What does $200\%$ faster mean? (How can something be more than $100\%$ faster?)
Solution 1:
"Faster" refers to speed (units of something per unit of time) or velocity (units of something per unit of time, with a directional component).
In this case you are talking about charging a battery in $5$ minutes. The speed of that operation is $(1.0\,\text{battery})/(5.0\,\text{minutes})$, or $0.2$ batteries per minute.
$100\%$ faster than $0.2$ batteries per minute would be $0.4$ batteries per minute, or charging a single battery in $2.5$ minutes.
$200\%$ faster than $0.2$ batteries per minute would be $0.6$ batteries per minute, or charging a single battery in $1\frac{2}{3}$ minutes, or $1$ minute and $40$ seconds.
Solution 2:
You're applying the percentage to the wrong magnitude. 100% faster means speed is increased by 100%, i.e., double speed. So, you don't reduce time by 100%, but by 50%. If it's 200% faster, you get 3x speed.
In your example, you'd charge the battery in 2.5 and 1.6 minutes, respectively.