Charging a battery with a hub dynamo is problematic. On the one hand, the power output of the dynamo fluctuates depending on the speed. On the other hand, the charger has to carefully monitor and adjust the charge voltage and current to assess the state of charge of the battery and avoid overcharging it. So you're trying to match a variable power source to a variable power sink.
An alternator with voltage regulation via the coils, such as is used on a car, produces a near-constant voltage regardless of speed as long as it's turning. That would help a lot with this problem, but I've never seen such a unit for a bike.
I think a better bet is forgetting about charging and using the dynamo to extend battery life, and the battery to back up the dynamo. If dynamo output is above battery voltage then the light runs off the dynamo, otherwise it runs off the battery.