running di2 from an eBike battery
has anyone successfully removed the di2 battery from 12 speed di2, and run the system permanently from one of the auxiliary outputs from an eBike battery?
obviously, the eBike battery is orders of magnitude larger than the 7.4v, .5 Ah di2 battery in the seatpost. assuming an always output is available and an appropriate step-up or step-down regulator is used, can anyone think of reasons why this is a bad idea? obviously running the battery completely down to zip would leave you without shifting as well, but nobody should ever do that on an e-bike anyway.
i'm thinking about a little project to hardwire lights and a varia to a bike that didn't come with those things, and it got me thinking about the di2 battery as well. why ride with four separate batteries, one of which is 10x the size of the others combined.
garmin varia battery - 3.7v, 1400 mAh = 5.2wh
knog pwr road 700 - 3.7v, 5000mAh = 18wh
shimano di2 battery - 7.4v, 500mAh = 3.7wh
ebike battery - ±36v, 242wh.
assuming the auxiliary output from the battery can handle the current of lights, it seems like it would not have any problem with di2, right? the accessory cable by the motor/battery manufacturer is a dead simple y cable splitting from the small proprietary connector to two pairs of bare wires, which certainly suggests they designed it to be wired directly up to simple DC loads....