[aeajr]

I was asked by so many new pilots to help them estimate how long a battery would fly their plane. So I worked up this little article. I hope you find it useful.



ESTIMATING BATTERY RUN TIME
by Ed Anderson


THE AMP HOUR CALCULATION METHOD

Note that a 1300 mAH pack = 1.3 AH pack

m = mili which means 1/1000. Just two ways of expressing the same numer.

Capacity in AH / amp draw X 60 = minutes of run time.

1.3 AH / 8 amps = .1625 hours

.1625 X 60 = 9.75 minutes at 8 amps.

This assumes you use up all the useful battery capacity, not that you are
running the battery to zero voltage. It also assumes that the battery can
actually deliver its total rated capacity before the LVC, low voltage
cut-off, kicks in to keep you from running it too low. See the end for more
on this.

Normally you don't run at full throttle all the time. For mixed flying that
is probably more like 15 minutes. I usually estimate mixed flying time at
150% of the calculation but your actual experience will differ based on how
you fly.

When estimating useful flying time out of a pack, be conservative, then
watch it over several flights to get your true number. This calculation is
for planning purposes.

If you are sizing a power system for a plane, part of that sizing should
include the duration of the battery pack.


THE AMP MINUTE CALCULATED METHOD

Another approach is to convert everything to amp minutes.

A 1300 mah battery = a 1.3 amp hour battery

1.3 amp hours = 1.3X60 = 78 amp minutes.

Your plane draws 12 amps at full throttle. How long will this battery last?

78 amp minutes/12 amps = 6.5 minutes.

Assuming you never fully drain the pack I would use about 75% of that or
about 4.9 minutes. This leaves some reserve and does not over drain the
pack.


ANOTHER QUICK ESTIMATE METHOD

Above is the more precise way to calculate run time. However I usually use
this quick estimate method.

If the battery can deliver 1.3 amps for one hour then it can deliver 13 amps
for 1/10 of an hour ( 6 minutes )

In this example, we are only drawing about 2/3 of that ( 8 amps) , so the
run time will be about 1/3 longer than 6 minutes, about 8 minutes. Just a
quick estimate method I use. Not as exact, just a quick approximation that I
can do in my head.

However, your actual run time will vary by battery quality, how hard you are
pushing the pack, the LVC setting on the ESC and how much time you spend at
what throttle setting.

For example, if you run your 20C pack at 20C you will get greater voltage
sag then if you run it at 10C. The greater voltage sag will cause you to hit
the LVC sooner than if you run the same pack at 10C.

In actual flying you will likely be flying at partial throttle at times
which will reduce the draw and extend the time. You might get twice the
estimated full throttle time if you do a lot of partial throttle flying.

When working with e-gliders, where we typically only run the motor for short
bursts the time could be much longer. My Radian's battery is only good for
about 3 minutes at full throttle. Since I have the motor off most of the
time and I know how to ride thermals, that battery typically lasts me an
hour.

THE EFFECT OF C RATING ON PACK PERFORMANCE

While several battery packs may be "rated" at a given C rating we can see
significant variation on how well they actually work at this rating. The
higher priced, higher quality battery packs tend to be better at running at
this extreme end of their abilty. The lower cost packs may not live up to
that rating quite as well. But it can vary from brand to brand and pack to
pack.

I typically don't plan to run my packs at greater than 80% of their stated
continuous C rating. So if that 1.3 AH pack I used in the example ( possibly
a Radian pack for example ) is rated at 15C then you would expect it could
run at 15 X 1.3 amps or 19.5 amps and maintain a good voltage of 10.5 to
11.1 Volts for most of its useful capacity. Well some can and some can't.

I would look at that pack and say that I would plan to never run it
sustained at more than 15-16 amps. This would put less stress on the pack
and give me more useful capacity in the range that I want.

If you are running in a situation where you only need full power for short
bursts, like a 30 second full power climb followed by running most of the
time at about 2/3 throttle, than the pack might handle 19.5 amps quite well
for those short bursts.

Some packs have sustained ratings and peak ratings. I usually ignore the peak
ratings.

[Max]

Great post.

I'm going to stickie it

[rf103]

I use spektrum telemetry to tell me my voltage with alarm. For the little extra expense I find it's well worth it. Has save many dead stick landings!

[aeajr]

That should be great with a pure glider and receiver pack. How would it work with an electric glider and a BEC in the ESC? are you reading receiver voltage or battery pack voltage?

[rf103]

The module plugs into your receiver to read receiver voltage. To read voltage from a flight pack battery, I solder wires to the battery output wires that also plugs into the module. The module comes with all the wires and plugs you need.

[RCBYAIR]

I hate taking my eyes off my aircraft so I just set my radio timer for 4 minutes for everything I own. Way better considering the first aircraft I owned were elastic powered and only flew for 15 seconds.

https://www.youtube.com/watch?v=ywMkDEVJ3iw

[Lazy Ace]

So many aircraft so many different situations. The only constants are the battery size most likely are going to be the same for each air frame and the type of flying for that air frame will be similar for each flight. The way I do it is on my gliders I will do say four climb outs to altitude then land. I check the battery with the lipo checker then charge it to full and note the amount put back in the battery. Now I decide how far I want to use my pack and what reserve I wish to keep and say I come up with 6 full climbs to altitude with a good amount of reserve in case I need to do a few go arounds.

Other aircraft like my Edge or MXS I fly like I normally would some full and some half or less for 6 minutes then land check and charge and calculate my flight time plus some reserve.

So far its worked well for me and I have never had to land dead stick. On my MXS with sound system this is critical because the esc will cut out and the sound system will still work in conjunction to throttle position which can be very confusing when the engine quits but the sound system still puts out throttle noises. This happened only once as I had originaly set my time without the sound system drawing from the pack. Talk about confusion darn engines running but the thing is not flying. OOOOOPPPPS throttle down pretend its dead stick lucky I was high enough and right over the field.


Dennis

[RCBYAIR]

Yes Dennis,

Could see how that could throw you off for sure! Yikes!!! Engine sound on but no prop pulling.

[dangerdan]

Use one of these.

LED 1-8S LIPO BATTERY VOLTAGE TESTER/ LOW VOLTAGE BUZZER ALARM (1S SUPPORT 3.7-30V)

Set the alarm for 75%. When it goes off its time to land.