See Artificer's Battery, Purifying Bolt, Static Hum, and Energizing Bolt.

i like this

It would probably be easier to balance at "You lose half your charge" rather than just one.

I feel this card and commons in general might actually be better designs/more interesting by themselves if they e. g. gave you one-shot two or three charge on ETB rather than adding one repeatedly. That way charge would be a meaningful concept even if you play only this one common with it.

So have something like this at uncommon, and Purifying Bolt and Energizing Bolt at common?

I wouldn't mind something like this at common either as long as it adds 2 charge and is cost appropriately.

Hmmm, on its own it's a 2/1 haste for 1. That's strong, but red gets that. If does it via the set mechanic; rather than flat - but that's not a reason to move it out of common.

That you can instead drop a couple of them and do nasty things slightly later in the game might be though. The possibility of this being 11/1 with haste seems like it would be a problem.

Hence "cost appropriately", I think my proposed version would not serve as a 1-drop.

But now that you mention it, at common you may want to use flat boosts e. g. "~ gets +1/+0 as long as your have a charge."

That would work perfectly.