Another posting on "C" and what it indicates

[kyleservicetech]

Some of my RRCC club members have had questions on just what "C" is and what those battery ratings are. The following was placed in the RRCC newsletter, February issue by Kyleservicetech.

Lets list those ratings:
Volts
Amps
Watts
Milliampere Hours
“C”
KV

Starting with “Volts” that is the voltage behind your battery, just like the common 12 Volt lead acid auto battery, and those 9 volt alkaline batteries.

Amps is the current pulled by your ESC and Motor combination. The amount of Amps pulled by your electric power system is determined by the KV rating of your motor, the voltage of your battery, and the propeller used.

Watts is the total power input to your electric motor. Watts is directly equal to volts times amps for DC power. Watts is directly proportional to horsepower, where it takes 746 watts to equal one horsepower. If you’ve got a model that is pulling 73 Amps on a three cell LiPo, that is some 750 watts or one horsepower.

The term “KV” is a number derived from how many no load RPM’s your motor will turn on a given battery. As an example, a 700KV motor will turn about 7500 RPM on a three cell LiPo battery.

In electric models, these numbers are critical in how much power your motor puts out. Increasing the propeller diameter by 25% can nearly double the power pulled by the motor. The same applies to adding one more cell to your 3 cell LiPo battery.

Based on this information, one piece of test equipment that is quite important on those electric models is some sort of a meter that can measure the current pulled by the motor and its ESC. It’s not wise to exceed the maximum current rating of either the ESC or the motor. Smoke can easily result.

And, no, the typical digital multimeters such as the $5.00 Harbor Freight meter can’t be used. They typically have a maximum current rating of about 10 Amps. Those electric models run 5 or 10 times that value.

A number of decent “wattmeters” are available, some inexpensive, some a little more costly. I’ve got an Astroflight wattmeter that has served the purpose very well for the past few years.

Another option is a clamp on ammeter. Problem is, most lower cost clamp on ammeters measure AC current only, not useful for our purposes.

I’ve got a Sears Craftsman # 82369 digital AC and DC clamp on ammeter that does the job. Not cheap at about $60, but in my opinion, worth every penny.

This Craftsman meter can measure currents in two ranges, 0-40 and 0-400 Amperes, AC or DC. (You can use it to measure the alternator output of your car, or even the starter
current) Its also got the usual volts, resistance, temperature, with a thermocouple, frequency and so on. Nice thing about those clamp on ammeters, you don’t need special adaptors to use them. Just clamp its jaws around ONE conductor of your battery.

So, now, what exactly is “C”?? The term “C” is a short hand calculation for how your battery will perform in the real world.

The formula is as per below:
C = Amperes pulled divided by the ampere hour rating of your battery.

That is C = Amps/Amp Hrs
Using a bit of 5th grade math, you can switch the stuff around on this equation, and have the following:
1) C = Amps/Amp Hours
From formula 1) we can derive the following two equations.
2) Amps = C times Ampere Hours

3) Amp Hours = Amps / C

And one more, flying time in minutes is 60 minutes divided by “C” as used by your model.

4) FT = 60/C
5) C = 60 / Flight Time

Lets use a typical example of a model with a three cell LiPo, pulling 54 Amps out a 2200 Mah (2.2 Ampere Hour) battery rated at 65 C.

C = Amps/Amp Hours = 54/2.2 = 24.5

Since the battery is rated at 65 C, that battery is operating well within its stated capabilities.

Flying time is 60/C, or 60/24.5 or 2.44 minutes.

So, now we have a battery rated at “65 C”. Let’s take a look at what happens when you actually discharge that battery at 65C.

Flying time in minutes is FT=60/C or 60/65. That’s 0.92 minutes, or about 55 seconds.

How many amperes are we pulling at 65C?
Amps = C times Amp hours, or
Amps = 65 times 2.2 or 143 Amps.

FYI, I’ve measured the number of amps pulled by the starter on my old Chevy S10. It pulls about 120 Amps.

That “C” rating on those LiPo batteries suggests the maximum current you can pull out of the battery. What it doesn’t indicate is how many charge cycles you will get out of that battery if treated like that. You might only get 5 or 10 flights before the battery is shot. A lot of electric flyers have had good success when not going over perhaps 50% of the batteries claimed “C” rating. So if you’ve got a battery rated at 65 C, actually running it at 33 C, or in the case of the 2.2 Ampere Hour battery in this discussion, that would be 2.2 times 33 or about 70 Amps. And, again, using that 33C rating, you’re going to get flights at continuous full throttle of less than two minutes.

Agreed, this is a bit of math required for figuring out these electrics, but once you’ve done it several times, it’s not to difficult. (Or, ask the fifth grader in your family!)

Now, lets say you want a model that will fly for 7 minutes while pulling 53 Amps out of the battery.

Previously we found that:
C = 60/Flight time
C = 60/7
C = 8.57

And:
Amp Hours = Amps / C
Amp Hours = 53/8.57
Amp Hours = 6.18 Amp Hours

This indicates that if you want to fly a model at a continuous 53 Amps for 7 minutes, you need a 6180 Milliampere Hour battery. Sounds a bit high, but for the most part, these electric models are never flown at full throttle for the whole flight. Just cutting back to 70% power can double your flight time.

What really kills these Lithium type batteries such as the LiPo’s, LiFe’s, or A123 cells is the effect temperature has on the battery life. Many sources indicate that the maximum safe temperature is about 140 degrees F for the INTERNAL temperature of the battery. (The internal temperature of the LiPo battery can be a lot higher than the outside battery temperature. It takes awhile for the heat to get out.) If the internal temperature of your LiPo hits 160 or 180 degrees F, you might only get 4 or 5 flights out of it before heading to the hobby shop for a new one. Much higher than that, and you need to worry about a LiPo catastrophic fire.