In continuation of selecting the best motor for my X8 (Octo coaxial) build for video/photography (see post here : http://www.diydrones.com/group/arducopterusergroup/forum/topics/tig...),
I must now compare and select the best battery in 6S votlage. I collected some information on various web site to compile a comparison for three categories of 6S batteries based on capacity : 5000 mah, 6000 mah and 8000 mah. I intend to fly with two packs in parallel, so I did not care to analyze bigger capacities like 10.000 mah , also because they are hard to find in these common brands : Zippy, turnigy nanotech and Kypom (i limited the comparison to these three brands because they can be found quite easily online in Europe and in the US, thanks to the Hobbyking web site, the onlylipo.com web site and the altigator.com web site, etc).
I defined five criteria as follows:
-price (in euro)
-weight (in grams)
-power density 1 (capacity in mah divided by total weight of battery). Higher is better. You get more capacity per gram.
-power density 2 (capacity in mah divided by total price in euro). Higher is better. You get more capacity per euro.
-Compacity. This is the biggest dimension of the battery in mm. Smaller is better. I chose only to compare on the biggest dimension because this is the constraint of my craft (battery holders are max 170 mm long in my design)
And the winners are :
-In the 5000 mah category : Zippy 30 C. It is best on 3/5 of the criteria and second best on 2/5 of the others in an unsignificant manner.
-In the 6000 mah category : Zippy 30C (again). It is best on 4/5 ot the criteria and second best on 1/5 of the others in an unsignificant manner.
-In the 8000 mah category : guess what... Zippy 30C (again again). It is best on 3/5 of the criteria and second best on 2/5 of the others.
Unless Zippy batteries are bad quality, Zippy seems to be a great battery. I would like people who have experienced Zippy batteries to give here in this post their testimonials about their quality and durability ?
This is great, thanks for sharing!
I noticed you called (mAh / gram) "power density," but wouldn't it be more correct to call it "energy density?"
It might be helpful to compare max current capabilities of these batteries too. For example, that Zippy 5Ah/30C can only support 5x30x2=300A (= 37.5A / motor). You're shooting for 40A ESC's so this is pretty good, but I would rather be limited by my motors and ESC than my battery. If instead you go with the Kypom 5A/35C, you can pull 5.1x35x2=357A (=44.6A/motor), and the limiting component is no longer the one that could burn your house down.
Then again, looking back at the price differences, I'd probably go with the same ones you picked.
So I'm doing the same for 3S. Was planning on evaluating:
o Zippy Compact 5000 25C
o Zippy FlightMax 5000 25C
o Zippy Traxxas 5000 30C
o Turnigy Nano Tech 3000 25C.
o Venom 7500 30C
All of the above batteries are via website data analysis their best for capacity/gram. All coincidentally also evenly go into 15000mAh (probably my max battery payload on most missions).
Will discharge each at between 20 and 25 amps from max change to min charge (3 volts). Tie the discharger into my computer and record the discharge data to calculate watts and work. Recharge and record the charge milli-amps (should be pretty close to the discharge amount). Possibly do that three times for each battery. Then do what you did and primarily look at discharge-capacity/gram.
Which Zippy did you test?
Was this done through actual test or analysis of website data?
Here is my data. Caution. Most of the data is taken from seller sites. My first experience with Zippy was that they were off by 12 grams (heavier) so don't know if they are inaccurate of not weighing the wire/plug. Also, capacities are only approximations and would need to be actually tested. The weight savings column is relative to the Turnigy Nano Tech 3S 3000 mAh. This was done in the early stages of preliminary design to gestimate the relative performance of 2S versus more cells (which only 2S proved to be an issue). 3S and up was all about the same.
so this is a nit, but Hugues is correct in calling it power, which incorporates a unit of time (milli-amp-hours). This type of analysis is dependent on discharge rages (time).
your insight into C is good. some might ask why even look at small 3000mah batteries if you know you are going to fly with 12000mah. the answer is two fold:
- if a manufacturer's most efficient battery happens to be the 3000 versus their 6000 then you get the added benefit of twice the maximum amp rate
- lower C batteries typically weigh less per capacity.
Per testing, Hugues knows that the max each motor will pull is 10 amps. So his max draw is 80 amps and his max C is 4 batteries x 30C = 120. So a good selection.
@Eric, about the C rating I did my selection based on the max amps I need which is less than 100 amps. Therefore even a 25C at 5000 mah would be enough. Since I plan to fly with two batteries in parallel, I am OK. Actually, taking lower C batteries will save not only on price but also on weight,therefore extending my flight duration.
@Forrest, the data comes from web sites. I only tested nanotech 4500 mah at 4S. I also tested a kypom 6600 mah at 4S.
Nice to see your table, it shows also zippy batteries as quite good.
We really need to testbench some of those! I am going to buy two 6S for a start. Still hesitating between 5000 and 5800 man.
I'm ordering resisters (or rheostat) to put across a connector so when i discharge the batteries in a bench test, it will discharge at approximately the same rate as the copter (many chargers will only discharge at a .5 to 1 amp rate depending on cell count). The Thunder AC6 has a nice feature where it plugs into the PC so you can analyze the discharge data data in excel.
Now I'm confused... maybe I shouldn't have brought it up, but why again do you think it should be power vs. energy density?
power * time = energy.
mA * hr = energy.
mAh / gram = energy per mass ~ energy density.
Am I missing something?
that does it ... Hugues ... you got room at your place. i'm moving to Belgium.
Bill - great put. you explained why it's important to do such tests relatively close to the discharge rate of your copter and the fallacies of even that test (it doesn't take into account the variances). so the only tests that i'll do, will be for three basic battery configs for the X2 Black Momba that draws between (when loaded) 16 to 25 amps depending on payload. i really don't know how useful that will be for anyone else. i'm hoping that it will allow the X2 to down select to the most efficient battery for a duration hover test. i'd do more, but my focus has to be on a stable photography platform after that.
Common energy forms:
o Potential Energy (mass x height)
o Kinetic Energy (.5MV^2)
So yes, you are right.
o Energy is the capacity for doing work (how much energy is stored in the battery)
o Power is the rate at which work is being done (the watts the battery is discharging)
So what are we interested in? We are interested in the capacity of the battery (energy) to release power at a rate (power) of our interest. What is non-intuitive to me is that I've been told that the actual 'capacity' of the battery varies in reality is based on the power discharge rate. So energy density might not be the right word because per the above, it depends. But certainly the phrase energy density for a specified discharge rate would be correct. And power density/capacity might not be right either as this term might imply a constant discharge rate over time, which in the tests we might not achieve.
I agree, it's a nit. But if you come up with a short term, I'll put it at the top of that column in the worksheet.
I know , like Caesar once said, the Belgians is the greatest civilization in the whole of his empire ( which included some northern barbarians across the channel ;) I wonder how the USA empire did not recognize that yet ;)
Back to our battery topic: I feared the good price of the Zippies was well for something. I had a nanotech though that puffed after about 20 cycles. I thought the nanotech description on HK was pure marketing bullshit but you confirm it indeed is true about lasting longer, lower sag.
You can still read on HK forum some customers complaining about false man capacity, especially on the 8000 man model: 12% lie (about 7000 real measured capacity). While the smaller model (5000 mah) was really 4500 mah: 5% lie which is better.