Watts (w) = volts (v) x amps (a)
Your charger has 50w limit. If charging a 6cell lipo (22.2v) the max amps your charger will have available is 50w/22.2= 2.3amps.
That's assuming 100% efficiency. In reality you will have about amps to charge with and it's not a goo idea to use stuff at 100%.
If charging a 2cell (7.4volts) you charger would have 50w/7.4v= 6.7 amps. However now you are probably hitting the chargers max amp limit. Most small chargers are limited to 5amps. So you can only get at 37watts of the 50 available.
When buying a charger check the max amps and power. Clear as mud?!?