Understanding wattage in relationship to charging

Understanding wattage

So what is wattage? Put simply, wattage is a measurement of the amount of work electricity is doing. It is used to quantify the energy used or needed for an electronic device to function. Now lets look at a few examples to help explain wattage.

Charger output in watts and why it is important
Many times I have seen posts on the forums that start like "My new charger says it can do 10A but it only seems to be charging at 3A ..." or "I can charge my 3s pack at 4A but when I put on a 6s pack, it won't go any higher that 2.2A ...". The simple answer is that you have to take into account the maximum wattage.

Many times this limitation is due to the maximum wattage of your charger. Many smaller chargers are limited to 50W or 80W. I know that number has no basis in reality to most, so below is how to understand how it effects your charging. 

First off, wattage is calculated by measuring the voltage and amperage of a circuit and then multiplying them together. The equation looks like

Watts = Volts * Amps

How do we use it? As it turns out I learn best by example so here comes some examples.

Say you want to charge a 2200mAh 3s pack at 1C. How many watts does that require? We know the voltage of the battery and we know that C for the pack is 2.2A, so that makes the 1C charge rate 2.2A. Now we plug them in.

Watts = 3s * 1C = (12.6V) * (2.2A) = 27.7W

So the charger will be outputting at most 27.7W. We say most because the voltage of the battery is changing the whole charge cycle. It will likely start in the mid 11V range and will finish at 12.6V. So in order to calculate the max wattage needed, we use the max voltage involved.

Now lets look at what it would take to charge a big pack quickly, a 5000mAh 6s pack at 2C. The charger will need to be set to 25.2V (6s) and 10A.

Watts = 6s * 2C = (25.2V) * 2(5A) = 252W

As you can see it takes nearly 10 times the wattage to charge this pack than the first example pack. 

Now that we have calculated watts, lets swap around the equation and find the maximum amps for a charger with a 50W output. We start by rearranging the above equation

Amps = Watts / Volts

Now we plug in numbers. Lets start with a 3s pack. We will use the maximum voltage for 3s.

Amps = charger wattage / battery voltage = 50W / 12.6V = 4A

Now for a 6s pack.

Amps = 50W / 25.2V = 2A

As you can see, there is a reason new chargers have more than 50W of output. With the average user charging 6s lipos, some as large as 5800mAh, more wattage is needed to allow the packs to be charged in a reasonable amount of time. 

Other places wattage shows up
A common question on the forums from is "How can I charge my battery at 20A if my wall outlet is only rated at 15A?" Well the simple answer is that you can not compare amps, you have to compare watts.

The average US household outlet is rated for 120V and 15A. If you take those numbers and plug them into the wattage equation above, you get

Watts = (120V) * (15A) = 1800W

Now lets compare that to a charger charging a 6s lipo at 20A.

Watts = (25.2V) * (20A) = 504W

Even charging at this high rate on a relatively large lipo, we see it is using a less than 1/3 of what the wall outlet is capable of outputting. There are of course other considerations such as efficiency losses but even with those calculated in, it is very apparent that the standard wall outlet is capable of powering any normal charging needs.

In conclusion
Wattage plays a crucial role in charging. Don't forget to take it into account as you choose chargers, choose power supplies and even plug into the wall.
Using wattage to help choose a charger

Now that we know what wattage is and how to calculate it, lets put to work to help choose a charger.

The best way to start is to take a survey of your needs. Figure out what battery sizes you plan on using and how you plan to charge them. Then do some simple math to figure out the how much wattage you would like to have available. Let me use my requirements as an example.

I have 3 sizes of lipos that I use. I have some 800mAh 3s packs for my 250, some 2100mAh 3s packs for my 450 and some 2600mAh 6s packs for my 500. I plan to charge all these packs at up to 3C, so that will need to be taken into account. I like to parallel charge (see Parallel Charging) all my packs, so I need to account for that also. Here is the math I would need to figure out how much output wattage the charger I choose needs to have.

Let me start with the 800mAh packs. The C for these packs is .8A and lets say I plan to charge them at up to 3C.

Watts = 3s * 3C = (12.6V) * 3(.8A) = 30W

That will cover the small packs, now lets do the same calculation for the 2100mAh packs.

Watts = (12.6V) * 3(2.2A) = 83W

And now for the biggest packs I use, a 2600mAh 6s pack.

Watts = (25.2V) * 3(2.6A) = 197W

So those simple calculations have shown me that my minimum wattage requirement for what I want to accomplish is just under 200W. Now to take my parallel charging into account. When you place packs in parallel, you are creating a larger logical pack that is the sum of the capacities but retains the cell count of the individual packs. As such we can calculate the wattage require to charge multiple packs by adding the wattage required by the individual packs together.

Let me calculate the wattage required for charging a pair of 2200mAh 3s together at 1C.

Watts = [(12.6V) * (2.2A)] + [(12.6V) * (2.2A)] = 55W

This can also be written as

Watts = 2[(12.6V) * (2.2A)] = 55W

Maybe it would be nice to be able to charge the same pair of packs at 3C. What would that take?

Watts = 2[(12.6V) * 3(2.2A)] = 166W

Ok now for the big one, what would it take to charge a pair of 2600mAh 6s packs at 1C and 3C?

Watts = 2[(25.2V) * (2.6A)] = 131W
Watts = 2[(25.2V) * 3(2.6A)] = 393W

Now you have probably noticed that all I really needed to do was calculate my maximum wattage by using my highest capacity pack. That is true in my situation but may not always be true. For example a 10s 2500mAh pack charged at 1C requires much more wattage than a 3s 5000mAh pack charged at 1C. This is because wattage is a product of both voltage and amperage. So don't forget to take the pack cell counts into account.

Watts = (42V) * (2.5A) = 105W
Watts = (12.6V) * (5) = 63W

In conclusion
There are literally 100s of different chargers out there. Some are small and some are very powerful. In order for you to decide which will cover your needs, you need to do a little math. The good news is that the math is very simple. Also remember that times change and needs change. So put some thought into both what you need now and also what you might need in the future.