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:

Fully centralized	Central with slaves	Distributed, banked	Fully distributed
A single board, wires throughout the battery (spaghetti)	A single controller, connected to a few slave boards distributed at various locations, each with wires going the cells (still spaghetti)	Cell Boards mounted on cells, connected together in a daisy chain, broken into a few banks; central controller with a cable going to each bank	Cell Boards mounted on cells, connected together in a single daisy chain; central controller with one cable going to the first Cell Board (christmas tree)

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