Efficiency I Can't Quite Explain

With the exception of the few months the Z3 has been off the road for repairs or upgrades, I've been driving the car for just over 26 months.  During that time, I've kept detailed records of every charge/discharge cycle of the batteries.  Every time I plug in, I note the mileage on the odometer, and the number of amp hours I've drawn out of the pack.  I then take that data and plug it into a spreadsheet that calculates a number of things for me, including how much money I've saved because I wasn't burning gas, how much the electricity I'm using is costing me, and most interestingly, how many Watt-hours per mile the car is using.

I've reported in the past the the car averages about 320 Watt-hours per mile on surface streets.  That's the number I've used to calculate the range of the car: 19,400 Wh / 320 Wh = 60.625.  This is why I've always stated the car has a 60 mile range.  And I've proved that out once, driving 62 miles on a charge once, in preparation for doing a bottom balance on the batteries.

Of course, once I go on the freeway traveling between 65 and 70 mph, that 320 Watt-hours per mile begins to look like a distant dream.  The aerodynamic drag on the car causes energy consumption to quickly rise to around 420 Watt-hours per mile at 65 miles an hour.  That meant that my round trip to work, a 23 mile journey, which includes 7 miles of surface streets and 16 miles of freeway, averaged between 370 and 380 Watt-hours per mile.  I've made this trip a few hundred times, I know the numbers.

It's no secret that I've had a few problems with the motor in the car.  The balancing putty has come off for a some inexplicable reason, twice.  Just recently it developed a short to the case that no amount of air blown through its guts could resolve.  George Hamstra at Netgain has been a champion through all of this and ultimately had a new motor sent to me.  In addition, we swapped out the brushes from the standard H-49 brushes used for high current applications like drag racing, to H-60 brushes which are better suited for street use.  A cool feature on Helwig H-60 brushes is the split, Red Top design, which helps to ensure better contact on the commutator.

At any rate, I got the new brushes seated in the motor properly, put the car back together and launched it back onto the streets about 3 weeks ago.  The car is my daily driver, so once I began driving it to work and other places, and recording the energy consumption, I was a bit surprised to notice it was more efficient.  At first I thought it was maybe just an anomaly, but it's clear something has caused the car to make much better use of the energy in the batteries.  My round trips to work are now averaging about 280 Watt-hours per mile.  Compare that to the older 370!  That's more than a 25% improvement in efficiency!  I made one trip where the average dropped to 266.

Today I took a bit of a longer trip out to Scottsdale.  A total of 38 miles, with 30 of those miles on the freeway, traveling around 70 mph.  The average consumption for the entire trip was 274 Watt-hours per mile.  In the past, I would have estimated this trip to be a 400+ Watt-hour per mile trip.  But that's not all!  During the entire trip, I had the AC system on (which draws about 9 amps) and of course, I've configured the power steering pump to run all the time now adding another 2 amp continuous draw.  BTW, the change to the power steering is interesting, but that's another post.  So there are more parasitic loads, yet, efficiency is up.

The one thing I haven't done yet is to see what average I would get if I traveled at 40 or 45 mph.   There's no reason to think the gain in efficiency that I'm seeing wouldn't appear there as well, but I simply don't have those numbers yet.

It seems like an obvious conclusion to draw that the increase in efficiency can be attributed to the new motor, or brushes, or a combination of them both.  But I really can't say that with certainty.  Perhaps I'd made some error when installing the drive line in the past which caused some binding or friction that I simply wasn't aware of.  I kind of doubt that, but who knows?  There's no question I've gotten better at disassembling the drive line of the car, but there really isn't much room for error here.  I have no reason to doubt the numbers the meter is giving me; after all, it's the same meeter with the same set up I was using before the motor swap.

What ever the cause, the car does seem to be more efficient.  At an average draw of 280 Watt-hours per mile, it's gone from a 60 mile range to nearly a 70 mile range.  I'll take it.

Update 5/20/2012:

I've continued to see the gains in efficiency I detailed above, but I've realized I've let myself fall victim to insidious creature that is over optimism.  I've always maintained that the Z3 had a 60 mile range.  That was based on the fact that it consumed about 320 - 330 Watt-hours per mile when driving at ~45 mph in normal traffic.  What I really hadn't done is take an average over multiple trips to get a more balanced number, a real world number you can take to the bank.  Well since I've seen this improvement in efficiency, I've gone back  to my spreadsheet to see if I could mine some more useful, accurate data from the numbers.  Here's how it works out...

