What difference does the c rating make on a Lipo battery?

[tigerman]

Need some help .I do not really understand all the jargon on Lipo batteries being new to electric flight (always flown IC till now)I have ordered the Freewing Venom and they recommend a "6 cell 4000 40c battery" I have some 6 cell 4500 30c batteries .Will this be OK in the Venom being only 30c and not 40c and what difference will having a 30c battery and not a 40c battery ,if any ,have on the flight time and power on the venom I know the 30/40c is the discharge rate of the battery but what does that mean in layman terms?Or would it better to buy some new batteries that are 40c rated?

[Dave Hopkin]

The C rating gives an indication of the current the LiPo is capable of delivering without undergoing permanent chemical damage - the current rating is found by multiplying the Capacity of the Lipo by the C rating over 1000

In your case 4500 x 30 =/ 100 = 135 Amps

The C rate has no effect on flight times

[iqon]

EDF`s require a high c rating, your batteries will fly it, but if you want the most out of it get bigger c rating, try it and see....

[Geoff S]

The C rating is an indication of the internal resistance of the battery. No battery is perfect. Ideally they should have zero resistance but they don't so when you use the battery and draw current the resistance has two effects.

The first effect is that the terminal voltage of the battery falls from its value you measure with a voltmeter so the whole power system has less output. You'll notice that particularly in the high current demands of a ducted fan, not so much in a model, like a trainer, with more modest demands.

The second effect is that heat will be generated inside the battery proportional to the square of the current ie double the current means four times the heat. I understand electricity quite well but battery chemistry not so much but I do know excess heat will damage them so it's best to keep them cool.

Hence, you need to take much more notice of C ratings in high current applications. eg my electric Tiger Moth has a 4S 4000 mAH battery but, because it only draws 40 amps flat out (and only 20 amps cruising) is quite happy with a cheap 20/25C pack because the nominal max current is 20x4 = 80 amps.

Having said that, C ratings are actually quite a lot lower than the manufacturers claim but that's another story.

Geoff

[Simon Chaddock]

tigerman

The required C rating depends on the current you require to fly your plane.

If your plane plane is say using 80 A at full power then a 4.5Ah battery is only being discharging at a rate of 17C.

It is worth remembering that battery C ratings tend to be over stated. There is no formal test on what the battery life will be if it is regularly discharged at the full claimed C rating capability. On this basis even at lower power outputs a 40C battery is likely to be less stressed than a 30C one but then it is also more expensive.

EDFs tend to work their batteries rather hard so good quality high C rated batteries are likely to be worth the extra cost..

[Trevor Crook]

All wise words above. All I would add is that the C rating can affect flight times if the throttle is being used. A high C battery will hold a higher voltage while being discharged at a given current. Since power=current x voltage, if you are cruising around using say 300W, a high C rated battery will have to supply less current than a low C rated one, therefore will run for longer. In my HK Vampire I have 2200 4s batteries rated at 40C or one rated at 60-100C. After a 5 minute flight, the 40C cells have used about 2000mAh, whereas the 60C one only uses about 1800mAh because I can use less throttle.

Having said all that, see how you get on with the 4500s, you may well be fine. Like the Vampire, the Venom has a big wing so you should only need to use full throttle for take-off and manoeuvers in the vertical plane.

[MattyB]

If your charger can measure the IR (internal resistance) of your packs do that and stick the result into this online C-rating tool - I warn you though, you may be very disappointed in the real world performance of your packs vs what it says on the label!

[Trevor Crook]

That looks a useful tool Matty. Unfortunately neither of my chargers has the IR measurement facility.

I'm not surprised about the variance from the manufacturer's label. The quoted C rating should be used as a rough guide only.

[Dave Bran]

Posted by Dave Hopkin on 19/04/2017 21:56:08:

The C rate has no effect on flight times

Say you have two 1300mAh packs, one a genuine 20C, the other a genuine 60C. If the power train on the model can suck even some of the extra juice available, surely the pack will be depleted faster and duration reduce?

Nobody so far has been as blunt as to say, but some manufacturers vie for maximum "C" discharge ratings per price bracket as bigger numbers sell, and whether that number is actually correct to claim is not easy to check.

[PatMc]

Posted by Dave Bran on 21/04/2017 09:04:22:

Posted by Dave Hopkin on 19/04/2017 21:56:08:

The C rate has no effect on flight times

Say you have two 1300mAh packs, one a genuine 20C, the other a genuine 60C. If the power train on the model can suck even some of the extra juice available, surely the pack will be depleted faster and duration reduce?

