Another posting on "C" and what it indicates
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:
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
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.
Good post as always, Denny.....perfect timing...lol....last night was reading about this in way older posts as I felt I needed a refresher:D:D
In your opinon, is this pushing the battery too much???.....
thunder pwr 3s 2250 45/90c pulling approx 43amps(actual 5 flights with this setup) previously batts have around 30 cycles never more than 30amps in the past....kept discharged,....usually charged right before flying....cells have never been below 3.60v unloaded... 1.5c charging
approx. 6 min flight times.....30secs 3/4 to WOT then glide with zero throttle for about a min then 30/WOT,...etc
within a couple mins of landing temp of battery 90.4(using infared thermo)
(inside motor 94.7,esc about 90,cant rembr exactly) physically battery feels warm, little puffing is noticeable, but within few minutes...completely normal looking and nominal temp.
I don't have anything that can tell me exactly how many amps I am actually pulling during flight( more or less...I don't know:blah:)
2.25ah x 45c=101.25amps,.... so I am pulling less than half of max amps on the ground.
Thanks in advance........just trying to get as much life out of a (2) $70 batterys.
But, with your setup, C = Amps/Ampere Hours, or 45/2.2. That's C=19.5 a value that should be pretty safe with any decent quality LiPo battery.
The actual current pulled during flight depends on the model type, prop selection, and a lot of other variables. Interesting, even whether the model is level flight, climbing, or diving has a HUGE effect on the current pulled in flight. I've found on a full throttle shallow desent of mayby 30 degrees or so, the current pulled can drop to 20-30% of WOT.
My experience has been that on my models with 16X12 APC-E or 19X12 APC-E props, the full throttle current during level flight is about 80% of the current on the ground.
IMHO, what can really shorten the life of those LiPo or A123 cells is running them until the ESC low voltage cutoff kicks in. Those LiPo cells are always going to have one cell little bit less capable than the others, and that cell can go below safe limits before the ESC LVC kicks in.
As for me, I never fly more than about 60-70% of the battery Mah during any flight. That allows for go-arounds, people on the field and so on. This way, never totally discharging those $$$$ batteries really reduces the chances of any cell going below safe limits.
so I can push a little bit harder.....right!!:D
Thanks for the info again!
There is great info at Batteryuniversity.com
I just added the direct link for what you've provided.
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