Since the car was put on the road, up until the motor/brush replacement, it has averaged 376 Watt hours per mile.  That is the real world average.  Sure there were many trips that were better, but there were also many that were worse.  I can't get out of my neighborhood without consuming something like 480 Watt-hours per mile.  It's all about stopping and starting.  With no regenerative braking, stop signs and stop lights really affect your range.  The more of them per mile, the worse your range.  Well, there's 6 stop signs on one of the routes it takes to get from my house to the main streets, so you can imagine what that does to energy consumption.

The average after the motor/brush swap has dropped to 321 Watt-hours per mile.  Compare that to the old 376, and you note a 14.7% improvement.  That is huge!  Interestingly, and I've mentioned this above, virtually all of the trips I've made in the car since putting it back on the road have been on the freeway at 65 - 70 mph.  Meaning, that as more trips on surface streets are recorded I expect that 321 number to drop even further.

The bottom line is that I've been misrepresenting what the car's range really is by skewing the data toward the happier, more optimistic lower numbers.  Not intentionally or maliciously mind you.  The old average was 51.6, the average now seems to be 60.4.  A painful thing to admit, but there you have it.  Just as I could have easily squeezed 60 miles out of the car before, sticking to surface streets and avoiding stops, I expect I could squeeze 70 miles out of it now, doing the same. 

I've brought all this to Jack Rickard's attention at EVTV, and just like me, he was skeptical and ultimately amazed.  He's been doing some tests swapping out the original H49 brushes for the split, Red Top H60's and he's finding the same results.  I have a feeling this is going to be a hot topic in the EV community for a while.

A Good Drive

Earlier in the week I made plans to go see a friend of mine who lives about 25 miles away in Maricopa.  I had planned on taking my gas car, but when my daughter absconded with it, I was left with the Z3.  Now, a 50 mile trip shouldn't be a problem for the car, after all it's full range is closer to 60 miles.  But this would be the longest trip I've clocked up and most of it would be at freeway speeds.  I asked my friend if I could charge up a bit when I arrived and he graciously agreed.

The ride down was pretty uneventful, and I averaged just over 55 mph.  At that rate, I watched about 2 amp/hours per mile (320 Watt/hours) click off the Link 10.  By the time I arrived, I'd used 52.1 amp/hours for a total of   8.34 kW/hours of the 19.2 available.  Some of you may remember that I've got 120 amp/hour CALB cells.

As I rolled into the driveway, the trip meter clicked to 25 miles, we ran the extension cord out and I gratefully plugged in.  I set the charger so that it would pull no more than 10 amps.  I watched for a minute, no breakers tripped and all was good.

When I went to leave, I saw the charger was off; uh oh.  While the breaker never tripped, the power strip that he'd plugged into was not happy with the current and it tripped.  I'd managed to pack in 1 full amp/hour before it tripped.  Should have checked it.  So, with 51.1 amp hours down on the pack I headed for home.  "Should be no problem, I've got up to 120 to use," I thought.

Apparently Maricopa is slightly down hill from my house, which now meant I was going back up the hill to Phoenix.  I watched as just over 2 amp/hours per mile clicked off the Link 10.  I was 95% confident that there would be no problems.  I guess you could call that 5% range anxiety (something I gave up on as silly some time ago).  For the record, traveling at 55 MPH in 4th gear, the motor turned at about 2800 RPM, and it drew between 2.0 and 2.1 amp/hours per mile, or between 320 and 336 Watt/hours per mile if you prefer.

By the time I rolled up into my driveway, I'd drawn a total of 106.5 amp/hours off the pack.  This marked the first time I drew the batteries down below 80% depth of discharge, 88.8% to be precise.  There was absolutely no noticeable difference in the way the car drove from when I pulled out of my driveway, apart for the Link 10 blinking at me incessantly that I'd used too much current.

I was curious to see how the batteries measured up after that drive, so I left them to rest for 15 minutes and then took a reading off each cell.  The highest cell was at 3.248V, and apart from one cell, the lowest as at 3.231V, but 80% of the cells were at 3.241 + or - 0.003V.  The entire pack was at 155.7V vs. 160.0V when it's fully charged.

There was one cell at 3.221V.  If I weren't careful, and I ran the pack down too far, that cell is likely the first that would die a horrible death.  I'm seeing more and more why bottom balancing the cells is a better strategy than following no strategy, and way better than top balancing.  If you're watching amp/hours in and out, not over charging is a snap.  But if you're interested in drawing the pack down beyond 80%, you'd be better off making sure all the cells hit bottom at the same time.  I need to re-watch some of Mr. Rickard's videos to get details on how to do it. 

All in all, it was a great learning experience and I was terrifically impressed with how well the car did.  These batteries really are amazing.

