Monday, November 4, 2013

BMS Testing and Troubleshooting

Wow, it's been a long time since the last post, though there isn't that much I've been doing other than driving the car and enjoying the nice weather.  I've racked up 870 miles so far and still going well!

A while back my 12v battery started to show signs of age.  Before I hooked up the DC-DC converter, I had to charge it up with a regular automotive charger every night.  I noticed the voltage was getting low even after just one day of driving, then when I hooked up the converter, I realized it was causing a much higher load on the main battery pack than I was expecting.  Just to refresh you, the DC-DC converter takes power from my big 230 volt pack and turns it into 13.9 volts to run all the original electronics on the car.  Power in equals power out, so when my gauges told me that 5.2 amps at 230 volts was going in, that seemed really high.  In fact, that equates to 77 amps, so something's definitely not right (230 x 5.2 / 13.9 x 90% efficiency = 77).  I checked the actual current coming out with a clamp meter and saw 45 amps going into the battery and only 16.5 amps coming out.  After the battery charges up a bit, that drops off, but it seems to always want to take at least 10 amps to float at 13.9 volts.  The biggest problem with this is it seriously eats into my efficiency.  4 amps or so turns into 4 Ah at the end of the day, which is about 20% more than I was using before I hooked up the DC-DC converter.  Time for a new battery.

The battery in all gas powered cars needs to be big enough to pump out 600 amps or so to get the engine started.  I don't need that, so I saved some weight with a little 18 Ah AGM deep cycle battery.  That should be enough to drive for 45 minutes normally on just the battery, and around half that with the headlights on in case of some sort of problem.  And here it is!  Isn't it cute?

Here's the original for comparison.

After hooking it up, the current draw is down to 18.5 amps in, 16.2 amps out, which seems about right.  From the HV pack it's around 2.1 amps, and today on my first drive with the new battery I saw a 3 Ah improvement in my power consumption.  My next efficiency improvement will be a vacuum switch that will shut off the pump 95% of the time.  That should cut the current draw to about 0.7 amps and I should be pretty close to the original efficiency I was enjoying this summer.

So the whole purpose of this post was to talk about my BMS and I haven't even got there yet.  Sheesh, I can't believe you're still reading.  Another quick refresher on the BMS is that it keeps track of the voltage of all the batteries and makes sure they stay between about 2.60 to 3.65 volts.  I started designing the system about 8 months ago, got the wires in place about 5 months ago, finished the circuit boards 4 months ago, and finally got it all hooked up about 2 weeks ago.  I came across a post from back in June where I was optimistic it'd be working within a few weeks, ha!

Apparently I'm way behind on showing you this stuff, so here's a quick summary of where I've come.  Here's the master and 3 slave boards all hooked up to the LCD.  I'm testing a 3 cell battery on it.

Here I've got the master and one slave hooked up to the all the cells under the hood.  They're all out of calibration right now.

Here's what I spent a few nights working on, it's the code for the system.  The master has it's own code, then the 9 microprocessors on the slaves (3 each) have their own.  Unfortunately, due to a number of reasons, the #6 and #9 micros have a slightly modified code from the others.

Like I said, I got it all hooked up a couple weeks back and started to test it out.  Initially I couldn't get it to work at all, but I soon realized that in the process of modifying and cleaning up Joe's code (his isn't quite the same hardware as mine), I made it a little "too clean" and it didn't work.  I almost forgot, that since I don't have a laptop I had to drag a spare desktop computer out and set it up on a table next to the car in order to program everything!  And pretty soon it got really annoying after doing it for the 5th evening in a row, but now it's all working and calibrated...well, sort of.

I works great when the car isn't on, but when you start up the motor there's so much electronic noise being emitted that the signal is totally messed up.  I've got shielded wire, but that doesn't seem to be helping enough.  Joe lent me his oscilloscope and I tried to find the problem.

Here's what the signal looks like when everything is working fine.  Not super clean, but clean enough.

Here's what it looks like when the motor is running.  You can't even tell it's a square wave.

And here it is with the charger running.  Definitely a square wave, but kind of dirty looking, which must be confusing the master board.  A simple filter should fix this.

I hooked up a 4.7 uF capacitor to the master bus line and it didn't seem to do anything, so I bumped it up to 47 uF and then 100 uF.  Then I took another look at the schematic and realized I hooked them up in the wrong spot...doh!  I fixed the connections on the cap and got a flat line...too much capacitance.  I backed off to 4.7 uF and this is what I get.  You can see the wave never actually gets up to 5 volts even with a long space in between the pulses.  Still too much capacitance.

Here I've backed off to 0.1 uF and I still have the shark fins running across the screen.

Unfortunately, the next size smaller capacitor I have is 20 pF, which is 4 orders of magnitude smaller than the last one.  Here you can see it's not enough capacitance because it doesn't look much different than before.

Tomorrow I'll have to stop in to radio shack to find a few caps to fill the gap in my hoard.  I'm doubtful my $1.30 purchase will be very exciting for them, and they probably won't know what a capacitor is even though they'll insist on helping me, but somehow they'll still stay in business.  Well, that's enough rambling for one evening.  I'll try not to go 6 weeks before the next one!


  1. Did you bottom balance your cells originally?


  2. Short answer: Nope, top balance.

    Long answer; I haven't been discharging them more than 30-50%, so the risk of killing a cell is minimal. Once the BMS is working properly I'd be able to drive further and really test the range. However, the original plan was to have the BMS working within a month of getting the car on the road, and it's been about 4-5 months now! I've been keeping pretty good tabs on the cell voltages, but it's not foolproof and certainly not very convenient since it takes a lot of time to check them all manually. The BMS has been working for a couple weeks in a limited capacity, so I can now easily check all the cells every time I take my foot off the pedal. Of course, that doesn't cover charging, and in fact just last night I bled off a little charge from two cells that are starting to rise above the rest at the end of charge. Long story longer, I feel I've mitigated the risk of damaging the batteries, but I get more nervous every day and am ready to get this thing working finally.

  3. I bottom balanced mine to 2.65 volts with a Powerlab 8 and just finished the first charge to 3.65 volts/cell average. (138.8 volts total pack voltage for the 38, 100 ah cells) Now I am discharging the pack so I can check them all as they approach 2.65 volts again to see if they are staying in balance at the bottom. When I charged, none of my cells went over 3.7 volts and the lowest were no less than 3.6 volts. I have talked to several people who have accidentally discharged their pack to almost 0 volts and have brought the pack back with no apparent damage. I have a small Ballistic lithium 12 volt battery that was bottom balanced and ran down to almost zero, from a parasitic load when installed in the car. It came back just fine when charged back up. I talked to Gary Bulmer,, at EVCCON and he had been using an Elithion BMS. He was having problems losing cells, so he took the BMS off, bottom balanced and has had no problems since.


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