But when you catch them at the traffic signal and stand there, mentally tut-tutting in a patrician fashion, you notice there is a red flasher hanging there on their seat-post, dull and darkened like a 7-eleven never is.
You rustle your messenger bag, or do a little throat clearing to get their attention, then politely advise re: the inert status of their light emitting diodes.
There will be an awkward pause after they advise that they have run out of battery. Don't tell them they should have charged it up. You're not their mum.
BONUS TIP: Unless in fact you are their mum, don't reach out and check if they're lying by testing the on-off button. That puts your hand awfully close to their behind and that could get you sued, arrested, slapped, or beaten to death (depending on how far from the legalistic cocoon of the inner city you have this chance encounter).