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Old 07-16-2012, 01:44 PM   #15 (permalink)
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Join Date: Dec 2007
Location: Germantown, WI
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So, I've been working on the finishing things up before the BMS goes into the car. Lately, most of the work has been in trying to increase the accuracy of the current sensor. To test the sensor's accuracy I simply wired it up and set it on my desk with no wire or current flowing through it. I count the Ah and spit out out via the serial print command.

Leaving it uncalibrated showed unacceptable accuracy. After only 10 minutes of testing the sensor showed .61Ah had been used. This gives us an inaccuracy of .06Ah/minute. On a 60 minute trip the measurement would be off by almost 3.7Ah. If you only have a 40Ah pack you're looking at nearly a 9% inaccruacy which means you should reduce the usable capacity of the battery pack by 9% to compensate. That is a big chunk of SOC to loose.

Manual calibration has been somewhat more successful. My best result was a .009Ah/minute variation over a 1.5 hr test period. This means that after 60 minutes the amp hour reading would be .54Ah off. Again, considering the 40Ah capacity, we're talking a little over 1% inaccuracy which should mean that you loose 1% capacity to avoid damaging the batteries.

This is much better, however, manual calibration is a pain in the butt. Every current sensor would need to be calibrated and this should be able to be done automatically by the software. That is what I am currently working on. I should be able to take a voltage reading from the current sensor with no current passing through it and set that as my zero. So far I haven't been able to get it to work. I think I am having some power ripple issues. Once I figure out if/where they are, I should be able to smooth them out and get more consistent readings.
Current project: A better alternator delete
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