicon, and put in a 0,8AHr Yuasa gel lead acid.
For some reason I choose a 12v lead acid instead of a 6v. ?
The front headlight, which was a xenon lamp design with a voltage regulator circuit that I replaced with a LED inside the reflector, same headlamp control circuit, seems to like it. Good light.
However, the rear red 'standlight' got dimmer, and dimmer. And now barely lights up at all. I think it was not designed to operate with such a continuous peak voltage around 12v and wondered what you thought.