Champaign, IL
Joined Dec 2009
3,639 Posts

Thanks again everybody.
I'm not shopping for a charger or power supply today. I was just trying to sort the math to answer a question for another member in my local R/C club. I've got it all figured out now. To that end, I wrote this...if nothing else just to help me retain the information better (I know it's extremely elementary, but writing it down helps me rememberand maybe somebody else!)...
I know this has been covered time and again in other posts and forums (and links in other posts and forums), but just for fun I thought I’d take a stab at it my self—that is explaining how to figure out—“powerwise,” how much charger to purchase for charging LiPo batteries. Anyone out there feel free to correct me where necessary...
(This does not take into account other factors such as number of charge ports, input capability (AC, DC), etc. I’m just talking about the critical numbers here.)
You have to know three things:
1. The rate at which you want to charge the batteries.
2. Your battery pack’s Voltage when fully charged.
3. The power, or “Watts” required to charge your battery at the desired rate.
1. Charge Rate:
The charge rate, or the rate at which electrical current flows into your battery is expressed in Amperes, or “Amps” (usually just “A”). So the rate you should use depends on two factors; 1) How long you want to wait for the battery to charge, and; 2) The size, or “capacity” of your battery.
Concerning the time, this is already pretty much decided for you—or at least strongly recommended. It’s best for those of you new to LiPos to charge at a current flow rate (again, “Amperes,” “Amps” or just "A") that is equal to the battery’s capacity. Were the battery fully discharged, mathematically it would take one hour to charge. This charging rate equal to the battery’s capacity is called a “1C” charge rate because it’s one “times” the battery’s capacity. Simply, if you have a 1,800mAh battery, a 1C charge rate would be 1,800mA, or 1.8 Amps (1.8A).
Note that we never fly our batteries until they are completely drained, so a 1C charge rate should actually take less than an hour to fully charge. And some LiPos can be charged as quickly as 30 minutes (2C) or even in fifteen minutes (4C)! But charging LiPo batteries this fast is best left to experts because it can be dangerous, not to mention potentially reduce the life of your battery. Most LiPo batteries will be “happiest” if charged at that 1C rate that will take about one hour.
Okay, now we know that for our 1,800mAh battery we need a charger capable of charging at a rate of at least 1.8A.
2. Battery pack’s fullycharged Voltage:
Battery packs have the “nominal,” or average Voltage printed on the label. But another factor we need to know to determine “how much” charger is required is the battery’s Voltage when it is fullycharged. To know this, first we have to know how many individual battery cells make up the battery pack. Sometimes the number of cells is also printed on the label—“2S,” “3S,” “4S,” etc. (By the way, “S” means that the separate cells making up a battery are wired together in “series,” adding the Voltage of the cells to each other. A 3S battery is 3 individual cells wired together to make a single battery pack.)
But if the number of cells is not on the label, it’s easy to figure out. Divide the battery’s nominal Voltage (always printed on the label) by the nominal Voltage of a single LiPo cell which is 3.7V. So, if the label on your battery reads “11.1V,” divide 11.1V by 3.7V to end up with 3. Now you know you have a 3S battery. Don’t worry, if you’re new to this, eventually you’ll just know that a 7.4V battery is 2S, an 11.1V battery is 3S, a 14.8V battery is 4S, etc.
Once you know the number of cells, multiply that by the Voltage of a fullycharged cell which is 4.2 Volts.
Okay, for our 11.1V (3S) battery now we know we need a charger capable of charging at least 12.6V. (Sometimes though, the specifications for a charger might state only how many cells it is capable of charging—not necessarily the Voltage—“charges 1 – 6 cell LiPos”).
So, now we know the charge rate required and we know the number of cells, or the Voltage required. But there is one more, easy, final calculation to determine the last of three criteria to look for in a charger: We need to find out how much “power” the charger must be capable of delivering. This “power” is expressed in Watts.
Volts x Amps = Watts
Let’s do an example:
If we want to charge our 3S 1,800mAh battery @ 1C, how much Watts must our charger be capable of delivering?
12.6V x 1.8A = 22.7 Watts.
We need a charger capable of charging 3S batteries (12.6V) @ 1.8A that can also deliver at least 22.7 Watts!
Now, you might be wondering, if the specifications state that a charger can charge 3S battery and 1.8 Amps, then why do we need to confirm that it has enough Watts? Well, sometimes the specifications state “charges 3S, 2A,” but sometimes it cannot do both at the same time because it doesn’t provide enough Watts. It may be able to charge a 3S battery, but maybe only at 1 Amp (12.6 Watts). Or, maybe it can charge at 2 Amps, but only at 4.2 Volts (8.4 Watts). That’s why you also need to confirm the charger’s output power in Watts as illustrated above.
Here are some of the specifications for a Great Planes Triton EQ charger:
AC Input Voltage: 110V 60Hz  240V 50Hz (detachable AC cord)
DC Input: 1115.0V DC, with large alligator clips
Number of Outputs: One, banana jacks
Battery Types, # cells:
114 NiCd, NiMH
16 LiPo, LiIon, or LiFe (3.6, 3.7 or 3.3V cells)
2, 4, 6, 8, 10, 12V Pb (2V per cell)
Fast Charge Current: 0.15.0A (1C maximum for lithiums)
63W max DC, 50W max AC
As noted in the specifications, the Triton EQ will charge a 1 to 6cell LiPo, and can charge up to 5 Amps. It can handle 63 Watts if connected to a DC power source or 50 Watts if charging from your AC wall outlet. Note again though, since its maximum Watts output (from an AC wall outlet) is limited to 50 Watts, the largest 6S battery it can charge at 1C is about 1,980mAh (50W / 25.2V (6S) = 1,980mAh). Put another way, even though the charger is specified to 6S, 5A, it will NOT charge a 6S 5,000mAh battery at 1C because 25.2V x 5 Amps = 126 Watts, but the charger is good only to 50W (if charging from a wall outlet). Again, this is why you need to calculate the Watts requirement for the battery you will be charging and compare it to the Watts output of the charger you are considering—don’t go by just the number of cells and the current.
In the case of the 3S (11.V) 1,800mAh battery we were using as an example, the Triton EQ will certainly do the job. Given the 50W limit it could handle a 1C charge rate for a 3S (12.6V) battery up to 3,970mAh in size—anything larger than 3,970mAh will be less than a 1C charge rate. However, when shopping for chargers you might also consider any larger batteries you could be charging in the future should you become more involved in the hobby someday flying larger planes.
This is just a guess on my part, but for most people (50%, 80% ???), a charger capable of 6S, 5A, 50W will be suitable for all of their batteries. On the other hand, the more and more threads and posts you read about batteries and chargers, the more demand you see for chargers and power supplies capable of 100 Watts or more.
One last, very minor calculation: This is for modelers using a separate, DC power supply that they can plug into their AC wall outlet to power their battery charger. If using a separate, DC power supply, you have to figure in an amount of loss, or inefficiency when considering the Watts output required from a power supply. Most agree this factor is approximately 20% loss (or 80% efficiency). So, if we’re looking for a charger capable of 25 Watts (to round the 22.7 Watt figure required from the charger), you’ll need a power supply capable of at least roughly 31 Watts. Using our 80% efficiency factor, 25 Watts (required) / .8 = 31 Watts. This means we’ll need a 31 Watt power supply to get 25 Watts from our charger if using a separate DC power source.
