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Is My Battery Suitable for Use With This Model?

Article by Andrew Gibbs

Ken Knox wrote to me, and says:

Hi Andrew, I wonder if you could give your professional opinion on a newish project of mine. Some months ago I bought a Multiplex Cularis electric glider. As you probably know, its made of that white high density Elapor foam. The wing span is 2,610 mm (102 in), the recommended motor is a Himax 3522-0700 driving a 12x6 folding prop, and the recommended battery is a 3S 2,100 mAh LiPo.

I have finally got round to finishing the plane which I have to say was quite a complex affair. I have fitted it with a MVVS 3.5/1200 cage out-runner motor driving a CAM 12x6 folding prop and the battery is a 3S 2,200 mAh LiPo.

When I ran the set up for the first time today I recorded a full throttle a current of 28 Amps at 256 Watts which is over the 25C rating of the battery.

I have three 25C 2,200 mAh batteries rated at 25C, and another 2,200 mAh battery rated at 30C. My question is should I not use the smaller batteries and stick to 30C batteries and how long could I expect to get the motor run given that it primarily a glider? Best Regards, Ken

Andrew's Answer
Hello Ken, and thanks for your question. It is excellent that you have measured the current of your model. Far too many modellers don't bother to measure the current of their set up, so they have no idea how it is performing in relation to the limitations of the various components like the battery, motor and ESC. Knowing the full throttle current is essential to be able to assess a power system's overall status.

The 'C' rating of a battery is a way of defining the maximum safe continuous current, expressed as a multiple of the battery's capacity in Amp-hours. A 1,000 mAh (or 1 Ah) battery can supply a current of 1 Amp for one hour. This is therefore known as the 1C current for this battery. Alternatively, this same battery could supply 2 Amps for 30 min. This is the 2C current for this particular battery. The battery could also be discharged at 10 Amps which would mean it was empty in 6 min. This would be the 10 C current.

Similarly, your 2,200 mAh battery can supply 2.2 A for one hour. This higher current is the 1 C current for this higher capacity battery. The 10 C current for this battery would be 22 Amps, and this would discharge the battery fully in 6 min.

You have measured the current of your Cularis set-up at 28 Amps. This is under 13 C for a 2,200 mAh battery (13 x 2.2A = 28.6A) so I see no problem using any of your batteries (rated at 25 or 30 C) in this model.

However, there is an issue with your ESC. This is rated at 25 Amps, but the current your set up is drawing is consuming 28 Amps. I always recommend a margin of safety between the ESC's rated current and the actual current it is used at. For example, for a current of 28 Amps, I'd suggest an ESC capable of least a 35 Amps. I strongly recommend that you change the ESC for a more suitably rated example, especially if your RC system is powered by the ESC's integral BEC. This is important because if the ESC overheats, it may shut down, causing an associated loss of control of the model. Personally, I would not risk flying any model with an under rated ESC. You also need to make sure that the ESC is adequately cooled.

These themes and much more about power systems are discussed in the newly available Gibbs Guides to Electric Power Systems and the Guide to LiPo batteries

Concerning the motor run time, theoretically, a 2,200 mAh battery should be able to supply 28 Amps for 3.75 minutes. However, in reality batteries used at high power will not supply their full rated capacity so in practice you will very likely achieve a slightly shorter motor run than this for a charge quantity of 1,760 mAh.

Your model, being an electric glider will be able to fly for much of the time with no motor power, so your actual flight time of course will be considerably longer than this.

Note that a C rating may be exceeded for brief periods, provided the battery is not allowed to become excessively hot or discharged below 3V per cell. This means for example that a 3-cell LiPo should not be discharged below 9.0 Volts. Note that the cells in a battery are more likely to fall below the safe 3V threshold if the battery is in a low state of charge. For general sport modelling use, it is advisable to limit flight time so that at least 20 % of the battery's charge remains in the cells at the end of a flight. For a 2,200 mAh battery thsi means a maximum dischareg quantity of 1,760 mAh.

This practice will help to avoid the cells falling below a safe voltage threshold when they are on load. Note that a battery that is already at say 3.2 V on an off-load condition can very quickly fall below 3.0 V when on-load i.e. when supplying a current.

You can find more information about battery C ratings here.

Reader's replies to this article

From Mike White:

Dear Andrew, I have just received Successful Electrics; very interesting. I read with great interest Ken Knox`s bit about his Cularis glider. A lovely flier!! Two points come to mind regarding this. One is that I always multiply the battery capacity by the C rating and make sure that the current drawn is below this figure. Being a bit 'agricultural' in my approach to these questions it seems to me to be the simplest way to go.

The other point is that the Cularis is quite a sophisticated design having flaps, ailerons, rudder and elevator. That`s six mini servos in all which, as you know draw, a goodly current and somewhat greater than six standard ones, oddly enough. In this case, if it were my glider, I would either add an Opto ESC and a 1,500 mAh receiver battery (or so) or install a SuperBEC or U-BEC.

So, back to the e-Mag…..

Regards, Mike White

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