I recently purchased a MS906BT w/V100.  The set appears to be 2018-2019 vintage.  I've updated the V100's firmware, but this problem I describe was present before and after the firmware update.

The button to light the white LED (a flashlight to help located the OBDII socket) does not light the LED when the unit is standalone.  If the V100 is plugged into a PC via USB, then the button lights the LED, but not when it's unplugged.

Additionally, the V100 gets fairly warm when plugged in.  I'm thinking this has a failed internal battery.  

Two screws are located under the label on the rear.  IPA loosened the adhesive, and then a spudger helped to remove the label.  The screws are 51P TorxPlus (ie tiny).

Unplugged from the PCB, the battery measures no voltage.  With the PCB powered via USB, the battery connector on the PCB shows a pulsing voltage, so I'm assuming it's trying to charge the (disconnected) battery and shutting down, repeat.

The battery is EU401230P (4x12x30mm).  I've ordered one via Amazon; several are offered.  Search term that works for Amazon is "lipo battery 3.7v 12mm x 30mm".

Pity that no pictures can be posted here.  Update to follow.

 The replacement battery via Amazon, seems to work OK; I can now press the button on the unit and the "flashlight" works for the first time since I've owned it.

I did have to cut off the V100's battery connector and solder it to the new battery's leads, as the battery I'd purchased came with a JST PH2.0mm connector which is ~2x the size of the V100's!  I think the V100 battery's wires are the smallest insulated wires I've ever soldered.

Measuring voltage at the battery disconnected = 4.03v

With it connected to the V100, and the V100 powered via USB, = 4.13v

So, it's charging the battery when it's powered.