Old 08-12-2013, 04:20 PM
  #260  
AEAJR
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ESTIMATING BATTERY RUN TIME
by Ed Anderson
Updated 6-6-14


Since this comes up so often it is worth posting by itself.


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 number.

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 delvier 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 ignore the peak ratings.

To understand more on batteries see chapters 5 and 6.

Last edited by AEAJR; 06-06-2014 at 10:25 PM.
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