Volta has no upper limit to how much charge she can hold at once, but once she gets above a certain threshold she is so “saturated” with electricity that holding onto it becomes more challenging. Not really painful, it’s not like holding your breath too long, it’s just this full-body feeling of fullness, of tension.
She’s in no real danger of getting hurt if she takes on too much but the more and more charge she takes on, the less precise control she gets over it, so she tries to save periods of holding extreme charge for when collateral damage isn’t a concern, because chances are she’ll be dealing quite a lot of it.