Andrew B. Wright, Ph. D., SM ’88
I am ambitious in my mechanical design efforts at this point. I have a 3D printer. I have Adafruit’s and Pololu’s electronic modules for small robotics. So, I’m going all in on designing a battery pack that does all the things I want a battery pack to do.
Because I can.
- supply regulated power to the motors (x4), the controller, and the sensors
- provide protection against excessive current draw
- allow for easy, flexible recharging
- provide the user with a method of instantly dropping power
- “look marvelous”
I went through many iterations, and I’m probably not done.
One of the first iterations is located on thingiverse.com. I plan to include all the final design information for this battery pack on that site once I’ve completed this project.
There are two stages in this design, the battery pack for the Beaglebone alone and the battery packs for the motor drivers. I decided to split the task because the power requirements for the motors is substantially bigger than for the Beaglebone. If the motors were to exceed the current draw, it might reset the Beaglebone during operation.
Further, by separating the design into three pieces, the complexity of each piece is dramatically reduced. The motor battery packs are duplicates of each other, so, one design can be replicated.
Battery Pack Design Process
Motor Controller Design Process
Power is the heart of any mechanical device. In mobile devices, this power is supplied by batteries.
I have used four of the Vexrobotics 393 motors, which can continuously draw 3 A at 7.2 V, for a power of 4*7.2*3 = 86.4 W.
Using 8 NIMH rechargeable batteries in series would give a supply voltage of 8 * 1.2 V = 9.6 V. If the batteries are rated at 2 Ah, they could produce 9.6 * 2 = 19.2 W for an hour. Not a good match.
This robot could run for only a few minutes at maximum continuous power, and it would pound the batteries into oblivion in a few cycles.
Increasing the batteries to 12 and wiring them in banks of six series and two parallel gives a voltage of 7.2 V and a capacity of 4 Ah. The continuous power is now 28.8 W for an hour.
Splitting the job into two sides allows the design to be replicated on each side, and drops the current through the power system in half.
Each battery weighs about 30 g, for a total weight of 24 * .03 kg = 0.72 kg. Not bad at all. The continuous current of six amps (3 A per motor) is a little higher than the desired maximum of 5 A for the fuse. But, it can be made to work.
What can you do with 100 W?
100 kg in a standard gravity yields about 1000 N. Lifting this mass through a height of 10 m requires 10000 J. If this is done in 10000/100 = 100 s, the power required would be 100 W. In other words, this little power pack could lift a 200 lb person up 3 stories in about 2 minutes.
|‘AA’ Male Battery Contact||Keystone Electronics Corp.||5221||1||Mouser|
|‘AA’ Female Battery Contact||Keystone Electronics Corp.||5222||Mouser|
|‘AA’ Dual M/F Battery Contact||Keystone Electronics Corp.||5212||Mouser|
|‘AA’ Rechargeable Batteries (nimh)||various||3||various|
|Low Profile Universal PC Auto Fuse Clip||Keystone Electronics Corp.||3557||2||Mouser|
|1200 uF capacitors||various||2||various|
|Voltage regulator (low power)||Adafruit||Powerboost 1000 basic||1||Adafruit|
|Voltage regulator (high power)||Texas Instruments||PTN78020WAH||2||Mouser|
|Power switch (low power)||Mountain Switch||107-1261B||1||Mouser|
|Power switch (high power)||Schurter||1241.6831.1120000||2||Mouser|
|Power connector||uncertain||2.1 mm barrel connector (3310)||1||Adafruit|
|JST power connector||uncertain||1||Adafruit|
|Terminal block||uncertain||2 pin terminal block (724)||1||Adafruit|
|Blade fuse||Bussman||amperage TBD||2||Mouser|
|Wire wrap tool||Jonard||JIC-22681||1||Mouser|