Why Does My iPhone Use Battery Even When I'M Not Using It? Here's The Fix.
For as long as mobile phones are turned on, it will consume power even when not in use. In your case, it’s probably from background running apps because a 99% or 98% drop doesn’t seem that big. If it was dropping to something like 90 in a short span of time, I would say it’s a battery issue.
If this is indeed a battery issue, I suggest bringing it to the Apple store for check up. Don’t forget to include the charger because there is a chip in the cord and phone that is supposed to regulate overcharging and this could pose a problem too.