Article #3

Using wattage to help choose a charger.

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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.

Let’s look at 3 sizes of lipos that I regularly use and their charging requirements. I have some 3s 4000mAh packs for my B-17 bomber, some 4s 6000mAh packs that I use (in series, (e.g. 8s)) for my Rascal 110 and some 6s 8000mAh packs for my big Eflite Carbon Z T-28 Trojan. For the purpose of better understanding charger requirements, let’s also choose to charge all of these packs at 2C, and since I like to parallel charge all my packs to save time, so we need to account for these issues as well. This will give us a total math workout for all possibilities for these packs.

Here is the math I would need to figure out how much output wattage my charger needs to have:

Let’s start with the B-17 3s 4000mAh packs. The “C” for these packs is 4.0A and remember, I want to charge them at 2C. Also recall that a fully charged Lipo cell is 4.2 volts and the charger will need to be able to provide this voltage. So, for each pack:

Output (Watts) needed by my charger = Volts X Amps of batteries being charged.

Watts (3s @ 2C) = (12.6V) * 2(4.0A) = 100.8 Watts.

That will cover charging a single 3s pack at 2C, now let’s do the same calculation for one of the 4s 6000 Rascal 110 packs.

Watts (4s @ 2C) = (16.8V) * 2(6.0A) = 201.6W.

And now for the biggest packs I use, the 8000 6s pack.

Watts (6s @ 2C) = (25.2V) * 2(8.0) = 403.2W.

So these simple calculations have shown that the maximum wattage requirement for what I want to accomplish is just under 404 watts.

Now, let’s take parallel charging into account which as you might guess, is going to increase demand on the charger. Here is an important fact to remember: When you place Lipo packs in parallel, you are creating a larger SINGLE pack that is the sum of the capacities but is still the same cell count of the individual packs. So, we can calculate the wattage required to charge multiple packs by simply adding the wattage required by the individual packs together.

Let’s say I plan to fly my B-17 four times and want to charge all 4 packs at once. That’s a reasonable option. Let’s calculate the wattage required for charging all 4 of the 4000mAh 3s packs together at 1C (which is actually the way I charge them).

Watts needed = 4[(12.6V) * (4.0A)] = 4 * 50.4 = 201.6W.

If I’m in a hurry to get to the field, it might be nice to be able to charge the same set of 4 packs at 3C. What would that take?

Watts needed = 4[(12.6V) * 3(4.0A)] = 4 * 151.2 watts = 604.8 watts total. (Of course, if you are beginning to “get” this you can see that you really only need multiply the above wattage requirement by 3!)

Ok now for the big one, what would it take to charge a pair of 8000mAh 6s packs at 1C and 3C so I can fly my T-28 twice?

Watts = 2[(25.2V) * (8.0)] = 2 * 201.6 watts = 403.2 watts total.
Watts = 2[(25.2V) * 3(8.0A)] = 2 * 604.8 watts = 1209.6 watts total.

Since my Powerlab 6 charger’s maximum output is 1,000 watts, I know that even if I set my charger to charge at 3C, it will not be able to provide enough power to make me happy. It will just take a little longer, that’s all… it will not harm the charger, batteries or power supply.

The main point here is to remember that the power (in watts) that your charging needs are going to be based on is a function of the product of the VOLTS and AMPS of your battery packs and your charger’s total output.

In conclusion

There are dozens of different chargers out there. Some have lower output 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.