BATTERY / MODEL / Flight-Time Calculations

9 thoughts on “BATTERY / MODEL / Flight-Time Calculations”

1. Howdy Kingman Eric,
   I found your “Flight Times Calculations” article interesting. I mostly agree with your watts-per-pound figures and have another method of determining flight time, though it requires the use of a hobby Wattmeter or clamp-on DC ampmeter.

   Battery potential formula is:
   Capacity in Amp hours X 60 minutes = capacity in Amp minutes

   Using the Apprentice as an example, 3.3 Ah X 60 = 198 Amp min.

   Now use the Wattmeter to measure how many amps your power system draws. The Apprentice with stock propeller draws approximately 20 amps at full throttle, so…
   198 Amp min. divided by 20 Amps = 9.9 min @ Full Throttle
   Of course an Apprentice is not flown at full throttle the entire flight so “throttle management” can provide flight times of 15 to 20 minutes depending on how aggressive the plane is flown.

   On the other hand, your F86 draws approximately 75 Amps, so…
   4500 mAh = 4.5 Ah X 60 min. = 270 Amp min.
   and 270 Amp min. divided by 75 Amps = 3.9 min. @ Full Throttle,
   which can be stretched with throttle management…but who wants to do that with a jet?!!

   What makes a Wattmeter worth having is being able to try different props and cell counts in your plane while measuring the current (amps) so as not to exceed the limits of the motor or ESC, and of course it calculates the watts accordingly.

   Your club’s website is excellent!

   Wayne Wilson…..Quartzsite Desert Flyers

   1. Wow… your numbers came out very close to “my reality”! … Thanks for that info! On my F-86, I need to be “on the ground” at the end of three minutes with that 6S 4500mAh 60C battery. I take-off at full throttle and then try to fly at a fat half-throttle for cruise-speed and I add a burst of throttle for any climbing maneuvers (or for a flat-out full-speed pass down the runway!)… chopping back to that half-throttle ASAP after each blast … I have ordered a 6S 6000 35C… I am hoping the smaller C value will allow the battery to discharge more slowly? … so am hoping for longer flight-time at 6000mAh 35C.

      1. A battery with a low “C” rating in a setup that requires high power will result in a over-heated battery resulting in “puffing” and risk of fire. Most EDF jets require high C ratings that enable the motor/fan to turn at their designed RPM. When I used a 35C battery in my 4 cell jet the battery got too hot to hold in your hand; the 50C battery was just warm to the touch. The flight times were identical for both batteries.

         1. Well… THAT’S disappointing… I DO have two 6000mAh 6s 35C batteries on order… hoping for longer flight-time by using lower C-rating… I DID figure the battery might get warmer… hot? … I will run it “on the bench” for a test run to see how hot they get … or could just go for it and see if my F-86 becomes a flaming T-33 “Shooting Star”!

            1. The owners manual for that F-86 (very Chinese) says that model requires at least a 6S 3700 25C … THAT’d get me a minute of flight time!

2. In your first calculation under the heading The Battery Potential you show the voltage as 11.1 when in fact a fully charged 3 cell is 12.6 volts. One contributing factor when trying to determine battery performance is the internal resistance of the battery. The higher the resistance the hotter the battery will get during discharge. Many good battery chargers will show the internal resistance during recharge. The lower the resistance the better the battery will perform. As batteries age the resistance usually increases. Two identical batteries can have two different current discharge rates, which will determine the amount of flight time from each battery. Throttle management also greatly affects flight times.
   The most important thing to remember is to never fly a battery down to the level of 3.7 volts per cell. That can cause damage to the battery, as well as getting to the cut-off point of most speed controllers, where motor power will be lost.
   Also, remember to keep your batteries warm in cold weather, and out of the sun in hot weather. Leaving them in the sun can raise the cell voltage way beyond the safe voltage of 4.2 per cell which can lead to a LiPo fire.

   1. Thanks, Lee… good info!

3. Our member, Bruno Russo, replied via email…
   Hi Eric,
   Good article, thanks for taking the time.

   There are a few reasons your calculations are basically theoretical but not working in real life.

   1. Most battery manufacturers don’t tell the truth about true capacity or true max c ratings. I target for 50% of max c ratings to keep packs from swelling. IF a packs says 50C it probably won’t swell if kept around 25C.

   2. There is a factor that drops the voltage and wattage because the max power and current is continually dropping over the duration of a flight. Online calculator: Battery discharge time depending on load

   3. The life of a pack decreases over time and worsens when not discharged back to storage voltage. (about 3.7-3.8V/cell for li-po)

   Here are a few other sites to help.
   Battery run-time calculator from PowerStream, The “How long will a battery last calculator”

   RC LiPo Battery Power & Configuration Calculator

   Calculate Flight Time of LiPo Battery

4. Here is probably more information than you want to know about LiPo batteries and their Internal Resistance properties. Personally when my batteries do not perform as they did when new, or the charge times become excessive, then it’s time to use them for target practice! I have been using LiPo batteries for the past 12-13 years and during that time I have never “Puffed” a battery or had a battery fire…knock on wood!

   https://www.rchelicopterfun.com/lipo-internal-resistance.html
   https://oscarliang.com/when-retire-lipo-battery/
   https://www.youtube.com/watch?v=wP0MhOcPmQY

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