Zach’s Electric Diablo

Most of the time I really enjoy working on my car.  However, sometimes those not-so-exciting jobs need to get done.  Cutting and punching all the battery straps is exactly that kind of job…

On a more exciting front, I figured out a good fix for my low-voltage battery monitor problem.  If you’re interested, scroll down two posts or just click here.

High power battery chargers and motor controllers are electrically noisy. I found this out first hand when I turned on my battery charger for the first time. All my battery monitor boards went crazy. I put an oscilloscope across one of the batteries and saw huge amounts of high frequency noise.

This was a problem. Fortunately, there were some toroids laying around my office.

After some experimenting it turned out that a combination of three chokes (one wired to block common mode) brought the noise down to a tolerable level.

While the batteries were charging, I took a few minutes to figure out the whole strapping issue. I ended up using some stainless steel strapping.

Having long unwieldy strips of metal flopping around exposed battery terminals is a recipe for disaster. So, I wrapped them in electrical tape. I drilled some holes in the strap and tensioned it using a 1/4″-20 nut/bolt . Super easy and clean.

I spent tonight writing software for the battery monitors.

Warning! Warning! Super technical discussion ahead. If you are put off by such talk, turn back now!!!

For the most part everything seems to be working well. There is only one outstanding issue. I designed the circuit with a Schottky diode on the positive battery rail. This diode serves a couple purposes. First, it protects the circuit if its polarity is reversed. Second, it helps with noise immunity by allowing the 100uf filter capacitor to power the circuit during brief power glitches on the battery rails (I am not sure this will be needed, only driving will tell). The problem with the diode, is the fact that it is a diode. And like all diodes, it has a voltage drop. In this case .4v. The batteries are basically dead at 2.5v and the processor is good to 2v. So, it is not a problem for the processor. However, the output pins on the micro seem to output at a voltage roughly 1.1v less then the supply. The problem shows itself when I send data out of the processor through the opto-isolator (connected to the UART for communication). The opto wants a minimum of 1.5v. This means that with .4v+1.1v worth of voltage drop, my comms start dropping out if the battery voltage is below 3v. I am trying to think of options. I could ditch the diode, but that might come back to haunt me. What to do?

Update: I should have done a little more upfront testing… If so, I might have caught the fact that my PIC was sourcing current to the opto-isolator instead of sinking current. (Thanks to Fran for pointing this out). Unfortunatly, there isn’t a clean way to implement this fix without major board modifications  (and I sure as heck don’t want to re-make all 50 pcbs). So, instead I decided to add a 6 cent transistor to boost the voltage between the output pin of the chip and the opto-isolator. There is a very convenient place to solder it that only requires a couple scratched traces. Now I just need to wait for that order of 50 transistors to arrive.

The mail man dropped off my BMS printed circuit boards yesterday.

I did a quick test fit to make sure they fit on my batteries:

I first built one, and tested it:

It worked perfectly. Here are the features it has:

1. Measures battery voltage
2. Measures temperature
3. Has a 3W 10 ohm resistor that can be programmed to shunt excess current during a charge cycle (balances the cells)
4. Talks over an opto-isolated serial bus.
5. Has a programmable RGB LED (used as a diagnostic tool and for adding some “coolness” to an otherwise boring battery pack)

Then I built 10. (This took about two hours)

Only 40 more to go…

Today, I sent off my Battery Monitoring System (BMS) circuit to the fabricators. If all goes well, I should have 50 of these boards in my hands by Friday.

I also mounted the battery box (which is now powder coated) to the chassis and bolted on the power electronics (the charger, controller, main fuse, and both contactors):

The next step is to install the batteries and start wiring everything up.  I want to make everything really neat and clean, so I will be taking my time.