Almost certainly a dumb question.
I have 6V cycle lights which came with a chunky 4.5Ah lead acid battery.
Now I can get a replacement for under £10 (or more - not sure what the difference is although those targeted at security alarms seem to carry a premium).
However there are USB backup devices which offer 3.2Ah or more at 5v at a fraction of the weight.
So I had the passing thought that running 6v lights at 5v might give a reasonable light level and much lightness.
However I have no real grasp of what the effect of under volting a DC light could be - in fact 12V car systems are probably running at 13-14V at times so the whole idea may be a non starter.
Almost certainly not worth buying a 5V battery (although I already have a backup battery for phone/tablet) so I was just idly wondering.....
Cheers
Dave R