Electric Flight – Calculating Max Battery Usage

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This is a great article written by Kevin Peltz that will help many electric flyers save both aircraft and battery packs. Thank you for taking the time to write this Kevin. It is very helpful!

 

One concept that many electric pilots grapple with is how long they can safely stay airborne on a lipo battery, without damaging it. Constantly over-depleting lipo’s severely shortens the number of charge cycles you’ll get out of them. Also, I don’t like to rely on the speed control’s LVC (Low Voltage Cutoff) function to tell me when it’s time to land. Once you reach the LVC, you’re usually only a handful of seconds from turning into a glider (or a brick, depending on the aircraft). This is what I prefer to do:

– Select your battery and determine what 80% of its capacity is. For example, for a 2200 mah lipo, this figure would be 2200 x .8 or 1760 mah. This will be your “do not exceed” number.

– Prepare the aircraft for flight with the above battery and set your transmitter’s timer to count 5 minutes (05:00).

– Launch and fly the aircraft as you would fly it normally, ie 3D, mixed flying, just cruising around, wide open, etc. The general idea is to get a cross section of what you would typically do with that particular machine in a five minute window.

– When your timer indicates five minutes, land, and make note of the total flight time in seconds. Write it down so you don’t forget.

– Recharge that battery and take note of how many milliamps was “put back” in by your charger. If you don’t have a charger that displays this, ask to borrow one that does. Most of the current crop of reasonable quality chargers have this function. Again, best to write this number down so you won’t forget it.

– Use the following formula:

initial flight time (secs) battery usage (milliamps)

=
total allowed flight time 80% battery used

– Example: We’re going to use a 1000 mah battery. Our “do not exceed” number will be 80% of 1000 or 800 mah. We fly our aircraft for five minutes (300 seconds) and land, noting that the total time airborne was 320 seconds. We recharge the battery that we used and note that 511 milliamps was “put back in” by our charger. Using the above formula, we can now plug in the numbers:

320 secs 511 milliamps

=
n 800 milliamps

– We do the math – “cross multiplying” (320 x 800) / 511 = n. n equals 501 seconds, rounded up. Dividing 501 seconds by 60 will give us 8.35 minutes, or 8 minutes, 20 seconds (08:20). This is the maximum allowable flight time we can expect flying this aircraft in a typical way and still not over-deplete the 1000 mah battery we’re using. Most cellphones will have a calculator function, handy for number crunching at the field.

– Once you have this information, you can set your transmitter’s timer accordingly and safely maximize your flight time on a given battery. I suggest setting your timer for one to two minutes less than the maximum calculated time – this gives you a buffer in case you need to go around the pattern again when landing, or flying in a breeze where you’ll be using more throttle than normal. So in the above example, we might set our timer to 7 minutes (07:00).

– If the function is available, slave your timer to your throttle, and set it to count up/down any time you’re above flight idle. Some Futaba radios, like the T7C, can be programmed this way.

– To maximize your batteries, remember “up and down” – up throttle on up lines and upwind, and down throttle on down lines and downwind.

~ Kevin Peltz