From my limited understanding, the rate you can get charge into a battery is proportional to its capacity. You see this in the size normalised charge rating tha

From my limited understanding, the rate you can get charge into a battery is proportional to its capacity. You see this in the size normalised charge rating that batteries have, like 1C means it takes 1 hour to charge and 4C means it takes 15 minutes. Adding more batteries in parallel is much the same.