There are certain things you expect when you're building drones for photomapping and journalism. First, you expect some setbacks. Perhaps a crash or two, or at least a few broken props. At worst, you expect a drone to take a fatal nosedive into a field and break into a hundred pieces, never to fly again.
There is a learning curve to this stuff. But you don't expect your drone to go haywire and burst into flames while you're working on it.
Last month, I was busy preparing an electric-powered drone in my basement for a maiden flight. With a wingspan of over 5 feet, and weighing a little over 7 pounds, it was the largest drone I've worked on yet, and it had a decent-sized power source to match.
Much larger drones have flown on the same basic technology, with power sources of twice the capacity used here. Most of our development to this point has focused on battery-powered drones instead of methanol-powered drones, because we want to keep the risk of fire down (even though fuel fires are rare). But that doesn't meant that batteries can't catch fire.
For my drones, I use lithium polymer batteries, or "LiPo," and they're pretty advanced as far as battery technology goes. They run today's electric cars -- the Leafs, the Teslas, the Fiskers and Volts. If you are reading this on a smartphone, you can thank a lithium battery.
Most other cells are contained in cylinders, but lithium polymer cells come in individual pouches. LiPo batteries are packs of lithium polymer cells that have been bound and tightly wrapped together. What really sets LiPo batteries apart, however, is the amount of energy they can store.
Whenever you're storing a great deal of energy in a compact space, and you suddenly release all that energy, you're liable to create tremendous heat. Since LiPos hold a lot of energy, under the right conditions,
they can also catch fire.
According to the instructions of the original balsa plane I was hacking into an autonomous drone, the motor and its speed controller required 5 LiPo cells. I did not have a five-cell LiPo pack. I did have two 3-cell packs, and one 4-cell pack (which is quite large). Thanks to fuzzy math, I somehow thought it was safe to use the 4-cell pack and a 3-cell pack, for a total of 7 cells.
Everything seemed OK at first. The motor whirred happily during testing. Then I took the drone back to the basement to finish mounting and calibrating the autopilot, and things got weird.
