Versions

Minecraft: 1.15.1
Fabric Loader: 0.7.2+build.174
Fabric API: 0.4.25+build.282
Tech Reborn: 3.1.0+build.62

The internal energy buffer of my cables doesn't appear to get saved -- each time I log out and come back onto a server, all of the stored energy in my battery seems to go into cables.