Getting Some Driving Time

The heat here in Phoenix is letting up a bit. Not enough to comfortably drive the car in the middle of the day, but early and mid-mornings have been very nice. Every morning I take the car out for a daily errand, and it's just fabulous. What a great car to drive!

Back before I had to remove the motor, I was giving a friend a ride around the neighborhood to show him the car. I drove by a neighbor that lives around the corner, he and his daughter were in the street playing catch. As we approached, I was worried they might not know we were coming, but they saw us and moved aside and we all waved as we went by.

When we came back by, I slowed down to be sure we were all safe, and the father called out "Is that an electric car?" I told him it was, and he said that he knew it had to be because it hadn't made any noise when we first drove by. I stopped for a few minutes to chat. He had a lot of questions about the car, and seemed truly interested. I answered them all as best I could and waved goodbye.

Over the past few days, since I've been driving it more, I've happened by him a few more times. Each time he eagerly waves and smiles. It's great to have made a connection with a neighbor, I hope to talk to him more and maybe offer a ride someday. But the best thing is that he's seeing an electric car driving around the neighborhood on a regular basis, and he has a positive attitude about it. I hope that it's situations like this that will slowly help to convince more and more people that an electric car IS a viable option for some.

I'm still averaging about 325 Watt/hours per mile, which isn't horrible, but I'm pretty sure that will get better with the modifications I've planned for this fall. New springs in the front, fix the alignment and replace the differential's fluid are what I have in store as soon as the weather really starts cooling off.

There is one small thing that's been driving me crazy, and that is a rattle coming from the rear of the car. A rattle whose source, I've not been able to locate. It only happens when I'm heading over bumps, so at least it's not constant. I've looked for it repeatedly, driving the car up on the ramps and poking and prodding, but with no luck. It has a real tinny sound, so it's something small, and I thought I knew exactly what it was. It sounds like one of the clamps used to compress the batteries in the rear battery box is loose. But I've checked them all, and they are all nice and tight.

It's definitely coming from the back end though. The funny thing is, no one else notices it until I point it out. Then they all say something like "It's not that bad." They're right, it isn't, but it drives me nuts. I'm going to have to drive up the ramps again and really give the back end a thorough inspection. I'm not worried that anything is going to fall off or come undone. I used self locking nuts on all the connections in the car; the kind that can't spin off on their own. Eventually I'll find it. I just hope it's before I go mad.

I'm going out to the Diamondbacks baseball game tonight with a friend. I'd love to drive the car there, but the heat would make that an unpleasant drive. The real sad thing is that the ball park has removed their electric car parking spaces! They had 2 spaces in one of the closest parking structures, right up front, with charging stations and everything. I called to find out if they were still there and what was involved in using them, only to be told that they'd been removed. Sadly, the last time I went there, I found they had indeed been turned into handicapped spaces. *Sigh* I can't really blame them, they were empty every game, but what lousy timing!

Didn't Make it to Laguna Seca

Well, the REFUEL event at Laguna Seca went off today, and sadly I could not get there to participate. Over the last couple months, I reinstalled the motor and redid the front battery racks, getting the car fit and ready to participate. Where it all fell apart was in trying to arrange transport to get the EV Z3 to the track to participate. I don't own the right equipment and the cost of renting it was just not in the budget.

I'm hoping that there will be similar events here in Arizona in the future, but I'll just have to wait and see. I'd really love to get the car out on a track to see how it does.

On a better note, I took the car out for an extended drive early yesterday and found some interesting numbers. While here in the neighborhood, the car averages about 350 Watt/hours per mile (dismal). However, when out of the neighborhood, on surface streets, it averages about 250 Watt/hours per mile (better). The only thing that I can figure is the constant stopping and starting required by the high number of stop signs in the neighborhood take their toll quickly.

I believe the numbers can be better though. The transmission already has Redline transmission fluid in it, which reduces friction in the drive line. After I sent the transmission away to have the adaptor plate made, I filled it with Redline. I haven't yet put Redline in the differential, so I still need to do that. I don't know how much difference it will make, but it can't hurt. I'm keeping the tires at 45 PSI. The tire's maximum pressure is 51 PSI, so I'm well below the threshold, but they are certainly more firm than the normal 32 PSI.

There are two big things I need to do yet to help with efficiency. First I have to get the front end re-sprung. That won't affect the drive line, but it will get the ride height to the correct level. Once that is done, then I can have the front end aligned and take out any toe-in that's there now. I expect that will make the biggest difference of all.

