Monday, July 9, 2012

10,000 Electric Miles

I pulled into the garage last night and noted the amp/hours I'd used and the mileage on the odometer so that I could record them like I do every time I charge the car.  The mileage read 144,214 which is significant because that marks exactly 10,000 miles since the Z3 was reborn as an EV.  I figured that this would be a good time to go over some of the numbers I've been collecting that last 2 years, 4 months and see what I could glean, and then share them with you. 

530        Number of Charge Cycles

18.9       Mean average miles driven between each charge
34.9%    Mean average depth of discharge
36.0%    Median average depth of discharge (half the data points are above, half below 36.0)

WARNING: Wild numbers and speculation will now commence. 

CALB says that these batteries are good for 2000 cycles at an 80% Depth of Discharge (DOD), and 3000 at 70%.  But how long will they last if I'm averaging roughly 35% DOD?  Of course no one knows.  However, if you use the 50% increase we see going from 80% DOD to 70% DOD as a baseline, and extrapolate that out, we might be able to conclude that we could get 4500 cycles at 60% DOD.  If we keep going and then apply it to my average DOD of ~35%, we come up with something close to 12,500 cycles.  At 18.9 miles per cycle, that works out to 236,250 miles. 

Of course that's all theoretical, but it's likely not too far off from reality.  But let's be conservative and say I only get half that number of cycles out of the batteries, that's still 118,000 miles.  However, we need to keep in mind that the original 2000 and 3000 cycles that CALB states is a bit misleading.  It's not as if the batteries stop working when they get to 2000 cycles.  What they really mean is that after 2000 cycles to 80% DOD, the battery will only hold 80% of it's original capacity, so they will still push the car as far as I need to drive on a daily basis.

</Wild numbers and speculation>

As you can see this whole "cycle life" or "life expectancy" question for the batteries is highly fluid with the real numbers determined by a number of factors all at once which are, for practical purposes, impossible to determine or track.  One thing that is certain is that these batteries will out perform a lead acid pack by at least an order of magnitude.  Seeing as they only cost 4 times as much as a lead acid pack, I call that a good bargain.  And that's not even taking into account the numerous other benefits they offer, like the 60 mile range vs. a lead acid packs 20 miles (at best). 

$1,402    Amount saved not buying gas
3639       Total number of kWhs used to charge the car

That $1,402 figure is derived by taking into account the price of gas when I charged the car and subtracting the cost of the electricity used to charge the car.  I always charge the car at off peak hours, and I add an extra 10% to the amount of electricity consumed to take into account the inefficiencies of the charger as it converts the 240 Volts AC to 160 Volts DC. 

So how has the car performed?  How well has it used that energy?

376        Mean average Watt-hours consumed per mile
388        Mean avg Watt-hours/mile with the old solid brushes
319        Mean avg Watt-hours/mile with the new split brushes

You can see there's been a marked difference in efficiency since replacing the brushes.  The old average of 388 Watt-hours per mile was experienced over 442 charge cycles and 8,184 miles.  The average has dropped to 319, and that has been over 88 charge cycles and 1,816 miles.  I don't know how one could dispute the claim that these Helwig Carbon split Red Top brushes are better. 

That increase in efficiency has moved the car from a 50 mile range using the 388 Watt-hours per mile figure to 61 miles using 319 Watt-hours per mile.  Of course most of you know how much range can fluctuate with an EV depending on how and where you drive.  I've been on 40 mile trips with the car where I saw the energy consumption average drop to 266 Watt-hours per mile, which works out to 73 mile range.  Is that useful data?  I don't know, but it's interesting.

Expanding on that, I found 4 data points that fit together nicely.   These are individual trips with the miles driven, the total kWhs used and the Watt-hours used per mile.

50 miles      17.23 kWhs    345 Watt-hours/mile
51 miles      17.89 kWhs    351 Watt-hours/mile
51 miles      16.49 kWhs    323 Watt-hours/mile
51 miles      14.57 kWhs    286 Watt-hours/mile

Guess which trip occurred after the new brushes were installed in the motor.  By the way, those were all trips to the same destination and back.

The real question at this point is how are the batteries fairing?  For those not familiar with the car and reluctant to go back and read the multitude of tedious posts, I bottom balanced the pack back in February of 2011.  The only way to find out how the batteries are doing now is to draw the pack back down to the bottom and make note of the amp/hours taken out and the state of charge on each battery.  Hmm... sounds like another post.  Stay tuned.


Joey said...

A lot of bloggers disappear once the car is on the road. I'm glad you are actively updating. I'm convinced that your brush change was significant.

Like you mentioned, I'm not sure that the lifetime curve can be extrapolated (no one really knows yet), but your point about graceful end of life is important. Sizing the battery a little bit bigger reduces depth of discharge, and allows you to use the pack more cycles once capacity starts to decrease.

Tim Catellier said...

I kind of figured that people would not be interested in anything that happened after the car hit the road. It wasn't until the car hit the road that I began to realize that there was plenty of interesting things going on that someone, somewhere might want to hear about. Turns out I was right. There are more people reading the blog now than ever.

In many ways, driving an EV is a life changing experience, and it's worth trying to share that with people that haven't been fortunate enough to drive one yet.

You summed up in one sentence what I was trying to say in two paragraphs regarding the "graceful end of life" aspect of the batteries. And you're absolutely correct that given the same usage, the larger pack you have the longer they'll last. People have asked me why I didn't use more batteries, and I tell them I used 48 cells because 49 wouldn't fit.

In any event, I expect that these batteries will outlast the chassis of the Z3. At some point I figure I'll be looking for another donor car to put them in.