Motor Battery Selection

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By Jake Rice

UFO Abducted must be able to fly around a classroom using ducted fans driven by brushless DC motors. To achieve untethered flight, a battery must be used to power the motors. In selecting a battery for the UFO, the three main considerations are weight, maximum discharge current, and energy capacity. A lithium-polymer battery will be chosen due to the low weight and high discharge current of lithium-polymer batteries relative to other common types. Because the speed controllers for the motors are rated for 14.8V, the choices are limited to lithium-polymer batteries with 4 3.7V cells in series.

The main factor limiting battery choice is the maximum current draw of the motors. To maintain safe operation, the battery must be rated to supply more than the maximum current potentially drawn by the motors. Each motor has a maximum current draw of 27A, and the 4 motors have a combined maximum current draw of 108A.

In addition, the energy capacity of the battery must be large enough to ensure that the UFO completes its flight. Based on previous research, the UFO will be able to maintain level flight at under 70% of the maximum motor throttle, and at 70% throttle the motors draw 44.68A. To meet the 2 minute minimum flight time requirement, the capacity of the battery must be higher than 1489mAh.

The following chart compares several acceptable batteries. The battery purchased last semester is at the bottom of the chart.

 

Name

Cost

Weight (g)

Capacity (mAh)

Maximum Discharge Rate (C)

Maximum Current Draw (A)

Maximum Flight Time (min @ 70%)

Capacity/Weight Ratio (mAh/g)

Turnigy nano-tech A-SPEC G2

$49.77

296

2600

65

169

3.49

8.78

Turnigy nano-tech

$34.47

268

2250

65

146.25

3.02

8.40

ZIPPY Flightmax

$31.27

368

3000

40

120

4.03

8.15

ZIPPY Compact

$35.57

350

3300

35

115.5

4.43

9.43

Sky Lipo

$64.72

349

3000

40

120

4.03

8.60

ZIPPY Flightmax

$25.78

329

2650

45

119.25

3.56

8.05

Thunder Power G8 Performance Pro

$94.99

284

2700

45

121.5

3.63

9.51

MaxAmps LiPo

$109.99

327

3250

150

487.5

4.36

9.94

 

Of the batteries compared, the MaxAmps LiPo has the best capacity/weight ratio, the second-highest capacity and a far higher maximum current draw than any of the others. Using the Thunder Power or one of the Turnigy batteries would reduce the weight of the UFO. However, due to the higher capacity and the cost advantage due to the fact that we already have it, we should use the MaxAmps battery to power the UFO’s motors.