Auxiliary power from Bosch system?
 

I have a Bosch Performance CX bike, and would like to draw a little auxiliary power from the big battery. Not much, but the 5V USB out of the display is probably not enough. It should cut power when the bike is switched off. 36V is fine, I can use step-down circuits. Is there a way to do this which is safe, not a kludge, and ideally doesn't void warranty? (Tall order, yes)

Purposes: Indicator lights, horn, conversion of manual Nuvinci to automatic Harmony, if possible.

You could probably use one of these.

Busch + Müller USB-Werk AC
Busch & Müller USB-WERK Charger

(cheaper if ordered from Germany).

See manual link.

Input: 0 to 70 VAC, 0 to 50 VDC
Output: regulated 5VDC, 5W

Apparently it includes a buffer battery, so you may need to add switches.

You'll have to look at the bike's wiring, but I would think there would be a place to get to switched power from the bike (before the throttle).

Thanks. Voltage regulation is not the problem, that can be had for far cheaper than the usb-werk/e-werk, and I won't need a buffer battery. (I'll most likely need 12V and 36V, not 5V.) The challenge is getting access to switched power in a clean, safe way. The Bosch is an integrated system, the controller and motor are in the same housing, right next to the battery. There is no throttle. Users are not supposed to need to mess with it, so there is no wiring diagram.

Do you get live reverse power out of the charge port? What does the charge port look like? Is it in the way?

No battery connections? Access to individual cells?

Another option might be to add a dynamo, although that would be a bit redundant since you should already have power.

There is no charge port on the bike, the battery can only be charged off the bike. The bike/battery connector is a complex custom 4-lead job which would be hard to interface with anyway.

No other connectors on the battery, no way to access the cells without opening it and voiding warranty. I'd not want to do that sort of surgery anyway.

Yes, a dynamo would work. I've actually seen ebikes specced with that. It's nice to have lights even if your battery is flat. It's a 15mm thru-axle front wheel, but afaik hub dynamos for that axle do exist, so it's an option. In that case the E-werk could be good.

Or I could of course just add a second battery to the bike, but I'd rather avoid that extra charging hassle. If I can't find a clean way to do the job, I'll just drop it. The bike is too good (and expensive) to risk hacking apart.

Thanks