Not quite sure why this thread is in here, maybe a mod could move it to the technical section.
Yamaha system does do a handshake with the battery. If there is no communication everything looks ok, you just don't get any power. The display is powered by it's own battery, if you connect a non Yamaha battery you can switch the power on, select power levels, the speedo works etc but the motor does not get any power. I know that the communication protocols have been successfully reverse engineered in Germany. Maybe the display is receiving a spurious message and switching off, I guess without any communication it just sits waiting.
I borrowed a second battery a couple of weeks ago and it worked fine at first, then after a long descent the display had switched off. Pressed the power button and display came on but no power, turned out the battery was full of water from hosing off the hire bikes with some slight damage to case allowing the water in.