This is about something people are ignoring when selecting a charger especially new comers to Electrics.Generally people take into account is the no.of cells the charger can charge and the maximum charge current it can handle.And completely ignoring to know how many watts the charger can handle, which is also a very important factor.
Like in the case of BC6 charger which is a very good charger at a great price and works very well in its limits. When selecting a BC6 or any charger we usually see how many cells it can charge( here 6S) and how many amps it can handle (here 5 amps) and generally assume that it can charge 6S Li-Po battery pack at 5 amp current rate. But it is not correct. BC6 is a 50 Watt charger.Which means it can handle only 50 watts. Wattage is the product of charge Current and Battery Voltage.
Suppose we want to charge a 6 S battery i., V= 6x 4.2 = 25.2 volts.@ 5 amps , we would need a charger which can handle 25.2 x 5 = 126 watts. which BC6 can not handle. because W = V x I,
we can see that the maximum voltage V = 50/5 = 10 volts. which is more than 2S but less than 3S.So BC 6 charger can safely charge a 2S (8.4 volts) at 5 amps.Similarly BC 6 can charge 6S(25.2) at 1.9 amps only.
By using the formula W= V x I or I = W/V we can see that BC6 charger can charge different cell count batteries as below.
2S battery (8.4 v) , Maximum charge current = 5 Amps
3S battery (12.6 v), = 3.96 Amps.
4S battery (16.8 v) = 2.96 Amps
5S battery (21 v ) = 2.38 Amps.
6S battery (25.2 v) = 1.98 Amps.
The above given figures are calculated for BC6 charger. But one could calculate for any charger using the formula Maximum Charge Current = Max Watts the charger can handle/Full charge Battery Voltage.