Miniature Lithium Polymer Battery

On the prior page, we looked at some of the virtues and problems with stacked PCBs. The cube certainly provides an interesting look for the head of a robot.

That leaves us with the issue of a power source. With the exception of my solar robots, most have a substantial battery pack of at least two cells. The need for a higher (multi-cell) voltage is usually because the robot uses field-effect transistors for motors drivers, which is not an issue with the Afterthought Cake robot.

An obvious exception is Monkey Mints, which required the power source to be small enough to fit in a candy container. I decided to use the same single-cell lithium battery as that robot, as larger batteries would take away from the appearance of Afterthought Cake.

Small Ultralife lithium polymer battery wrapped in black masking tape for strain relief

Small Ultralife lithium polymer battery wrapped in black masking tape for strain relief

The battery suffers from thin wires that can break off easily where they meet the battery. That may be one reason the battery is obsolete. I can’t find an exact replacement (20 mm x 30 mm x 4.8 mm producing 3.7 V for 180 mAh), although even smaller batteries are available at SparkFun (PRT-00731, PRT-09142).

I replaced the delicate battery wires with thicker gauge and then looped them back over the battery body. Black tape holds the wires in place to prevent damage due to flexing.

Lithium polymer battery nestled between PCBs

Lithium polymer battery nestled between PCBs

The battery fits in between PCB layers. The battery can’t come out in the direction of the Molex connector, as the connector blocks that direction. It can’t accidently come out the other direction because the wires don’t reach that far when attached to the connector.

The battery isn’t loose -- the fit is fairly firm. Avoid using battery wires as the primary method of retention.

Voltage Regulation and Power Usage

The lithium cell provides a voltage from about 3.4 V to 4.1 V. Anything lower than 3.4 volts means less than 10% of the power remains.

That voltage range is appropriate for the Atmel ATtiny861 microcontroller, the escap MU916L motors, the bipolar transistor motor driver, and the color LED. The voltage is more than enough for the trimpot, pushbutton, and infrared emitter/detector. Therefore, there was no reason to install a voltage regulator in this robot.

Had the voltage been greater than 6 V, or had the voltage varied enough that a constant known level would be advantageous, then an MCP1702 could have been installed in the holes ready-made for it in the motherboard PCB. However, the voltage regulator would have consumed additional energy and would have limited the current draw to the motors.

As it stands, the current usage is as follows:

  1. Color LED (20 mA)
  2. Motors (15 mA each when moving steadily on a tabletop)
  3. Infrared LED (13 mA)
  4. Microcontroller (6 mA @ 8 MHz)
  5. Motor driver (3 mA each when moving)
  6. Trimpot (0.4 mA)

Even the tiny 180 mA battery should last more than an hour of robotic operation.


This robot provided many hours of fun in experimenting with different parts, arrangements, and alterations. It also caused hours of frustration with motor driver challenges and machining thin metal.

Due to lack of available microcontroller pins, the robot lacks a purpose other than to showcase a few demonstration movements. To free up some pins, I could multiplex the pushbutton and eliminate the transmit (TX) pin used for serial communication to support debugging. Removing the color LED would immediately free three pins.

But, that was never really the point of Afterthought Cake. I am satisfied in creating a novel-looking robot that works.