Hi all,
I want to start by saying thanks for all the info in this thread. Because of this, I 3D modelled a replacement case so you can open up the original and replace the battery on the 15 kits we have at our school. I've done a write-up and shared the model here:
https://www.printables.com/model/1762970-lego-energy-monitor-battery-case
Hope it helps anyone else trying to revive these kits. They're too good to let them die.