Understanding Charger Specifications

As discussed in RC Today Show Episode 89 and in the forum.

Electronic Principles
Watts = Amps * Volts
Ohm (resistance) = Volts / Amps

On a charger pay attention to:

  • Input voltage range
  • Max charge current
  • Max watts

For instance with an iCharger 3010B, their specs are:

  • Input Range = 4.5-38V
  • Max Charge rate = 30A
  • Max watts = 1000W @ 24V, 500W @ 12

Max charging amps of this input voltage for 6s pack is…

  • 500W / 26.2 = 19 amps
  • 1000W / 26.2 = 38 amps

Full voltage of different cell count packs (number of cells x 4.20vdc)

  • 1S = 4.2vdc
  • 2S = 8.4vdc
  • 3S = 12.6vdc
  • 4S = 16.8vdc
  • 5S = 21vdc
  • 6S = 25.2vdc

The equation that we are going to be using is watts / volts = amps.

(The output of your charger in watts) / (number of cells x 4.2vdc) = charging amps.

So applying the equation:

  • 1S battery is: 90watts / 4.2vdc = 21.43A charging output
  • 2S battery is: 90watts / 8.4vdc = 10.71A charging output
  • 3S battery is: 90watts / 12.6vdc = 7.14A charging output
  • 4S battery is: 90watts / 16.8vdc = 5.36A charging output
  • 5S battery is: 90watts / 21vdc = 4.29A charging output
  • 6S battery is: 90watts / 25.2vdc = 3.57A charging output

The charger will also have an amperage output limit, let’s assume that it is 10A. This means that when charging a 1S or 2S LiPo battery the output amperage is going to be 10A not the 21.43A or 10.71A as above.

With the charger amperage output when the battery is fully charged we can estimate the charging rate, aka the “C” rating. The equation is:

(Charging amperage when the battery is full) / (battery Ah rating x 0.8 aka 80% discharged) = charging C rating

So applying the equation:

  • 1S battery is: 10A / (2.08Ah) = 4.81C charging rate (using the 10A charging limit)
  • 2S battery is: 10A / (2.08Ah) = 4.81C charging rate (using the 10A charging limit)
  • 3S battery is: 7.14A / (2.08Ah) = 3.43C charging rate
  • 4S battery is: 5.34A / (2.08Ah) = 2.57C charging rate
  • 5S battery is: 4.29A / (2.08Ah) = 2.06C charging rate
  • 6S battery is: 3.57A / (2.08Ah) = 1.72C charging rate

You can plug in the numbers and get your charging rates when you are using a DC power source and being able to output 150 watts, but remember to apply the charger’s maximum amperage output limit.

If you want to try parallel charging, just add the Ah ratings of the batteries into one battery for the equation. The Ah ratings of the batteries can be different, but the cell count of the batteries must be the same.

For example parallel charging two 3S 2.6Ah batteries:

7.14A / (2 x 2.08Ah) = 1.72C charging rate