I'd just like to add:If it's this battery your considering then you will be able to charge it using the charger that came with your bike. I have been using one of these batteries for 2 years and have had no problems and it's still performing well.
I also bought one of those for the wife's ebike around 2 years ago, and it also is still performing well. (touching wood!)
However as the battery LEDs never ever extinguish at the end of the charge, I was a bit worried, so I stuck a wattmeter between the mains and the charger to see what was going on (and whether it was continually pumping current into the battery).
What I've found is that the LED's will stop flashing, and it will LOOK as if its finished the charge fairly quickly, however it does this with (I forget the exact figure), something like 20 or 30 watts still being drawn by the charger. Over a period of another few hours, the wattage will slowly ramp down, and stabilise at around 6w (and stick at this level forever).
So it looks as if the battery either is doing some final trickle charging, or more likely, its this last stage when the cell balancing is happening. If it is the latter, then its possible switching the charger off before its actually finished the full charge (even though it LOOKS as if its finished!) could eventually end up with a problem and maybe even cause the battery to go faulty or drastically lose capacity.
With only 6w final draw, I don't think there is any danger, and I'd be tempted to advise making sure you leave these batteries charging for MUCH longer than you would normally do - maybe overnight, on a fairly regular basis to make sure they remain balanced.