Here is your problem: You are storing the energy in the Instance of the Item ...
In Minecraft Items are designed to be Singletons, which means, for each Item there only exists one Instance at all time.
If you set the energy in the Instance of one of your CapacityCards, it will be set for every card, because they share the same Instance.
To store data only on one card, without affecting the others you want to store it on the ItemStack's NBT.
In you CapacityCard#setEnergyCapacity you have to write the energy to the stacks NBT data instead of the local variable
When ever you want to read the current energy you have to read it from the stacks NBT data
Nice Greetings
TechMage