Thursday, March 4, 2010

Real World Data: The Good, the Bad, and the Ugly...

The Good

I've been driving the car for three days now, trouble free. Everything works as designed. The vacuum pump for the brakes kicks on every second or third time I press the brake pedal. The speedometer and tachometer work properly. The heater works brilliantly, getting the cabin toasty warm in a very short time.

The car is very quiet. When sitting still, the only thing heard is the fan blowing air into the motor. As I mentioned above, the vacuum pump kicks on once in a while and it makes quite a racket, but nothing compared to the traffic noise around me. When I turn the car and the power steering pump comes on, there's a high pitched whirring sound, but it can only be heard when there's absolutely no traffic noise.

The power steering system is working quite well. It takes a bit of getting used to in that the first 3 inches I move the wheel, it's a bit stiff, but once the pump kicks on, it's easy as it was before I removed the engine. This can make for a bit of a jerky turn unless you're prepared for the sudden drop in resistance. It won't take long to get used to it. The good news is that I can change lanes at speed easily and the pump never comes on, which means I did a good job positioning the proximity switch.

So far I've put 62.5 miles on the car. With the local price of gas at $2.75 and figuring in cost of maintenance on an internal combustion engine, I've saved $6.10 so far. It costs just under $.03 per mile to drive.

The Bad

A couple days ago I mentioned a noise coming from the power steering rack on full turns. While it's still there, it seems to have diminished quite a bit. Not sure why, but I'll have to looked into that.

The Link10 meter is still a bit of a mystery to me. While it measures and displays the voltage quite nicely, the other measurements are either wrong or confusing. Because it's designed for battery packs under 50 Volts, I had to install a voltage pre-scaler. That means a voltage of 161.0 reads as 16.10. OK, that's no big deal. As expected, it's scaled everything else as well. Again, not a bad thing, but I need to learn how to read it. The kWh dial showed that I'd used 0.53 after my last trip. Based on the time it took to recharge the car, and the amount of current the charger draws, I'd think it took about 6.25 kWh to charge it up. If I'm dealing with the same decimal point error then that means the 0.53 translates to 5.3 kWh. That's about 15% higher than the 6.25, some of which undoubtedly is lost in efficiency converting AC to DC. Still it's impossible for me to know for sure because I don't have a meter that allows me to measure the current going in accurately. I may need to do something about that.

The most inaccurate thing the Link 10 displays is its "fuel gauge". If I really used 5.3 kWh (which I think is accurate), the "fuel gauge" showed that I was empty. I'd set it up to allow me to use about 16 kWh before it would display empty. So that aspect of the dial is worthless for now.

That brings us to range. Most everything I'd come across in my research lead me to believe I could expect to use between 225 and 275 Wh/mile. In reality, I'm using about 380 Wh/mile. That translates to a straight reduction in range. Now instead of the 60 to 65 I had calculated, I'm looking at more like 42. Not ideal, but I can certainly live with that. Now that's maintaining a 20% reserve in the batteries to protect their longevity, so the full range is closer to 53. Over time I may be able to increase that a bit here and there, but for now it looks like I'll be staying within 20 miles of home.

I haven't had the car weighed yet, but the front end is riding 1 inch lower than stock. Next week, I'll have it weighed so I can compare and decide what to do about the suspension.

The Ugly

Well, there's no getting around it. The car is slow. It accelerates like a 90 horse power economy car. I can't for the life of me figure out why that's the case. The controller is set to allow a full 1000 amps to the motor, which should produce enough torque to spin the tires! Instead, if I floor it from a stand still, it accelerates like a weighed down golf cart.

I've checked and I don't seem to be dragging an anchor, so I'm really not sure what's going on. I know it's not a friction problem. The car roles forever. If I simply let off the throttle pedal while doing 45, it will coast for over a quarter mile losing less than 5 miles an hour. It's like putting a regular car in neutral. I've gone over the Zilla's configuration again and again, but it's OK.

Now, I do have the controller set so that it can draw no more than 500 amps from the batteries. I did this for two reasons; first the Link 10 meter's shunt is a 500 amp shunt which means that if there's more than 500 amps going through it, the Link 10 can't read it leading to further inaccuracies in the meter's data. The second reason is the fuse I have on the battery pack is only good for 400 amps sustained. It will tolerate 1000, but not for more than something like 10 seconds. Still, throwing caution to the wind, I tried setting the battery current up to 1000 to see what would happen. I started the car gave it some juice and found that it was only slightly better than before. Not enough of a difference to warrant getting a bigger fuse and leaving it that way.

So there you have it, good news and bad. I'm going to continue looking into the problems I've encountered, and hopefully I'll find something. I think I'm going to have to ask a few experts some questions. Stay tuned.


htfiles said...

Looks like a great EV build, nice work!

Regarding your poor acceleration, you really need to get some additional data to hone in on any solutions.

2 key things to find out are:

* What's your pack voltage drop under acceleration?

* What's your peak current under acceleration?

For the first measurement, you can verify that whether or not your Link10 is reading your pack voltage correctly by sticking any multimeter across the + and - pack leads heading into the controller. The reading should be the same as your Link10. If it is, just check out how low the pack voltage sags when you accelerate from a dead stop and note it down. You may have a sick cell in the pack somewhere.

For the second measurement, you need a DC current clamp meter with a peak hold function. Here's an example:

It's non-contact so you just pop it over one motor cable and it will measure and display the peak current that flows through (using a hall effect sensor). You may even be able to rent one locally.

It will tell you if your controller is actually drawing it's rated current.

Tim Catellier said...

Excellent advice! You may notice from my latest post that I'm heading down that road. I hadn't thought of using the additional meter, that is a great idea. I'll see if I can lay my hands on one. It will help me verify if the Link 10 is showing the correct amount of amps drawn.

I know that the Link 10 is displaying the voltage on the pack correctly, it matches my multi-meter. I also believe it is displaying the kilowatt hours used correctly. I know what my charger puts out, and the times to charge it vs. the amount it says I've used all add up.

The Link 10 shows the voltage on the pack drop from 160 to ~150 under hard acceleration, which isn't too bad. I don't think that's too much, but I'm not positive.

If you have any other insights, or advice, I'm all ears. Thanks!