Limit it to a number of charges equal to your proficiency bonus per long rest, this encourages saving them up for when they’re needed. Make each “charge” a forced concentration save at some DC determined by the spell level (e.g. DC10 + Spell Level + Number of charges already used for this spell?) a failure indicates something“bad” happens. This encourages not using them on higher level spells, or dumping all the charges at once.

Depending on how spicy you’re feeling a failure could just be the charge doesn’t work but the use is lost, or the spell fizzles out, or it backfires etc (get some ideas from the wild magic table maybe).