mschwett 

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....

rm -rf

I'm pretty sure that the battery holds the Di2 setup configuration. (I'm familiar with 11 speed Di2, not 12 speed.) I don't think it can be substituted with a generic battery source.

Bald Paul

I think there are a lot of people that can find reasons this is a bad idea. In fact, a VERY bad idea.
I suggest you do some research into Di2 system operation before you go any further.

mschwett 

i thought i had read that with 12 speed, shimano moved all the brains into the rear derailleur from the battery, but i infer from your reply that this isn’t the case.

ang suggested technical sources other than bettershifting or shimano’s manuals?

at some point inside the BT-DN300, whatever brains it has receives power from a dumb battery…

Bald Paul

Although my experience is with the 11 speed Di2 system, it still sounds like there is some type of 'black magic' inside the 12 speed battery. If it was just a 'dumb' battery, why would the dummy plugs be needed? This is what the manuals say:
When using the product, be sure to connect the electric wires to the center E-TUBE port. The product will not operate normally if nothing is connected to the center E-TUBE port.
and:
Be sure to attach the dummy plugs to any E-TUBE ports that do not have electric wires connected to them. The product will not operate normally if the dummy plugs are not attached.

Iride01 

Instead of replacing the Di2 battery with the e-bike battery, why don't you just charge the Di2 battery from the e-bike battery? Voltage on your e-bike is likely different for the motor than it's accessories. So make sure you know what voltage you are tapping into. Although I'd tap into the main voltage and use a power supply (transformer) that is appropriately rated for that input voltage and produces the correct output voltage and amperage for the Di2 charging. If you tap into the other e-bike voltage that might be the same as your Di2. Then you need to know it can supply the needed amperage.

mschwett 

i didn’t think there was an “internal” way to charge di2, since the input is at the RD. but that’s an interesting idea too.

since the big battery has 5v accessory ports, that’s a safe way to charge or power accessories rather than stepping down the 36v again…

mschwett 

yeah, i read that too which strongly suggests the little cap part of the battery has electronics that function as a sort of hub. i think i’ll try and source a defunct one to take it apart.

mschwett 

it might also be worth looking at how shimano does it themselves, although i’m guessing their main battery is heavily customized/designed to do all the other di2 functions.

”Utilizing SHIMANO e-bike’s centralized battery power source and integrated smart systems, SHIMANO CUES Di2 ”

Bald Paul

FWIW, the number of times my Di2 battery has required a charge just wouldn't justify all the modifications to the wiring of an e-bike. They last a pretty long time. If you're really wanting electronic shifting, just install it as a stock system. At least that's what I would do.

mschwett 

i have two bikes with 12 speed di2 - dura ace and ultegra - and it's great. love them. one is an e-bike, and charging both batteries at once, plus a varia, plus a front light finally put me over the edge of "why is this like this?!?!?!" this was then exacerbated when i had to pull the battery out of the seatpost to change the seatpost, which is a very tight fit! still love di2, would never buy another mechanical group again, but i think there's an argument to be made for a centralized battery even on a non-e-bike, if implemented properly.

given that the lights need charging very often, i'll start with hard-wiring those.

Iride01 

You change the seat post often? I wouldn't let one little frustration with that set you up for even more frustration with doing something the is essentially going to be a one off thing with no engineers behind it to say that it's going to perform perfectly and not leave you endlessly troubleshooting issues.

mschwett 

no, i don’t imagine i’ll ever need to change the seat post again! i just think one battery is better than two, especially given that it gets charged fairly regularly.

Bald Paul

But....
What if you took a longer than expected ride, and your e-bike battery died? Yes, you can still pedal, but now you wouldn't be able to change gears.

mschwett 

the odds of that happening are waaaay less than the odds of forgetting to charge the di2 battery after a few months. e-bike batteries don’t just suddenly die, i’d turn the motor off (which is how i normally ride it anyway) long before it got too low to shift.

blacknbluebikes 

I ride Di2 on one of my bikes. I do love it, but it doesn't lead me to avoid the other bike with campy record, I'm quite happy on either one. But I'm not sure I'd make the investment to carry Di2 on an e-bike. Somehow seems "outta spec" to me.