Following on from my previous post (Tesco Hudl2 battery problems and teardown) I ordered a replacement Li-Ion battery (from eBay) to compare and test if this was the problem.
Again on the black and red wires I was getting around 3.9v which looked good. I then tested the yellow and green wires. As you may remember these were the two I was most interested in as I think they were 'sense' wires to the Hudl2 to perhaps control if it should power on, check for 'safe' state of the battery, etc.
These did the same as the original battery, same low voltage output and same floating values, however after a bit of thought I tried the test but against the red (positive) wires, and lo and behold I got values! I got the same 3.9v on both of these wires. Comparing to the old battery, that did the same, so now I know, both batteries are giving the same output.
Therefore I wasn't very hopeful at this making the Hudl2 work.
Connecting it up, and sure enough the Hudl2 wouldn't power on. Connecting the charger up it again didn't light the charge light. No luck.
I then took the main board out of the Hudl2 to look for any other signs of problems, couldn't find anything, many many probe pins are on the board underneath marked power, signal, etc, and probing almost all of these showed voltage and signal so the board was getting power, etc, and all looked OK, so it appears to be something a lot more fundamentally wrong with the board now.
My suspicion is that when the fault PSU/wire started shorting out the usb input, there wasn't enough protection on the board and it damaged some key components (Not just in the charge circuit/components) and effectively bricked the unit.