Flight Lifetime on LiPo Pack
 

Hey Guys,

A friend and I are embarking on a project to build a UAV. One of our many criteria is the flight lifetime, which needs to be around the hour mark. We have looked at LiPo batteries, and are confused about discharge rates and capacity. Somebody suggested that the discharge rate is relating to capacity, so a 45C 5000mAh battey can supply 225A for an hour. This is totally different to every other bettery I've seen, so I thought I would ask some experts to clarify.

The need for so much power comes from the fact we are using large EDF motors to get off the ground. We are planning on using four EDF motors with 5kgf thrust each. These will be powered by two of the 45C 5000mAh 6S batteries wired in parallel.

If you want more information about the craft, we have a 10 - 15kg payload, and a 2.9m wingspan. The reason to using the EDF motors over regular props is they have more thrust for less cost. A bonus is they wont break on a crash landing (yet to be confirmed!).


Thanks,

MM

http://phantomtwo.blogspot.com/

RE: Flight Lifetime on LiPo Pack
 

Hi fellow Kiwi,

You are well astray on your arithmetic... a 5000mAh pack can supply (ideally) 5A for one hour [in practice a bit less than that because you cannot/should not extract every last mAh out of a pack]. The C-rate (45C) only comes into the rate at which you can discharge the pack... in this case you could (again, ideally) draw 225A from it... but of course it would only last 5000/225000 x 60 = 1.33 minutes = 80 seconds.

As for using EDF over props... whoa!... EDFs are far LESS efficient at producing thrust!

Even a slow flying trainer aircraft with very light wing loading and a big, efficient prop needs a minimum of around 110W/kg to fly reasonably... if you have 15kg (is that payload only or AUW?)....you need at least 1650W... on 6s that is perhaps 80+A... so to fly for an hour you'll need 80,000mAh... that's 16 of your 5000mAh packs.

I just found a reference to an EDF producing 5kg thrust: The motor in the 120mm edf can deliver 2200watts of power producing 5kg of thrust.
. That suggests that if you go the EDF route, you'd need 8.8KW... whew!

Cheers, Phil

RE: Flight Lifetime on LiPo Pack
 

Hey Dr Kiwi,

I knew I was off! It seemed crazy to think that was how it worked, but my friend was adamant. I'll make sure to tell him how it is.

A lot has happened in terms of UAV design. We have selected a different plane with a net weight of 350g, and a flying weight of up to 1300g. We are still looking around, as we want a good large cockpit for the electronics...


Thank you very much,

MM

RE: Flight Lifetime on LiPo Pack
 

At 1300g you probably need around 300W-400W minimum since a UAW isn't going to fly on wing alone.

RE: Flight Lifetime on LiPo Pack
 

What would be the best way to calculate what's the best "C" rating to use?

RE: Flight Lifetime on LiPo Pack
 

To decide the "C" rating needed - -
The motor short term maximum current under load is the major factor. If it were to require 20A at maximum (short term) load, I'd use that number, along with the rated battery capacity.
Most reasonably priced LiPo's have about a 25-30 C rating. A common capacity might be 2200MAH for smaller R/C planes. So, 2.2 A x 20C = 44A maximum short term load. (to be on the safe side for a 25c rated battery)
Finding out how much total battery capacity is needed is another thing entirely. You need to know what the average motor current draw will be, and the total flight time required.