I'm very much enjoying the mod and have somewhere around 150 trains running. For the life of me, I haven't been able to find a good way to avoid trains running out of fuel, despite having more than enough batteries "in the wild." I'd love to see a way to have the train stop read the number of batteries (both charged and discharged) so it could be added to the circuit network.

If there's already a simpler way to ensure some trains don't end up with 60 batteries and others run dry, I'd be happy to hear it. Thanks

Not sure if adding a circuit condition is an easy task since I haven't done that before lol. Another idea to "fix" this would be, as suggested by another user before, add higher tiers of batteries with more capacity when running Space Exploration. This way you'd have more juice in them battery packs, making it less likely to run out.

You may not be able to read how many batteries are currently in the train, but you can count how many drained ones came out and only put that many back in. Which can be done with 2 combinators.
Here's a blueprint: https://pastebin.com/ycVBUVKc
Check the locomotive in the blueprint for stop conditions. You don't have to have the charger right there; you can replace it with belts or chests or whatever.

I haven't gotten to space trains yet in my actual game, but I did notice and appreciate that the batteries have the same energy density per stack as all other fuel sources (except nuclear, which gets a slight boost for the sake of the 1.21 GJ joke), so I don't feel like more energy density is really necessary especially since there's already an option to double it in the settings. Plus, an extra layer of management like this is very on-brand for SE.