Since I started driving the car, I have had two cells die. I'm not sure what the cause of the failure is, but the cell goes to 0V but still conducts current. Obviously, the cathode and the anode are being shorted together internally but I'm not sure if it is something I am doing by stressing the cells or if it is a manufacturing defect. The cell doesn't explode or do anything nasty, it just doesn't work anymore.
As far as terminology goes, I have found myself standardizing on what I call the battery parts when I'm explaining my project to people. In the interests of staying consistent:
Cell - 1 3.2V LiFePO4 60AH cell
Battery - 24 cells connected in series = 72V (really 80V) nominal 60AH battery
Pack - 4 Batteries connected in parallel = 72V nominal 240AH pack
So, with the idea that I need more information, I started working on my BMS. There are some good ones out there right now, but for something as simple as measuring the voltage of each cell, keeping track of cell performance, and alerting when there is a problem, most of the stuff is in the $15/cell range. So for my pack, that is nearly $1500 bucks. In the interests of saving money (yea, right!) I decided to build my own.
I found a cool IC from Linear Technology: the LTC6802. This chip is a stackable, 12 cell battery monitor. It was designed to work with series cells up to 1000V so my 72V pack is nothing. But the nice thing from my perspective is that two of these guys on one circuit board will monitor one of my four batteries.
In my previous post about my year end progress, I posted a picture of the circuit board I designed for this BMS. And here it is in real life:
It doesn't look like much in the photo, but SMD soldering is something new to me and it is harder than I thought. Thanks to YouTube, I found some great videos of how to do it, but those guys make it seem easy. One lesson I learned: if you are going to hand solder SMD parts, don't use the 0402 stuff. They are TINY!!
The next step is going to be interfacing one of my Netburner MOD5213 microcontrollers to the 6802s via the SPI bus (more stuff to learn). Hopefully it won't take me too long but it probably will.