Once we had our first powered test drive (hooray!) It was time to address the most obvious problem it unveiled: the power supply. The test had used a small 2-cell lithium polymer battery pack that was over ten years old. Originally purchased for small lightweight remote-control aircraft, its capacity has diminished over time as lithium chemistry battery cells tend to do. Sawppy will need a higher capacity battery pack.

Remote control aircraft battery packs like the old unit on hand tend to be small, lightweight, and correspondingly expensive. Fortunately there is another remote control hobby niche that is less weight-sensitive and has power consumption profile nearly identical to Sawppy: remote-control off-road vehicles. The 1/10th scale monster truck category seems to have standardized on using a pair of 2-cell lithium packs so that’s what I bought.

A few accessories were purchased alongside the battery. First a pack of power switches so Sawppy can be turned on and off more elegantly than reaching in and unplugging the battery. To protect the battery, we’ll want an always-on and visible volt meter to monitor its voltage. To protect the rover, we’ll add a fuse holder in line with the battery and a variety pack of fuses. The battery pack is advertised to deliver up to 30C which is far more than any Sawppy component would want (or can tolerate.) Hence a fuse to cut power if there should be an unfortunate short-circuit.

Once all the parts were on hand, the next decision is where to put them all. Since Sawppy is intended to be a mechanically faithful model of actual Mars rovers, the answer is obvious: we’ll put our power system in the back, sticking out at an angle just like real rovers have their radioisotope thermoelectric generator (RTG). Sometime down the line we might print a cosmetic cover that makes it look more like the actual RTG, but for now we’ll settle for matching its position and orientation.

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