Volts * amp = Watts.
So a single lipo is nominal 3.7v, and 4.2 fully charged.
So you set the charger to charge at 5 amps. (Assuming a 5000mah battery and 1C charge rate here). 5a*7.4v = 37W. So at the start of the charge, you need 37 watts to charge a 2s at 5 amps. However, as the battery charges, the voltage goes up and the amps stay mostly constant. For a balance charge, somewhere around 4.1v to 4.15v per cell, the charger will start decreasing the amps. Somewhere around 4.0vps, the charger will see peak wattage. So, 8v * 5a = 40W near the end of the charge cycle. Add in a 20% safety factor, you are at needed 50W capacity to charge a 2s pack at 5 amps. (Which is why there are so many 5a/50w chargers out there).
So, easy math is each cell needs 20w for a 5a charge rate.
2s = 40w
3s = 60w
4s = 80w
6s = 120w
Plus add the 20% safety factor.
But that is only for a 1c charge on 5000mah cells. For those that like to run those big 7400mah cells, 1c is now 7.4a, and you need 30w per cell. Now that 6s 7400mah battery needs 180W to charge at 1c, and that is still not counting a 20% safety factor.
TLDR;
Big power toy cars with big power batteries need big power chargers.
So a single lipo is nominal 3.7v, and 4.2 fully charged.
So you set the charger to charge at 5 amps. (Assuming a 5000mah battery and 1C charge rate here). 5a*7.4v = 37W. So at the start of the charge, you need 37 watts to charge a 2s at 5 amps. However, as the battery charges, the voltage goes up and the amps stay mostly constant. For a balance charge, somewhere around 4.1v to 4.15v per cell, the charger will start decreasing the amps. Somewhere around 4.0vps, the charger will see peak wattage. So, 8v * 5a = 40W near the end of the charge cycle. Add in a 20% safety factor, you are at needed 50W capacity to charge a 2s pack at 5 amps. (Which is why there are so many 5a/50w chargers out there).
So, easy math is each cell needs 20w for a 5a charge rate.
2s = 40w
3s = 60w
4s = 80w
6s = 120w
Plus add the 20% safety factor.
But that is only for a 1c charge on 5000mah cells. For those that like to run those big 7400mah cells, 1c is now 7.4a, and you need 30w per cell. Now that 6s 7400mah battery needs 180W to charge at 1c, and that is still not counting a 20% safety factor.
TLDR;
Big power toy cars with big power batteries need big power chargers.
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