Rescue Hexacopter Project

 

Design - 22 Volt System

The hexacopter will have several different voltage systems. A 22 Volt system will provide power for the motors and indirectly the flight controller. A 12 Volt system will provide power for the video and lighting systems. The 5 Volt system will power the receiver, flight controller, FPV camera, and the landing gear retracts.

The 22 Volt system consists of a battery wired to an Electronic Speed Controller or ESC which is wired to the motors.

22 Volt Electrical System Diagram

Batteries

There are many types of batteries on the market, from dry cell batteries, lead acid batteries, and lithium polymer batteries. For large craft, however, nothing provides the balance of high power and light weight like lithium polymer batteries. Lithium polymer batteries feature high discharge rates and are usually bundled in groups of cells, each cell having 3.7 Volts. The cells are wired in series and are sold by the number of cells. For example a 3S battery has three 3.7 V cells wired in series or 11.1 Volts. A 6S battery has 6 cells or 22.2 Volts of current. It might also be listed as a 6S1P battery which means 6 in series, 1 in parallel.

Lithium polymer also have a maximum discharge rate which must be taken into consideration in the planning process. For example, we will be using the U7 motors which are rated at a maximum current of 40 Amps. Since we have six of these, we can expect a maximum draw of 240 Amps at full throttle. We need to know that the batteries can handle a draw like that. The manufacturers of LiPo (Lithium Polymer) batteries provide a C (or Capacity) rating to determine the maximum discharge the battery is capable of. The formula for total capacity is C-rate times Amps per hour. Thus, a 5000 mAh battery would have 5 Amps per hour (5000/1000) and if it had a C rating of 25, then it could handle a maximum discharge rate of 125 A. In our previous example, if we wanted to use a 5000 mAh battery to power our motors, then we would look for a battery with a C rating of at least 48 C (240 A / 5).

Initially, we planned to use four 5000 mAh batteries wired in parallel (6S4P). However, each 5000 battery weighs over 1 Kilogram or 2.2 pounds and costs about $90 per battery. That would be about 8.8 pounds at a cost of $360 providing 20000 mAh. By using three 8000 mAh batteries wired in parallel (6S3P), we would have 24000 mAh and the weight would be roughly the same at about 9 lbs with a cost of $330. Thus we could save money, increase our capacity by 25%, and still keep the same weight with less footprint by going with 8000 mAh batteries.

The battery we chose to use is Turnigy nano-tech 8000mAh 6S 25~50C Lipo Pack and will be purchased from Hobby King.

Turnigy Nano-Tech 8000 mAh Battery

We will be using three of these batteries wired in parallel. The batteries will be posititioned on the top of the Top Plate and secured to the plate using two velcro tie-downs per battery. Since the battery will be resting over two screws, we designed a balsa wood plate with holes where the screw heads are located. This was made from two pieces of balsa wood 1 3/4" x 8" x 3/32" glued together and then epoxied over the holes. The balsa wood will be purchased from Midwest Balsa. The balsa will protect the batteries while still allowing access to the screws if necessary.

Battery Plates Installed

We will also want to provide a means for the batteries to be quickly and easily connected. The batteries come with 5 mm bullet connectors with a female on the positive side and a male connector on the negative side. The bullet connectors are problematic on a carbon fiber frame as carbon fiber conducts electricity. Therefore we will be replacing the bullet connectors with EC5 connectors which feature an insulated and protected connection. We will be purchasing the EC5 connectors from Hobby King.

EC5 Connectors

The EC5 connectors will connect the batteries to the parallel circuit which will mounted under the top plate. A 1/2" circular hole will added to the top plate between the batteries to allow the wires to pass through. The wires from the first and second battery will be connected to a home made splitter made from three 5.5 mm bullet connectors. The third battery and the first splitter will be connected to another splitter. The 5.5 mm bullet connectors will be purchased from Tower Hobbies.

