RE: Upgrading RX Battery
Your charger should charge it fine. As for charge times with a 1650:
1650mAh/50mA = 33 hours.
You will not need to charge it for 33 hours every time you recharge it because you will most likely not be using the entire battery in a day. You just need to replace what you used after a day of flying.
You could get a cycler/charger. One nice thing about that would be that you can take the battery you have now, fly with it a couple of times, then throw it on the charger at the field for a fast charge and then keep flying. Then you do not need to add extra weight to your plane or buy a new battery.