My thinking:
- normal ammunition should have a 95% chance to spawn "drained" cells when fired. Bulk and Overcharged ammo should be about 80%, and Maxcharged ammo should be about 60%.
- drained ammo has the same weight as charged ammo. You're taking electricity out of it, not mass.
- recharging ammunition requires a source of power: a fission battery. A fission battery might recharge 200 ECPs. (They're very heavy, so it has to be worthwhile to bother with them)
- ammo conversion recipes require empties of the target type, and leave empties of the source type. In other words, if you convert 30 ECPs to MFCs, you will need 10 drained MFCs in order hold the charge, and, you will be left with 30 drained ECPs afterward.
- charged energy weapon ammo should be about 3 times more expensive, and even drained cells should have value
The overall aim of such changes would be to differentiate energy weapons from guns, in that the ammunition you're using is effectively batteries. You're not "using up" an energy cell by using it to power a laser, you're just removing its charge, and it can be recharged without any fuss. For an energy weapon user, the problem is less about finding enough cells to use in their weapons and more about finding sources of energy to keep their cells charged.