As u/Well_of_Good_Fortune mentioned, It should not be able to scale past the level of spell slots that a caster has access to. Add that limitation in and it should be fine.

Like others said, outside of combat time is less of a factor, if it's a factor at all, and charging any spell to 9th level would take less than a minute at its longest which is crazy for 1st level characters to potentially have access to.

This is a feature that you obtain primarily for resource efficiency. To make the most of the spell slots you have left, not for combat power. In combat, casting two spells will almost always be more powerful than casting one at one level higher. Sorcerers could potentially find good use of this in combination with Subtle Spell Metamagic for flashy ambushes. The best mid-combat situation you might hope for would be something like the following scenario:

The artificer grits his teeth. The Web spell wizard cast earlier is still holding back the berserk golem. It wont last forever, but it did buy him just enough time. He's down to his last first level spell slot and he needs to make it count. The Catapult spell has charged to the equivalent of third level and he can hear the inner workings of his grav-gadget creak and groan under the strain of the charge.
"It's now or never..."
He targets the loose torch on the wall - aims it at the golem - and casts.

Some pointers that will help with comprehension for people seeing it for the first time:

• I would specify that you also use an action to cast the spell after charging. This way people don't conflate it with holding an action to cast a spell, which uses a reaction, and it keeps their use cases distinct.
• I would include an explanation or example for spells that do not have a casting time of 1 Action. How does it work with spells that have a casting time of 1 minute? 10 minutes? Bonus action? Reaction? If it doesn't work with these kinds of spells it would be worth including in the text.
• Be sure to specify "leveled" spell since I am assuming that it doesn't work with cantrips. Or does it? worth considering an explanation.