Silly flying time math quiz - Warning Math inside

[Jermo]

Ok, 2100mAH battery with 25C rating. I'm guessing I'm missing something really silly but here goes.

2100mAH battery is rated to supply 1C discharge for 1 hour (60 minutes), I've used excel to calculate runtime in minutes using a ratio formula of (60minutes * 1C Amps)/Discharge Amps. Of course my calculations show linear discharge and we all know that a real heli flight won't discharge in a linear manner. I was just amazed at the low flight times by calculation.

AH.....Discharge Rate C.....Discharge Amps.....Runtime Minutes
2.1....................1....................2.1... .....................................60
.........................2....................4.2. .......................................30
.........................5....................10.5 ......................................12
.........................10..................21... ......................................6
.........................15..................31.5. .....................................4
.........................20..................42... ......................................3
.........................25..................52.5. .....................................2.4

so in a perfect world with a lightweight heli, packs at max efficient temp, no wiring losses or drag..etc what's the average current draw for normal flight? for 3d?

tnx in advance. Please correct any mistakes /misconceptions I have.
Jermo

edit's to fix formatting

[Pinecone]

No errors I see. Other than the common recommendation is to use no more than 80% of the capacity.

My Trex pulls about 1450 mAH in 7 minutes of mainly hovering. And crashing. :) Or about 12.5 amps average.

I have heard numbers like 4 - 5 minutes of hard 3D for 80% draw, which is 1680 for a 2100 pack. Or 25 amp average for a 4 minute flight.

[spork]

Your math looks right to me. I agree that we don't want to assume draining the pack 100% though.

[Jermo]

wow, that seems quick to me. I'm assuming 100% capacity is 3V per cell. Most folks set their lipo alarms to 3v? or more like 3.2? or??
Jermo

[spork]

wow, that seems quick to me.

I don't follow. Do you mean those flight times seem short?

I'm assuming 100% capacity is 3V per cell.

If we use 100% capacity we take the cells to 0.0 volts. Without a discharge curve handy I don't know what percent is used at 3.0 volts.

I set mine so that I put 80% back in. I think that falls somewhere around 3.2 volts/cell.

[Jermo]

yes..seems to be quick flight times. Understand it takes me time for preflight :) I'm old and careful with $800+ flying out there.

I'm making a lipo alarm, I had it all done on project board using point to point wiring. It's way too messy for me so I'm going to pickup the mats today to etch a board out. Adjustable and will connect to the balance connector. Design seems pretty solid and has a piezo buzzer alarm. someone elses design I've modified slightly. I'll probably etch several boards at once just so I don't waste the board. I could cut the section off first but I doubt i'll be etching any other boards anytime soon. hehe.
Jermo

[spork]

Your pre-flight time will only count slightly against your flight time if your blades aren't spinning.

[Jermo]

:) I guess we need more effient ESC/Motor and BIGGER+Lighter batteries!!!! :D
Jermo

[Pinecone]

Actually my packs after flying and cooling off are about 11 volts puttin gback in about 70%

As for lipo alarms:

http://www.helifreak.com/album_personal.php?user_id=7914

Great guy to deal with, fast, even from NZ and work great.

[MarkD]

Actually my packs after flying and cooling off are about 11 volts puttin gback in about 70%

Im putting back in approx 1800mA with a pre charge V of 10.5V on a 2100 pack - done about 15 flights on both packs and they still are OK

No sign of damage yet - maybe Lipo's are getting better

[Jermo]

http://www.helifreak.com/album_personal.php?user_id=7914

Great guy to deal with, fast, even from NZ and work great.

I've seen those and they are nice looking. I just prefer to make my own. With calibration you really need a meter to get it exactly where you want it and I wonder why you need LEDs if you have a buzzer *shrug*. RS DOES sell an LED that flashes w/o any additional circuitry. The advantage to the lipo design I'm using is I can fix it, change it, do anything with it because I know how it works 100% AND I get the fun of building it. I was just trying to help folks out with a board since that's the hardest part AND economies of scale make it cheaper if you make 10 with the kit.
Jermo

[Striker]

Can you do some math for the 4s A123 batteries?

See here for more:
http://www.helifreak.com/viewtopic.php?t=26850

claiming up to 12 mins of flight time :D

Thanks,
Eric James :shock:

http://www.expertmagic.com/

[Jermo]

The rates would be simular but because the voltage is higher the discharge rate should be lower. 4s pack of A123 is 2300mAH at 16v, 1C is 2.3A, to estimate time find the average discharge amps and read the runtime minutes. Note that the runtime column stayed the same but the discharge amps column changed.

AH.....Discharge Rate C.....Discharge Amps.....Runtime Minutes
2.3....................1....................2.3... .....................................60
.........................2....................4.6. .......................................30
.........................5....................11.5 ......................................12
.........................10..................23... ......................................6
.........................15..................34.5. .....................................4
.........................20..................46... ......................................3
.........................25..................57.5. .....................................2.4

Jermo

[spork]

I think the homemade LiPo alarms are great. I'd would caution that an LED alone can be hard to see. I would also caution that it's hard to find a Piezo buzzer that's really easy to hear - but you learn to hear it.

A good homemade design can be found here:
http://www.rcgroups.com/forums/showthread.php?t=221018&highlight=microscream+

It uses the best piezo buzzer I'm aware of.

Also, I think it's better to calibrate your alarm based on capacity put back into the batteries than to try to calibrate it by voltage. By the way, the resting voltage of your LiPo will almost immediately seem much healthier than that same battery under load 30 seconds earlier.

[Pinecone]

OK, so what we REALLY need is an alarm that works such that after you pull X mAH, it goes off.

Cool, design one of these.

And no problem if you want to roll your own, I do that for some things. Just making sure you know what is out there. For something like this, I would rather buy one and spend my time flying. :)

[spork]

OK, so what we REALLY need is an alarm that works such that after you pull X mAH, it goes off.

The only problem with that approach is that it will end up taking 100% out of the battery when the pack's capacity eventually goes down to 80%. I like to adjust my voltage threshold by making sure a new pack takes 80% back, and then keep that voltage threshold (whatever it happens to be).

[Jermo]

The design I'm using is with off the shelf parts at radio shack, the parts for the one you referenced isnt' available for most of us (Ca is blessed with Fry's). The piezo buzzer could be easily replaced with a louder one.

Since I started looking at having the boards commercially etched I'm further looking into having the boards commercially stuffed ...but with surface mount components. This would cut weight.

Here's the circuit (props left in to the origional designer)

http://www.helifreak.com/album_mod/upload/7fea908919719804536f4192f1bed310.jpg
and the unmodified circuit board layout
http://www.helifreak.com/album_mod/upload/057f02d08053889f9f9801aadde4679e.jpg
Jermo