Parallel voltages are the same according to wiki,
VT = V1 = V2...= Vn where n is the nth voltage
The leds's fitted by SIMS LEDS on my MK use the same battery as the pre amps and is a great indicator of dying battery before the distortion cuts in.
I really don't understand why the extra battery, I would have thought an in line LED with the pre amp battery and appropiate resistor to prevent it burning out would be a better indicator.