How do you calculate wattage?
Answer: This is a common call we get at Venom, and most of the time it goes like this: “I’m trying to charge my three-cell 5000mAh battery at 5.0A, but it only gets to 3A.” The charger allows you to set the 3S 5000mAh pack to charge at 5.0A, but it never actually gets to 5A. The reason for this is a result of the maximum wattage available in the charger. First, let’s start with the following equation: Volts x Amps = Watts. Let’s say that you want to charge a 3-cell 5000mAh battery at 1C. Calculate that charge rate first by dividing the mAh rating of the pack by 1,000. In this case, we get 5A.
We now need the maximum voltage required to charge that pack. Take 4.2V and multiply it by the amount of cells in your battery. In our example, this would be 12.6V (4.2V x 3 = 12.6V). Multiply the maximum voltage by the charge rate and you’ll get the wattage. In our case, it would be 63W (12.6V x 5A = 63W). So if you were charging with a 50-watt charger, the charge rate would be less than 5A. Let’s now take a look at calculating the maximum charge rate based on the wattage information. For that, we need a new equation, which is: Watts ÷ Volts = Amps.
Using our same example battery (3S 5000mAh LiPo) and keeping our charger at 50W, we get approximately 4A (50W ÷ 12.6V = 4A, rounding up). Remember, we calculate voltage by taking the maximum voltage of 1 cell (4.2V) and multiplying it by the number of cells in the pack (3).
Using the same 50W charger, let’s see what happens when we make the battery a 6S pack. For a 6S pack, the maximum voltage is 25.2V (4.2V x 6 = 25.2V). Plugging the numbers into the equation, we get approximately 2A (50W ÷ 25.2V = 2A). As you can see, wattage plays a large factor in your charger’s ability to charge higher-voltage, higher-capacity packs. By using this information, you can determine what charger will meet your individual charging needs.
We’ve discovered how wattage affects your charge rate and how to calculate the maximum wattage required for the particular battery that you’re charging. On a related topic, we are often asked to compare charger amperage to the amperage in your house. The question goes something like this: “If my wall outlet is only rated for 15A, how can my charger work at 20A?”
The answer to this question is that you can’t compare amps if you are not factoring in the wattage as well. North American homes have 120V at 15A, so using our math equation, the wattage in your home is 1800W (120V x 15A = 1800W). Compare that to your charger charging a 6S LiPo (25.2V) at 20A, which turns out to be only 504W (25.2V x 20A = 504W).
We hope this puts into better perspective the power that your wall outlet can produce.