Troubleshooting Kindle Keyboard 3 (D00901) Battery

[beardedfool]

A blast from the past here but would be grateful to check my thinking please
Especially from knc1

I've had this old kindle stuck on the child under tree graphic
(I vaguely remember it having the battery warning and resetting it though holding on the power button some months ago but wouldn't totally trust that memory)

Orange light comes on whilst charging ok
I've today disconnected the battery and confirmed that it's giving 4.2V at the battery terminals
The battery has .08V
It's now back on for a 24 hour charge but a shorter charge today didn't seem to raise the battery voltage. Multimeter does read slightly higher .2 or so and starts dropping to .08 or less
Device won't turn on whilst charging. Tried sliding power for 40secs+

Aims:
Be nice to have it as a backup but it's not worth the money to buy a new battery for over here in the UK - is it definitely the battery?
Or I'm tempted to hack the OS (if I can get it turned on) to do something similar to https://ponnuki.net/2012/09/kindleberry-pi/ but first I need to get it turned on consistently

Is there anything else to try please?

The reason I'm slightly confused is I believe I saw a post saying that the recovery mode for the battery was low mAh, not voltage.
So the device should still turn on (?)