**sticky** Understanding Lithium Polymer Batteries

[CatalinaWOW]

Sounds like a battery issue.  I have some that are rated 15C, but show extreme voltage drops at rates over 5-7 C.  That is not all bad.  Those batteries are now over 7 years old.  I bought 4 at a bargain price, one died in a crash in my sons plane and one died of heat death from an overaggressive flight on a hot Arizona summer day, but the remaining two soldier along and give long flights in trainers.

C rating (what the vendor says) and C capability (what the battery can actually deliver) are related to internal resistance, the it isn't a straight one for one relationship.  So, when as Bartman points out above, internal resistance goes up, then C capability goes down.  Older batteries lose their punch.

If you are going to get deep into electrics (really meaning if you are going to buy more than a couple of batteries) it is well worth getting a wattmeter.  They run $30-50 bucks and by monitoring current, voltage and power with a restrained plane on the ground you can learn a lot about what is going on in your setup.  You can keep track of your batteries and save the weak sisters for the fun flies and lazy afternoons, while setting aside the good ones for the 3D days.

A high C rated battery can make a dramatic performance improvement in some setups, but usually adds cost,  weight and volume to the battery.  Nothing in life comes for free, but for me this trade has almost always come out good for me.

 

[Bartman]

i was going to say basically the same thing, knowing watts or amps will tell you whether the battery is properly sized for the plane.

you can start by looking for a spec sheet for the motor you're using.  if you're using a prop and battery within the specified range for the motor then the motor spec sheet might tell you roughly what current you're pulling when flying.  you can also do what @catalinawow; said above and hook up a wattmeter or multimeter to measure actual amps that the motor is using.

i'd guess your 31" pylon racer is using maybe a battery with a capacity of 2000mah battery, 2000mah milliamp-hour is the same as calling it a 2 amp-hour which means the battery can supply 2 amps of current for an hour and then it's fully depleted. that's the theoretical limit based on the rating the manufacturer's claims.  it can make more than 2 amps but the trade off is it will deplete more quickly.

the max current that the battery can supply (in theory without causing damage to the battery) is the C rating (yours is 20) times the capacity (yours is 2000 mah or 2 ah).......20*2=40 amps max current.

measure what the motor is actually drawing and compare it to that number (or whatever you have for the actually battery you're using).

just guessing but 20C is low for a smaller pack flying an e-racer but it depends on what the motor is actually pulling.  what is throwing me off here is that you're saying the battery isn't warm when you're done flying which is usually the first thing to look for when performance is poor due to undersized batteries.

Bart

 

[Ohio AV8TOR]

Jetpainter said
CatalinaWOW said
All good stuff.  Another thing to think about is that the discharge rating (C) is very differently (and sometimes creatively) applied by various vendors.  Some will give the highest rate that won't damage the battery.  Others give the highest rate that won't drop the output voltage below 3.3 volts, or sometimes 3.0 volts.  You probably won't damage batteries using these various definitions, but your performance can vary noticeably.

I've been wondering about that. On a low C rating battery can the power noticeably drop off even though the battery is still over 50% charge?

I've got a little 31" scratch built pylon racer I just swapped to electric. It makes awesome power for the first minute or so then seems to slow down quite a bit for the next few. After landing I've checked the battery and it's never been under 40%. It's a 20c battery. I've been wanting to try a higher C rating to see if that helps. The battery and ESC stay nice and cool so it's not a heat issue.
Disable the ESC's low voltage cutout.  On a low C battery you may be dropping down in voltage then the ESC sees this and reduces the power so you can land.  Reduced power = reduced heat that is why you are not hot.  

 

[Jetpainter]

I hadn't considered the low voltage cutoff. I just checked and it's set to the reduced power mode, with a 3volt cut off. I find it hard to believe the voltage is getting that low, and I would think if that was the case, it would slow down a lot more, but who knows.

 

[Bartman]

have you set up an ESC for LiPo's before?  which ESC is it that you're using?

 

[Jetpainter]

Bartman said

have you set up an ESC for LiPo's before?  which ESC is it that you're using?

I've set up few.

The ESC is a Cobra 33 amp. The motor is a Cobra 2213/12 2000kv with an APC speed 6 x 5.5. The battery is a 3s 2200mAh 20c.

 

[Bartman]

at the Cobra website

http://www.cobramotorsusa.com/2213-12.html

they show 3S power and a max current of 30A (330W which is volts X amps).  your battery is 2.2 amp-hour and 20C so the max theoretical amps the battery can produce without damage is 2.2 X 20 or 40.4 amps. 

if the prop is correct for the motor, which it probably is, then you shouldn't be seeing an issue although the discharge C rating is not always an honest rating depending on the brand.  I've had Zippy packs that seemed to be half of what the wrapper claimed.

the settings in the ESC might be the actual problem though.  if you need any help getting them set right just holler.

 

[Jetpainter]

Cobra has a very helpful static prop data chart for the motor that I used to select the motor in the first place. Using their figures I should be at 23amps, 255 watts, with a static pitch speed of about 90mph, but real world results always vary. I've never checked it.

Their data with that motor on 4S is interesting as well. If I move to 4S, drop the prop to a 5 x 5, it lowers the amps just a tad, raises the watts to 331 and the pitch speed to 112. The downside is the loss of thrust, and being a hand launch airplane if you get the static thrust too low the hand launches get way too exciting. I experienced that with this airplane an the glow motor on a small diameter high pitch prop almost 20 years ago.

I may pick up a 4S pack and prop and give it a try. The little thing is fast enough already, but it would be interesting to experiment with it.

 

[angelob]

Great write-up! Thanks for taking the time to do it.

 

[rcpilotacro]

A word about this Discharge Limits, the capital 'C' rating written on some batteries (Incl nanotech), i would take it with a pinch of salt, more often than not they are overrated, !! IMHO