Tuesday, August 10, 2010

Interesting Results

Over the weekend, I ran the final test (for now) on the high voltage system. I hooked up the last piece of equipment to be tested, the DC to DC converters, and let the car sit for 3 days. By the end of the three days, the Link-10 showed that 2.18 kW-hs had been drawn off the pack. For those of you keeping track, you'll recognize that number as being very close to what we've seen in the past. Indeed, previous tests have all come up showing 2.15 kW-hs over the same time period. I'd mentioned that I expected this test to be a bit different because the fans for the DC to DC converters run continuously due to the ambient air temperature. So, at first glance it looks like we're seeing that difference. The interesting thing is what happened when I charge the pack.

On prior tests, the meter showed that the pack had lost 2.15 kW-hs, only to find when I charged the pack it would only accept around 200 W-hs. When I've worked out the math each time, I found that the Link-10 was introducing a about a 700 W-h per day error. If you want to be precise, it's actually 650 W-hs, but I rounded up 700 because it's easier for me to remember and for a few other reasons that would simply put you to sleep if I enumerated them.

Anyway, this time proved to be a bit different. I plugged in the car, started up the charger and stood there waiting for the charger to indicate it was done. I figured it would take 5, 10 minutes max. Well, after waiting for 15 minutes, and watching over a 1 kW-h be pushed to the pack, I realized that this wasn't going the way I'd expected. By the time the charger finally kicked off, it had pushed a total of 1.53 kW-hs into the battery.

Even though the Link-10 showed nearly the same draw over the 3 day test as it did on every other test, there had actually been a real draw off the pack that amounted to 511 W-hs per day! I'll talk about what the real power draw means to me and the car in a moment, but for now lets talk about the meter. The only problem is that I'm not so sure I can put it into words.

Essentially what we see is the meter failing to register the draw off the pack. Arguably, and presumably, if you increased the draw up to that magic 700 W-h per day error that seems to be inherent in the meter, the meter's reading wouldn't change much. For now, we can only speculate that would be true. What would be interesting to see would be if you increased the real draw on the batteries up to 800 W-hs per day. Would the meter begin to register that and show the real number? It's interesting to speculate, but I have no idea for now.

So what does the 511 W-hs per day that the DC to DC converters draw mean to me and the car? First, I'd like to know how much of that we can attribute to the fans. I can't easily get to the fans to see what their labels say with regards to power draw, but they seem to be ordinary 80 mm 12 VDC computer fans. One that I have laying around here says it uses .075 amps at 12 VDC. If we use those numbers (which are close enough for now), keeping in mind we have two fans, that comes out to a total of 43 W-hs per day. That seems like a pretty small fraction of the 511 W-hs that is actually being consumed. I can only assume that the remaining 468 W-hs is being gobbled up by the DC to DC converters as some sort of offering to the gods of inefficiency in the form of heat.

I think what that means is that it really would be worth while to look at turning the DC to DC converters off when the car is off. You may remember that I looked into that a couple months ago, installing a couple relays to turn the converters off when the car was off, and I ended up welding the relays shut. I contacted Ryan Bohm at EV Source and told him my dilemma. He recommended an "inrush limiter" for each of the converters. I ordered them and have them here. They really should be installed in the unit itself. Only downside there is that I have to take the converters out of the car to do so. Another task to add to the list of things to do. Mean while, the car still runs great and remains fun to drive.


Anonymous said...

Hi Tim,

I'd like to suggest that the input and output current & voltage of the DC to DC converter be measured. Then you will know the efficiency of the DC to DC converter. Theoretical eff. of most DC to DC is from 80 to 85%, some up to 90%. If you're loosing 511Whrs per day, that means about 21.3 Watts(511/24hrs). If you are using a 200 watt DC-DC conv., 21.3 seem to be reasonable at 90% eff.

These calculations are from the top of my head so, they may not be all correct.

Anyway, that's all I can say.


Tim Catellier said...

That's a good suggestion John. Thanks!