I have been having a lot of trouble lately, most times getting low voltage errors on compression tests e.g. the scope only seeing 7.2 approx volts and telling me to check connections.
I was checking the connections to the battery terminals, using different leads (in case it was a lead fault) etc.
Same thing happened yesterday.
I touched the USB lead where it connects to my laptop and hey presto the voltage suddenly jumped to NBV.
The USB port on my laptop is worn, moved to another port and no problems.