Friday, February 20, 2009

Charging issues

I made another long trip 26+ miles but it hasn't been all fun. I'm still working on a good cell balancing solution and now I've realized I have some charging issues.

I noticed that when I try to crank up my charging amps (above 6 or 7 amps) the BMS starts freaking out and misreporting cell voltages. Many just go to zero and the BMS triggers for the charger to shutdown.Two things will fix this. First, remove the serial cable from the charger to the BMS. I can turn my amps up to max and the BMS happily reports all cell voltages but of course I now have no protection from over voltage and other alerts. The charger will only charge based off full voltage. The second thing that got it working was completely removing the charger from the trunk and setting it on the garage floor. Now I can leave the serial cable plugged in and it all works fine. Put it all back in the trunk and it's crazytown. It seems I'm getting some kind of EMI going through the serial cable to the BMS which is causing problems but not sure why only in the car.

I didn't notice any of this before because I wanted to start out charging the pack at low levels. I need to be able to charge at much higher rates when making long trips (like going to work) or I might have to sleep over. Oh well all part of being the the few first to do something like this.

This is now fixed. I had to completely isolate the charger from the car ground. I used some foam stripping that is sticky on one side and placed that between the charger feet and the rack it mounts to. I then placed rubber grommets in the holes on the charger feet so the bolts couldn't touch metal on the charger. Last I used rubber washers between the charger feet and the metal washers. Basically just make sure no metal on the charger touched the car metal. I'm now able to charge at 24 amps!


Jeff Dinges said...

bummer, maybe you'll need some advanced software to analyze each cell for proper charging and proper usage during driving.

Are all the cells being discharged equally?

Brian said...

I hope to get this system I already have figured out. Can't afford another one at this point.

I believe they are discharging evenly, however, since they've never been truely balanced (yet, since I'm still building the balancing circuit) it's hard to say how evenly.

Philip said...

the BMS does the balancing and switches the charger on/off with the serial data link? Could it be that the charger is not properly connected to the cars earth when in the trunk? Or not properly isolated? Since it seems to work out of trunk?

By the way, in some older post you report on having different voltage run length from the front and rear cells to the BMS. Have you checked them for voltage drop, which could interfere with correct balancing?

Brian said...

This BMS doesn't balance, it's more of a monitoring system than management system. It does notify the charger of over temp, over current, under voltage and over voltage.

I'll be creating my own cell balancers which will be on each cell so that voltage drop won't be an issue. They will simply shunt the energy when the cell reaches a certain voltage given the other cells a chance to catch up.

The charger was originally mounted to the rear battery rack which is attached to the car so grounded. I then placed it in the trunk and tried to make sure nothing could touch it except the carpet in the trunk and the problem was still there.

Philip said...

thanks for the update.
It is again and again interesting what a complex electric system is willing to throw at you.
Somehow reminds me of the "magic switch" story.