White Paper - Spaghetti? In my battery?
A centralized BMS can add a whole lot of wires to your batteries
Normally, Li_ion Battery Management Systems (BMS) sample the battery voltage at every single cell. Most of them use centralized electronics, with a wire between it and each connection between cells. That is fine for a small pack (think "laptop battery"). But it can become problematic in a large battery pack. For example, a battery with 100 cells in series must have 101 wires running between the cells and a centralized BMS. All those wires can be hard to route. Worse, the more wires there are in a battery, the greater the risk that one of them will ease the path for a plasma discharge between two end of the battery.
One wire per cell in the BMS for a Prius BREM.
BMS is box on foreground, right. Batteries are in 2 gray boxes.
A few BMS use distributed electronics (Cell Boards), mounted directly on the cells, but each board has a cable between it and the centralized BMS controller, so the problem is actually compounded.
Even fewer BMS (including Elithion's) also use distributed electronics, mounted on the cells; however, they use a single communication cable daisy chained between adjacent cells. This results in having only one or two cables between the centralized controller and the Cell Boards: the Spaghetti is gone!
Just like christmas tree lights of yore would turn off if a single light bulb burned up, a daisy chain system could result in an entire BMS not working if any Cell Board quits. For that reason, even in decentralized systems, it is recommended that, instead of a being wired is single long string, the Cell Boards should be electrically divided into a few, smaller banks.
Here is a summary of some possible distribution of BMS electronics in a battery:
Depending on the application, Elithion produces either one of the last two topologies, though it recommends the "Distributed, banked" topology as a good compromise between simplicity and improved reliability.
"Spaghetti? In my battery?" by Davide Andrea is licensed under a Creative Commons Attribution-Share Alike 3.0 Unported License. Permissions beyond the scope of this license may be available by contacting the author.
Davide Andrea, Elithion, 9/17/08