Batteries...

With the bulk of the mechanical work done, it's time to start thinking about the electrics. I've chosen the Lithium Iron Phosphate batteries from Thunder Sky, which don't catch fire like some lithium batteries do when overcharged. Nevertheless, the batteries can be damaged if they are overcharged or overdischarged. In a long string of cells wired in series (I'm using 96) some cells are inevitably stronger than others, and will end up being overcharged. The string voltage remains constant, so this means that some cells get severely discharged (even negatively charged). Both of these destroy the affected cell, so I need a way to keep the voltage of each cell between 2.5 volts and 4.25 volts. There are several ways of doing this. Charging each cell individually, then monitoring all the cells for undervoltage during discharge is the best way, but constructing a charging system to individually charge 96 cells, then connect them all in series to the drive circuitry would be a difficult design challenge, as well as a wiring nightmare.

Instead, I'm taking a simpler approach. Each cell will have a shunt regulator across its terminals. A shunt regulator is a device that conducts no current until the voltage across it reaches a set value. When the voltage rises above the set value, the device begins to conduct just enough current to maintain the voltage at the set point. This takes care of the overvoltage protection. Undervoltage is a bit trickier. I think I've come up with a clever way to signal the controller to shut off when the voltage of any cell drops below the setpoint using a single circuit (instead of 94), but I need to develop it a bit first.

The idea of building 96 circuit boards is a bit daunting though. Each board will have ~10-15 components and cost about $5 for parts. It only adds about 5% to the cost of the battery system, so is well worth it, but spending my evenings hunched over a soldering table doesn't sound like fun. Maybe I will send it out for assembly... with 100 pieces, the prices should be pretty good. I just need to prototype one or two to get everything right.