Lipo choice - Lower C rating with higher capacity

[Dave Y]

Hi All,

I'm still looking at electrifying my Limbo Dancer, just waiting for Giantcod to get some stock in.  I have chosen the motor (Epower Motor) and am trying to figure out the most suitable battery.  The following are rough calculations using Timbos formulas in the Sticky:

Battery 1:

4S 5000mah 20C Lipo

5.0A * 20 = 100A Max Discharge

Max draw from motor using 4S = 45A

Flight time:
5.0A / 45A = 0.11
0.11 * 60 = 6.67 minutes flight time at 45A

Battery 2:

4S 3000mah 30C Lipo
3.0A * 30 = 90A Max Discharge

Mas draw from motor using 4S = 45A

Flight time:
5.0A / 45A = 0.08
0.08 * 60 = 5.14 minutes flight time at 45A

Again these are rough numbers - the motor wont be run at 45A constantly for real!

Both lipos are the same price on Giantcod and i'm struggling to see the situation in which you would use the 3000mah 30C battery over the 5000mah 20c battery.  Although it has a lower C rating the 5000mah battery gives a higher theoretical draw and longer flight time.

Am I missing something here, or is the 5000mah 20C battery a better choice for me with this setup?

[nasa_steve]

hi dave

       the 5000 looks better on paper but it depends how many cycles the 5000 packs will last generally speaking the higher c rated packs seem to have a longer useable life probably because the pack technology is newer i would try to get some info from someone who has operated these cells in the long term to see what they're really capable of

steve

[Bruce Richards]

I think the most important differences between the two packs will be weight and physical size. Unfortunately GC do not list these details for these batteries yet. The 5000mah is likely to bigger and heavier than the 3000mah.

[Tim Mackey]

Slight correction.

4S 3000mah 30C Lipo
3.0A * 30 = 90A Max Discharge

Max draw from motor using 4S = 45A

Flight time:
5.0A / 45A = 0.08

0.08 * 60 = 5.14 minutes flight time at 45A

 

It should be 

3.0A /45A = .06

 

.06 X 60 = 3.99minutes at WOT.

 

Basically the C rating has little effect on flight duration, and simplistically viewed, it means the packs can be hammered a bit harder thats all!  They will normally hold votage better under load, but thats not a big deal in most cases.

pack capacity is what matters when aiming for duration. Of course, higher cell counts mean lower current ( for the same watts ) and that really helps duration .

 

 

[Simon Chaddock]

Dave Y

The 5000 mAh battery will weigh more than a 3000....er..... pretty nearly 5/3 (1.66) times more!

This extra weight will require more power to fly so the actual endurance will be longer but not 1.6 times longer.

The actual effect the extra battery weight has will depend on the total weight of the plane. For example an extra 4 ounces say on a 3 lb plane would not be very noticeable but finding the best set-up can involve a degree of trial and error.

I hope this helps.

 

 

[Dave Y]

Ack my mistake on the calculations, not sure why I put 5 in there!

 

Thanks for the replies, I think I will email Giantcod and find the specs of the batterys.  I don't think the Limbo Dancer will mind a little extra weight!

[Simon Chaddock]

Dave Y

Just to expand on Timbo's point a bit more.

Lets say you have a set up with a 2s 4000mAh battery providing 40 amps (280 watts). Using an appropriate motor and prop you could get the same power (280 watts) using a 4s 2000mAh battery with just 20 amps.

The batteries would weight more or less the same so is there any advantage in using the 4s layout?

Basically the answer is yes. The losses (usually in the form of heat) are proportional to the current squared. So as the 4s layout only uses 1/2 the current it has only 1/4 the losses.

As leccy flying is pretty efficient to start with the actual gain is likely to be small but worth considering if all other things are equal.

[Dave Y]

Well I have all the bits now

 

Old Powertrain (Enya 25) :

 

 

New Powertrain :
EMAX BL2826/06

4S 5000 20c Lipo

Hobbywing 60A ESC

 

The subject of the conversion! :

It will be a tight fit I think! :

 

 

I hope to get the bits installed in the next few days and maiden when the weather is right, will post the results!

 

[Tim Mackey]

Nice one Dave, that setup should be fine, You haven't mentioned the prop choice yet - so if I may, let me suggest you will need a fairly good sized one to get the power out from the motor. If it used to fly on a 25, then a very rough and ready conversion would mean you should be aiming for about 500 watts. Perfectly do-able on that set up providing you go for something around the 13 x 6 mark. On the 4s 5A pack expect to hold at least 3.5V per cell under load, so reckon on at least 14V. This means for 500 watts you will be pulling around 35A, which on that battery is about 6C, so you should have a nice lifespan out of it as it is being used very gently compared to its potential 100A amps.

6 C also means at WOT your duration could be up to 10 minutes

( 60 minutes divided by 6C = 10 ) Of course, in practice you should get quite bit more, as depending on various factors, you are unlikely to be using WOT for the whole flight. Also, the maximum current draw will fall when actually flying.

My self made little spreadsheet deducts 20% of static current draw for real in the air figures, and adds 30% to flight times at WOT figures.

So.... your current in the air will be about 28A at wot.

Flight times would be 60 / 5.6 = 10.7 and after adding the 30% = average flights of 14 minutes.

All theoretical data, based on my actual figures achieved with many different models over several years, in various conditions, so not gospel, but would interested to hear what your results actually are.

[Dave Y]

Well we did the maiden this morning, this motor is great!

 

Flew it first with the 4S pack and a 12x6 prop (the LMS is sold out of 13x6 atm) - WOT was almost like a 40 glow engine with this plane, amazing.  The battery is still on charge so yet to see how much power was used to work out flight times.

 

Because the plane is so light we then tried it with a 3S pack (12x6 prop) and it flew with what seemed like slightly less power than a 25 glow, still plenty enough to beat this plane around.

 

Got a watt meter over christmas so here are a few figures, I havent done the 4S pack figures yet so will post those soon.  Still waiting for the recommended 13x6 prop.

 

Battery: 3S 2300mah

Prop:  12x6
Amps:    25A
Watts:   290w
Volts:   11v

Prop:    13x8
Amps:    35A
Watts:   380w
Volts:   10.6v

 Oh we took some video too so will post that once it's edited!

 

Edited By Dave Y on 02/01/2010 14:35:09

[Tim Mackey]

Nice one Dave, glad it worked out for you

You may well find that the 12 x 6 on the 4s will be fine then, although the 13" may just give it that extra "punch" and make it more "flyable"  IYKWIM