One of the most common calls we get at Venom goes something like this: “I’m trying to charge my six cell 5000mAh battery at 5.0A but it will only charge at 2A.” In other words, the charger allows you to set the 6S 5000mAh pack to charge at 5.0A, but it never actually gets to 5A. This is a direct correlation to the maximum wattage available in the charger.
What is wattage? Wattage is a measurement of how much work the electricity is doing. It’s used to identify how much power a device or system needs to function.
So how do you calculate wattage?
WATTS = VOLTS * AMPS
So how do you use this equation with LiPo batteries? Lets say you want to charge a 3S 5000mAh battery at a 1C charge rate. We know that the 1C charge rate is 5.0A (See our article on determining charge rating if you’re unsure how we got this number.) So how many watts will we need?
Using the equation above, we take the maximum voltage that pack will need (Take 4.2v and multiply it by the amount of cells in your battery.) and multiply it by the 1C charge rate.
(12.6V) * (5A) = 63W
As you can see, in our example battery, if you were charging with a 50 Watt charger it would not be able to handle a 6S 5000mAh pack at 1C.
So now that we’ve figured out Wattage, lets take a look at calculating the max charge rate based on the wattage information. For that we need a new equation:
AMPS = WATTS ÷ VOLTS
So using our same example battery and keeping our charger at 50W we get: 50W ÷ 12.6V = 4A
Now, using the same 50W charger, lets see what happens when we make the battery a 6S pack: 50W ÷ 25.2V = 2A
As you can see, wattage plays a large factor in your chargers ability to charge higher voltage, higher capacity packs. Use this information to determine what charger will meet your individual charging needs.
Another common question about wattage 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 cannot compare AMPS if you are not factoring in the WATTAGE as well.
For your home:
WATTS = (120V) * (15A) = 1800W
Compare that to your charger charging a 6S LiPo at 20A:
WATTS = (25.2V * (20A) = 504W
This puts the power your wall outlet can produce into a little better perspective.
We hope this article takes some of the mystery out of those numbers and letters printed on your charger! Stay safe and have fun!
Special thanks goes out to Jeff Simon of Venom for the article above!