Monday, November 8, 2010

Battery Management System

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.

--Side Note--

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

--/Side Note--

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.


  1. Hi,

    Read through your entire blog and your experience. Fascinating work. Came across your blog b/c I want to do the same as my dream car since 16 has been the BMW 2002 tii. If I convert, that won't matter as much :) I just finished and about to sell my biodiesel run 1987 Mercedes 300D Turbo. Looking fwd to this project.

    Now if you had to redo it again, and based on what you've learned about conversions, would you do it the same? Or is there an alternate approach you would take?

    I think most people including myself do not have the technical/software experience that you do, thus making the control issue a challenge. But great to see you resolved/resolving it.

    Any future updates?

    Much compliments,

  2. Bob,

    I'm heading off to work, but I'll post a reply tonight.

  3. Bob,
    I think that if I did it again, I would probably do a standard conversion instead of trying to make a 4 wheel drive car.

    Most of my expense was in converting the wheels to direct drive. Then, I could have spent more on larger batteries and a more powerful motor. I could have kept the transmission and then my torque issue wouldn't have existed.

    As far as updates, I've been working on my battery management system. I wasn't happy with the first try so I redesign it with different ICs and I'm currently putting together the first prototype boards. I started on the software, but that has quite a bit more work to go.

  4. Incredible amount of work. I love checking in with your project from time to time and wish you the best success. Please keep writing.

  5. In order to maximize the battery's capacity, and to prevent localized under-charging or over-charging, the BMS may actively ensure that all the cells that compose the battery are kept at the same State Of Charge, through balancing.

  6. All the Battery Management System models allow for charge and discharge control of every single cell, cell-temperature control and cell balancing of each single cell in charge, discharge and idle mode.

  7. Any updates? I've been learning about battery management over the years by reading about peoples experiences. How did yours turn out?

  8. Hi, name's Andre'. Love what you've done here. I myself was moreover interested in your one off RWD idea. I drive a 2006 Scion tC and have been wanting to make it electric RWD for track days with the various Scion FR-S and Subaru BRZ crews. Any thoughts from you on the matter would be greatly appreciated and feel free to email me directly if need be. Once again, great project build and I keep my eyes open for any details/updates in the future.

  9. Andre, glad you liked it. I had a gearbox failure on the rear drive system, so I have been reworking it for a couple of years (darn job is getting in the way). Basically, what happened was the coupling between the large sprocket and the flange connecting to the half shaft wasn't strong enough. So after driving it for a couple of months, the pockets holding the hardened steel pins, rounded off and the two halves wouldn't stay rigid.