Sunday, August 26, 2007

PakTrakr is Online! DC-DC blown again...

I spent most of yesterday and a few hours today wiring up the PakTrakr system. It's a bit of a pain to disconnect the most negative lead before attaching the display, but that's minor nuisance. It seems to measure pack voltage well; however, the voltage measurement on each battery seems to be slightly inaccurate. The Zivan charger is balancing the batteries well and each of them is within an 8.48V-8.50V range (20mv difference is pretty good!).

I chose to mount the display on the central console as shown above. It doesn't have a backlight but is easy to read during the daytime and the placement makes the buttons easy to access.

I had to greatly extend one of the PakTrakr remote module cables so that the rear battery pack could talk to the remote modules in the front of the car. That involved cutting one of the 3-wire cables on one remote module and splicing in an 8-foot extension of additional 3-wire cable. I added heat-shrink tubing to make the splices clean so they could be pulled through the right-hand heater duct under the passenger-side door.

The system seems to work fine. Having a backlight would be nice, but not necessary. My biggest gripe is that double sided tape seems to have a big problem sticking to the back of the PakTrakr modules, even though they tell you to use it to mount the modules.

On a downer note, the CCPower DC-DC converter blew again (fuse was vaporized and FETs shorted out). I actually caught the system in a working condition just before driving home and found out that it had blown when I got home. The only difference was that the DC-DC converter was under a heavy load on the drive home because the headlights were on.

Based on the circuit inside the DC-DC converter, I suspect that there is a significant amount of current (required from the headlights) going through 144V side. When the FET opens, the current rushing through the inductor causes a high voltage spike (300-400V is not unheard of) that gets added to the +144V side and just wipes out the 500V FET. I'll have to double check to see if the inductive kick-back diode is properly functioning. The CCPower website only has the input voltage of their DC-DC converters listed as 80 volts so, although these are designed for EVs, the underlying design may not be able to handle the higher voltage, especially with spikes from regenerative braking.

In the meantime, I'm using a 12V charger to charge the accessory battery as well. Maybe I can integrate that into the overall charging circuitry when I plug in the car.


pjorg said...


Are you sure there isn't a short in the 12 vdc system somewhere?

Roger Daisley said...

When I spoke to Mike Brown and mentioned the spec'd max. input of 80v, he told me they had contracted with CC Power for a special unit, good for up to 144v. I dropped the idea that perhaps CCP had shipped the wrong units.

Since I could find no placard on the unit and the data sheet accompanying the unit was devoid of much useful info, I have no way of determining if that is it.

TimK said...

I'm pretty sure there isn't a short in the 12V system because the system works fine after the DC-DC dies.

While the system may work fine up to 144V, I'm concerned that the regen braking will push in pulses up to 180V.

I talked with a power supply designer at work today and he felt this was clearly an overvoltage condition caused by inductive kickback and not an overcurrent condition that blew the FETs. These FETs are some of the largest available and they dissipate into a very large heatsink. The DC-DC was heatsink was relatively cold, so I'm tending to agree with him. I'm taking a pile of fast-acting high-current diodes home to add to the circuit to see if that helps. Heck, I voided my warranty, might as well cause an even bigger fire... :)

Roger Daisley said...

rogerTim: You may have technically voided your warrantee, but if there is a basic design problem (not likely, based on Mike's statement, "we've sold a lot of them and they are a solid unit") or CCP/EA provided the wrong units, then I think "someone" (I'm thinking EA) has a good faith duty to make good on them. Based upon the fact that mine also quickly failed ... well, you see the point here.

TimK said...

Hi Roger. Yes, I agree with you that "someone" has a responsibility to make good on the provided product. What's really going on here is an ego battle in my head. I'd much rather "solve" this issue and prove my own worthiness as an engineer than return the product. I think the following link (scroll down to "Ego" at the bottom) explains the situation:

"There's a fine line between a hobby and insanity" - anonymous

Cheers, Tim

Ross Cunniff said...

I'm going to push as much of my final build to completion this weekend as possible - and I'm considering leaving out the DC-DC converter for now and just going battery. Any drawbacks you can think of?

TimK said...

Hi Ross, I don't see any major drawbacks. I've found the 12V battery to be sufficient for at least half hour drives and more. If you charge the 12V along with the 144V pack at the same time, I think you'll be okay. It'll last even more if you just drive during the day so you don't have to drive the high-current headlights.

I've been e-mail with Mike at Electro Auto to try and root cause the problem in the DC-DC. When I get back from vacation this coming weekend, I'll probably rig up a 180V DC power supply in the lab and see if I can put a 30 amp load on the output to try and measure all the waveforms. As I mentioned in my e-mail to you, I'm rather shocked (pun not intended) that the high-current DC-DC doesn't have kick-back diodes on any of the inductors driven by the FETs. In my book that causes major high-voltage issues.
Any suggestions for how to debug the beast?