5.5 mm Bullet Connectors

The red wires from the three batteries are connected to a battery shutoff switch, which will be purchased online from Del City. Normally when connecting a high amperage battery to a closed circuit, there is a substantial spark as the final connection is made. The shutoff opens the circuit and eliminates the chance of the batteries arcing as the final connection is made. A cutout will be added to the bottom plate to accommodate the shutoff switch.

Battery Shutoff Switch

The red wire from the Battery Shutoff and the black wire from the second black splitter are connected to a 5.5 mm Bullet to 6 X 4 mm Bullet Multistar ESC Power Breakout Cable purchased from Hobby King. This will be used to connect to the Electronic Speed Controllers.

5.5 mm Bullet to 6 X 4 mm Bullet Multistar ESC Power Breakout Cable

The Electronic Speed Controllers we chose were from Tiger Motors as well. The Tiger T60 is a ESC which is rated at 60A and runs at 400 MHz right out of the box. We looked at several others but they all had to be reflashed in order to reach the 400 MHz speed. The ESC controls the minute changes necessary to keep the craft level and provide navigation.

T60A Electronic Speed Controller

We will need twelve 4 mm male Bullet connectors to solder on the battery side of the ESC. These will also be purchased from Hobby King.

4 mm Bullet Connectors

On the motor side of the ESC, we will need a total of fifty-four 3.5 mm Bullet connectors - three per ESC, three per socket assembly and three per motor. These will be purchased from Hobby King as well.

3.5 mm Bullet Connector

The final component for the 22 Volt system is the U7 Motor from Tiger Motors. It will be mounted on the motor mounts we designed earlier and will be attached to the ESC through wires run through each boom.

Tiger Motor U7 490Kv

In additon to the components listed, we will also need several different wire gauges for the system. For the connections to the batteries including the parallel wiring, we will need 8 AWG silicon wire. Silicon wire is extremely flexible and allows a greater amperage due to the thousands of individual wires in its design. Most suppliers only carry it in 1 meter lengths. We will need about 3 meters red and 3 meters black. We will be purchasing it from Hobby King.

Black and Red 8AWG Silicone Wire

For wiring the motors, we will need 14AWG wire. We will need 1 1/2 meters per boom and an additonal meter of red and black for the interior wiring. The wire for the motors, since it will never be seen, can be all one color. Again, the wire will be purchased from Hobby King.

Black and Red 14AWG Silicone Wire

Finally, we will also need 1 meter of red and black 10AWG wire which also will be purchased from Hobby King.

Black and Red 10AWG Silicone Wire

 

22 Volt Electrical System Component Drawings
Part/Assembly PDF STEP
4 mm Female Bullet 4 mm Female Bullet.pdf 4 mm Female Bullet.stp
4 mm Male Bullet 4 mm Male Bullet.pdf 4 mm Male Bullet.stp
5.5 mm Female Bullet 5.5 mm Female Bullet.pdf 5.5 mm Female Bullet.stp
5.5 mm Male Bullet 5.5 mm Male Bullet.pdf 5.5 mm Male Bullet.stp
5.5 mm 2F/1M Splitter 5.5 mm 2F/1M Splitter.pdf 5.5 mm 2F/1M Splitter.stp
5.5 mm 2M/1F Splitter 5.5 mm 2M/1F Splitter.pdf 5.5 mm 2M/1F Splitter.stp
Battery Plate Battery Plate.pdf Battery Plate.stp
Battery Strap Battery Strap.pdf Battery Strap.stp
Battery Disconnect Battery Disconnect.pdf Battery Disconnect.stp
EC-5 Connector Male EC-5 Connector Male.pdf EC-5 Connector Male.stp
EC-5 Connector Female EC-5 Connector Female.pdf EC-5 Connector Female.stp
T60A ESC T60A ESC.pdf T60A ESC.stp
Turnigy 8000 mAh Turnigy 8000 mAh.pdf Turnigy 8000 mAh.stp
U7 Motor U7 Motor.pdf U7 Motor.stp

 

 

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