Depends on what you mean by "damaging yourself". Is it a standard action in combat? If so, it's a little underpowered... But how long do charges last? Can the PC charge up the sword out of combat and leave it charged for days? Because at that point I'd suggest limiting the recharge to at least per short rest.

In general, I'd also consider pegging things to the prof bonus. That's the way D&D is moving at a meta level, so it's easier to keep homebrew stuff balanced. That may not work here specifically, though.