In that situation the duration would only reduce because you are increasing the power, flying faster perhaps. OTOH in most situations the 20C battery would have less duration because it will have a higher internal resistance than the 60C battery. This would result in greater heat loss within the battery.

[Bob Burton]

Quote 1 : "The C rate has no effect on flight times"

Quote 2 : " in most situations the 20C battery would have less duration because it will have a higher internal resistance than the 60C battery."

So, does C rating have an effect on flight times or not ?

[Denis Watkins]

Your flight time is governed, in my view, by the Lipo capacity, namely the mAh,

These milliamps are quoted to be available at a C rating, that is ambitious by all accounts

And to make things more complicated, these milliamps are used up by movement of a stick, by your thumb

Controlling a model in surrounding air that may be moving slowly or very quickly.

The major player in airtime would be the pack size, measured in mAh.

[Trevor Crook]

As I mentioned in my post above, because a battery with a lower internal resistance (higher C rating) will allow the model to be flown at a lower throttle setting than a battery with a higher internal resistance, therefore a higher C rating will increase flight time. Of course, if you use the extra performance of the better battery by flying more "enthusiastically" then you will not see much improvement in flight time. However, as Pat points out above, a battery with a high internal resistance (low C) will generate more internal heat, which is energy that the motor won't see, so this is another factor that will drain a low C rated battery more quickly than a high C rated one.

So it's not a completely straightforward answer, but in general the higher the C rating, the better the duration.

[Dave Hopkin]

Posted by Dave Bran on 21/04/2017 09:04:22:

Posted by Dave Hopkin on 19/04/2017 21:56:08:

The C rate has no effect on flight times

Say you have two 1300mAh packs, one a genuine 20C, the other a genuine 60C. If the power train on the model can suck even some of the extra juice available, surely the pack will be depleted faster and duration reduce?

Nobody so far has been as blunt as to say, but some manufacturers vie for maximum "C" discharge ratings per price bracket as bigger numbers sell, and whether that number is actually correct to claim is not easy to check.

The C rating is a measure of the batteries ability to deliver current on demand and is governed by the internal resistance of the battery (which will vary between packs and degrade over time)

The motor/prop determines the demand placed on the battery to supply current (Amp)

Capacity / demand = Duration IF the C rating is high enough to deliver that current

Once we raise the C rating high enough to meet the demand flight times will not be changed by increasing it further

BUT dont forget C ratings are a measure of the batteries ability to discharge without causing internal chemical damage - all batteries will deliver much higher currents but internal damage may well result

[Ben B]

Re flight times don't forget if you're pulling a reasonable load with a low C battery the voltage will drop which will trigger the LVC - or just damage the cell. So the useful capacity is less. Other than that C doesn't make a big difference to flight times- energy used during flight is energy used. IE think about it not as volts and amps but as Joules/Watts.

[Dickw]

Posted by Dave Hopkin on 22/04/2017 09:32:46:.....

....................BUT dont forget C ratings are a measure of the batteries ability to discharge without causing internal chemical damage - all batteries will deliver much higher currents but internal damage may well result

I sort of agree with this, but I think it is more the temperature rise caused by high currents which causes the damage not the current value itself. If you use the very high currents for short periods with cooling time between then it won't be too big a problem.

As an extreme illustration of this I have attached a chart from a data log showing some 1500 cells providing peaks of up to 250A (166C) and hardly dropping below 100C while running and yet still maintaining over 3.35v per cell. No damage apparent after some months of use, and I expect to get a full years competition flying out of these cells, but the temperature is never allowed too get too high. (about 2 secs ON and 18 secs OFF)

I think these Turnigy Graphene cells are rated at about 60 or 70 C which is similar to the stated rating of other cells I have tried which don't perform so well, so I don't get too hung up on claimed C ratings.

Dick

Edited By Dickw on 22/04/2017 10:27:41

Edited By Dickw on 22/04/2017 10:42:09

Edited By Dickw on 22/04/2017 10:43:25

[Simon Chaddock]

C ratings and flight times?

The question really is just what proportion of a LiPo battery's total discharge energy is 'lost' due to its internal resistance.

A typical internal resistance value for a LiPo cell is about 10 mOhms, high C cells are about half that.

To keep the maths simple lets say a 1000 mAh LiPo cell is discharged at 10A (10C). At an average of 3.3V gives an average energy release of 33 W

The internal resistance loss (I squared R) would be 10*10*10/1000 = 1 W. For a high C cell it would be 1/2W.

Yes the low C cell would get warmer but unless the cells are being worked really hard surely it would need accurate testing to detect a difference in actual flight times between otherwise identical 20 and 40C batteries.

[Ben B]

Of course just to confuse things when the batteries do warm up the IR goes down