The main thing that people don't understand about the 50W chargers out there is they say they can do up to 6S, and they say they can do 5A, (and this is true), but they can't do both at the same time.
Power = Current x Voltage
You can do the math from there, if you only have 50W power handling you can see how much current it can handle for each voltage e.g. 2S, 3S, 4S, etc. And this isn't even calculating in losses (nothing is 100% efficient, so the true capability will only be 80-90% of what you calculate)