As a D&D player, one reason we get gold is because we can turn the currency into what we really want. Non-gold treasure either falls into things that can be converted to gold (art or gems as two examples), or items that we can use (potions, scrolls). Also, depending on the setting, there are plenty of adventurers just in it for the money and fame.
If you're looking at a post-apocalyptic setting, then you'll have to consider why your group is adventuring in the first place. Much like the fantasy setting, people have motivations for adventuring, and that will affect what they desire more out of treasure. Put in broad strokes:
- People aiming to help their settlement are likely on the lookout for treasure that will help their home or gear that will help them be better at that goal
- People on a personal quest will look for treasure that will help them achieve their goals
- People out for money and fame will seek treasure that is valuable and/or has an epic story behind it
Improving the Homestead
If they are adventuring to help their home settlement, then anything that they could bring back to help their settlement is good treasure. Even damaged or broken items potentially hold value as somebody back in the settlement might be able either reverse engineer them or outright repair them given the time and opportunity to do so. Such things may include:
- Energy cells to power the machinery in their settlement until they can build something more permanent -- as per what you have in the question
- Alternatively, empty cells can be recharged and the settlement just needs more cells to run more machines
- Parts and/or schematics for machines would be valuable -- doubly so if those particular items were lost during the apocalypse
- Materials to fabricate things are not worthless, and would effectively be Trade Goods in this settings
- Gold is still an excellent electrical conductor, so even if it is not being used as currency, it may hold value in other ways
- Salvaged materials from broken machines can be repurposed into new items
- Depending on the level of apocalypse, food plant seeds might hold some decent value and be useful to bring in new foods to the settlement as a luxury good
- Gear to better analyze salvage so they know what is better to return home would not go awry
For the Loot
Alternatively, if they are in search for riches, then the value of individual items will be on their mind. As such, the quality of the loot is just as important as what that loot is. Also, as they gain experience, they will know who will buy what for what price and will be on the lookout for those things, preferably in good quality.
- Scouring old ruins for artifacts and art from Before the End may prove lucrative if they know a buyer that would pay for such a unique (if functionally useless) item.
- Old machines, or their schematics, may prove valuable depending on their condition.
- If they have a particular rare schematic, being the only group with a particular machine to sell may also prove valuable.
- Anything from the first section is still useful, though they'll hold onto it to get a better price if they feel they can
As another thought, as it's post-apocalypse, it is entirely possible that schematics for machines are not always perfectly legible from one source. It may require finding the same item multiple times and effectively piece together a complete schematic from multiple sources. This idea could be used for a particularly powerful machine so a lucky find doesn't derail the game.