Significantly off-topic: Mods plz to move if appropriate.
My 12V, 1.3 Ah LeadAcid cells weigh 650g each. My lights were visibly (?) flagging in the hour before dawn on the Tan Hill ride, as the cells were powering twin 6V LEDs at the front, and the same again at the rear.
I'm informed that there's a Li-Ion cell available, weighing 40g, rated at 3.6V, 1.8 Ah. Two of those in series, and re-molish the various bikes' wiring looms so that the lights are in parallel, and I should have six-fold increase in energy-per-kilo, at the sole disadvantage of not having 12V available to blow up my airbed.
If said Li-Ion cell were USB-chargable too, I'd make the changeover straight away.