Beaglebone: Battery Pack

Andrew B. Wright, Ph. D., SM ’88

9/20/2018

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.

Functional Requirements:

  • 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.

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.

Using a pololu 2890 https://www.pololu.com/product/2890 up converter on each side allows all power requirements to be met. (I went with a TI voltage regulator.)

Part Manufacturer Part Number Quantity Supplier
‘AA’ Male Battery Contact Keystone Electronics Corp. 5221 1 Mouser
‘AA’ Female Battery Contact Keystone Electronics Corp. 5222 1 Mouser
‘AA’ Dual M/F Battery Contact Keystone Electronics Corp. 5212 7 Mouser
‘AA’ Rechargeable Batteries (nimh) Energizer tbd 8 Mouser
Low Profile Universal PC Auto Fuse Clip Keystone Electronics 3557 2 Mouser
5A automotive blade fuse Bussman-Eaton BK/ATC-5 1 Mouser
E-Stop Switch (big red button) – 5A EAO 51-256.026 1 Mouser
1200 uF capacitors various various 2 Mouser
motor controller Pololu 1213 2 Pololu
power supply tbd tbd tbd Adafruit
power supply tbd tbd tbd Adafruit
power supply tbd tbd tbd Adafruit
power supply tbd tbd tbd Adafruit
power supply tbd tbd tbd Adafruit
Posted in: Robotics

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