So, why haven't I had the front end re-sprung yet? A couple reasons really. First, I had to take the car apart recently, you may remember. That hampered my ability to drive it anywhere. Second, the shop that can do it is in the middle of Phoenix, about 23 miles away. Now, the distance isn't a problem for the car, it can do 60 miles. However, the heat is a problem. It's been over a 110 °F lately, which makes for miserable driving when you don't have AC. Second, the little radiator and associated fans used to cool the Zilla have a difficult time keeping it cool in such high temperatures. I could drive the car there early in the morning, but by the time the car was done, I'd have to limp back home in the heat, doing my best to protect the Zilla. At this point, I'm inclined to wait until cooler weather comes so that I can get it there without any risk to the controller. After all, the car works great now, it just uses a bit more energy than I think it needs to.

Oh, and did I mention how much I love that new vacuum pump?

Good News Everyone!

I've received word that the batteries have arrived in the country, Seattle to be specific. The company I ordered them from is going through the shipping container and portioning out everyone's order. I should have the batteries here within a couple weeks.

While I'm talking about batteries, it's occurred to me that I haven't really gone into too much detail about them, the range they should provide, and other such pieces of data. If you're interested in the nitty gritty numbers read on. Other wise... you've been warned.

I'm getting 48 of the Sky Energy SE120AHA batteries. They have a nominal voltage of 3.4 volts and provide 120 Amp/Hours. The batteries will be wired in series. This is just like the batteries in a normal TV remote are set up (positive to negative, positive to negative, etc). What that means is that all of the batteries become one big battery, and the voltage of that one big battery is the total of each added together. So, the math looks like this:

3.4 x 48 = 163.2 Volts

So that whole pack will have a nominal voltage of about 163 volts. Each battery offers 120 Amp/Hours of current, so to figure out the total number of Watts provided, you multiply the Volts by the amps:

163.2 x 120 = 19584 Watt/Hours Or roughly 19.6 kilowatt/hours.

Now, I don't want to draw all the current out of the batteries, because that will shorten the life. If I use only 80% of the power, or less, I'll extend the life of the batteries to upwards of 3000 cycles. So, if I want to leave at least 20% of the power in the battery, that means I'll get to use this much power:

19.6 kWh x .80 = 15.68 kWh

The car will likely use between 225 and 250 Watts/mile. Erring on the conservative side, lets say it will be 250. So then we figure out the range of the car, using only 80% of the current:

15680 / 250 = 62.7

So conservatively, I should have a range of 62 miles. Of course if I'm flooring it when I leave every stop light, that number goes down. If I'm on a freeway going 65 miles an hour, that number will go down. If I travel conservatively on a surface street, traveling no faster than 45, I should be able to extend that a bit. Or in the event I need to tap into that remaining 20% reserve energy for some reason, I could get as much as 16 more miles out the car.

You can see why people that drive EV's cringe at the question "How far can you go on a charge?" (Would you really want to explain that every time?) Or why the manufacturers that are just getting into the market of promoting EV's that they hope to build at some undetermined time in the future are reluctant to give out those numbers. Well that's not true of all manufacturers. Some promise 100 miles when practical experience or simple math like I've done above shows that the range would be more like 50.

To charge the car is simply a matter of how much electricity I can get a hold of and how fast I can dump it into the battery. The charger I have will run off of anything from 110 to 240. It has a dial that will allow me to limit the current (amps) it draws from the socket so that if I'm at a friend's house and they let me charge the car, I don't draw more than the outlet's breaker is rated for. The charger can only draw 20 amps off the service line which is probably the biggest limiting factor I face. But if the battery pack voltage is less than the outlet's, the charger modifies the power by decreasing the voltage to the battery pack, while at the same time increasing the amps it puts out to as much as 30. Such will be the case with the service line I'm running to the garage now.

For the sake of simplicity, lets assume I've drawn the battery pack down by 15 kWh's and the charger opeates at 100% efficiency (more on that in a moment). The charger will essentially be putting out 172.8 volts (the charge voltage for the battery pack) at 30 amps. Here's how it pans out:

172.8 volts x 30 amps = 5184 watts

15000 watt hours / 5184 watts = 2.9 hours

A 120 circuit is a different story. A 120 volt circuit putting out 20 amps equates to 2400 watts no matter how you cut it. The charger will modify that by raising the voltage to 172.8, but that means the amperage output will drop correspondingly to 13.8.

172.8 volts x 13.8 amps = 2400 watts

15000 watt hours / 2400 watts = 6.25 hours

As you can see a 120 volt outlet will take a while longer, but 6.25 hours is still quite acceptable. Now there is some loss when the charger is modifying the power from the wall and pushing it to the battery pack. It won't affect the time on the higher voltage charge, but the 120 volt charge time will likely go up by an hour or so.

So, there you go. Way more data than you ever wanted to know. When ever anyone asks me about range I'll simply refer them to this page. :-)