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Twitch - Power

Twitch is a battery operated vehicle with a relatively straightforward power system. This was my first project using lithium batteries, and as is tradition, I burnt my house down bricked a few. My current pack is the KyPOM 3300 mAh 3S1P Lipo pack. This pack is rated to 35C continuous, which translates to 115A, more than my motor controllers might ask of it.

Power electronics schematic

Two pairs of 16AWG wires connect the master switch and the main current hogs of the system: my Pololu TReX motor controllers. These are DC motor controllers with two bidirectional channels, capable of 13A continuous per motor. The 16 AWG wire was too thick to comfortably fit into the screw terminals of the motor controllers, confirming my suspicion that I have no clue how to properly determine the ampacity of a wire. Oh well, I suppose too thick is better than to thin.

From the same lines that feed the motor controllers I spliced in leads to power my Battery Elimination Circuits (BECs). These circuits supply power to my servos, one for each box. They are necessary since I can't run the servos from my Arduino (they draw up to ~3A each), nor can I power them straight from the battery pack (input voltage of 4.8-6V). One of the BECs performs double duty as the power supply for my Arduino. This is somewhat suspicious practice. When the servos are moving, the voltage provided by the BEC will sag somewhat. If the sag drops sufficiently low, the microcontroller can lose power and reset, which obviously isn't good. To combat this I added a monster 3300uF capacitor just before the Arduino voltage input. The cap helps fight the voltage sag and has kept me out of microcontroller trouble so far.

Battery life wasn't a big concern in this build since the bot serves no practical purpose. At 3300mAh and a peak current draw of 4*13A = 52A, the bot can run for about 4 minutes flat out. Of course I'm never running close to that consumption, and have found in practice that 15-20 minutes of play between charges is more typical.