Replace the battery in your meter
As for Pi not booting, the faint light of green LED is usually an indication it wants to boot but something is not right with the SD card (or the image on it). Do you have another card to try with?
! the faint light is caused by the built in pullup of the GPIO started as input on reset, when no code has run to initialise it as an output.
Therefore it means absolutely nothing, other than no code has ran yet!
I see that the tip to replace the battery was already given, that was my first thought too. I'm sure your supplies do not all output 8V!
When not doing (being able to do) a clean shutdown, its possible (but not likely) that open buffers on the SD card were written to the flash in the card, and that can corrupt the card contents!
Wipe the card completely clean, so that the cards logic is reset, and install the OS again.