sorry for late post.
but i reverse engineered the yamaha battery a year ago.
in case you want to build your own.
Just get a Arduino compatible board and a few discrete components.
Add a 5V source, i usually use a LM2596HV dc dc converter for this. A USB powerbank or even running it from two or three Cells on the pack (VIN pin of the arduino) will work.
Optional: To monitor the voltage add a voltage divider set to a ratio of 7.401 (42v in 5v out) and connect it to the A0 pin of the Arduino.
Edit this code https://pastebin.com/34SdehGt
Edit line 35 to match your voltage divider. calculate or test to find out ADC values.
Change line 44 and 45 (the number 20) to a value you want. 20 means it averages the current voltage 20 times to smooth it out. 20 is 5 seconds.
Remove line 50 if you dont want your indicator to stay at 10% even if its lower. this prevent turning the motor off when voltage drops driving up a hill.
After that, Flash it in Arduino IDE
Wire it up like this.
Some people including me allready did it to the haibikes, works great. See german pedelecforum.
If its too much for you, a guy called Cosas in pedelecforum is selling modules based off my code that does exactly this, basically dongles you put on the Battery connector and can connect any ordinary 36V Battery onto it.
Havent done anything on Bosch so far, i dont have Bosch.