voltage of lighting system
You most probably damaged the rear light and are risking damage the voltage regulator on the front, too, since the huge majority of generator hubs provides 6 Volts.

If the regulator on the front is sophisticated enough, perhaps it is using just enough current and voltage to feed the light, but I think most circuits use a Zener diode, that actually make the excess voltage to "leak". This is not big issue with generators, whose alternating current waveforms just go above 6V during very short fractions of a wave, but with a battery, it's possible that it is working a bit like "short circuited". If the front light is over-heating, that most probably might be happening.

As for the back light, the light-emitting elements of leds are very sensitive to higher-than-normal voltages, and tend to "burn" due to overheating and chemical/physical damage.

I advise you to check the current voltage of your system, and switch back to a lower voltage if that is the case.

Hope that helps!