I just tested with the console, and saved in the interim, and it retained the change in charge I made.
Fair enough, maybe the max charge is fixed (as in constant, rather than repaired). Are you tempted to make it?

I'm a big fan of this idea. I've seen similar ones, where the enchantment is restored by Magicka (actually that should really have prompted me to realise that it's possible to set the charge...) which is similar, but not the same. Think it was called Soulfire? Yes: http://skyrim.nexusmods.com/mods/9852
That said, it's old, so I can't vouch for stability. Haven't used it in a while.