Thursday, July 19, 2012

A Look at the Bottom

Juvenile jokes aside, at the end of the last post I alluded to the fact that I was interested in seeing how the batteries were doing since I bottom balanced them in February of 2011.  Quite a lot has happened with the car since then.  The batteries have been through 353 cycles.  They've put out 2209 kWhs of electricity and then had it stuffed back in.  I've driven a total of 6324 miles.  The car has been out of commission twice for motor problems, and off the road for a grand total of 7 months.  During those occasions, the battery pack was partially disassembled with half of the cells out and laying on the floor of my garage, while the other half remained connected together in the car.   That last point has concerned me a bit.  I've wondered all along if having them apart might have introduced some variable that may have caused cell drift.  I kind of doubt it, but I simply wasn't sure.  So, I set out to find out. 

Last Sunday I had a chance to drive down to the brand new Tesla store in Scottsdale and take a first hand look at the Model S.  I even passed someone that was out for a test drive in one of the demo cars.  I don't believe I've seen a bigger grin on someone's face while they were driving.  It was a great trip, but I'll write about that later.  At any rate, with a round trip to Scottsdale, and a couple errands thrown in, I'd used about 95 amp/hours out of the pack's 120 amp/hours available.  I decided to make a quick trip out to run the batteries down a bit more.  I figured I'd get up to 115 amp/hours or so and then run the rest down by running the heater in the car.

I set off for a quick 10 mile lap that would do it, when I had a second thought.  I remembered that the last time I'd charged the batteries, the charge had cut off a bit early with the top cell being about 3.40 volts.  That equates to the pack starting off about 7 to 10 amp/hours down, so I decided to cut my trip short.  Turns out that was a good thing.

As I was driving adjacent to my neighborhood heading for a specific entrance, I noticed the car was not really accelerating any more.  A half mile before that I'd accelerated to 40 mph with no problem but now, it was acting dead.  I turned into the neighborhood quickly and headed for home nursing the car the whole way.  I'd brought my multi-meter along but I was afraid if I stopped, I wouldn't get going, so I coasted (running a couple stop signs along the way) and turned the final corner to my house.  As I was heading up to the garage, I was hoping the door made it open in time because if I had to stop, there was no way I was going to get it up the hill of my driveway into my garage.   I made it in, but the car was dead.  It would barely move the 6 more inches I wanted to go.  I quickly jumped out and measured the cell I know to have the lowest capacity and it was at 2.043 volts.  I started measuring others and they were in the 2.5 to 2.7 volt neighborhood.  Well, that doesn't seem balanced to me!  I decided to let the batteries rest for an hour or so and come back to measure them.

By the way, when I pulled into the garage, I'd used 113 amp/hours (for those of you keeping score at home.)

Now, I'll elaborate on this more in a moment, but take note.  The lowest cell was 2.043 volts right after it had had a load on it, which is just above what CALB considers dead.  The car would barely move.  Yet no cell was below 2.0 volts and no cell was ruined.

I came back an hour later and measured the cells and found that the gaps, or differences I'd seen in the voltages had closed up dramatically.  The lowest cell that was 2.043 volts had bounced back and was now 2.684 volts; the highest cell was 2.937 volts.   I should let the numbers speak for themselves.

You can see that the cells were mostly between the 2.700 and 2.800 range, with a few just 1/100 off in either direction.  But there were 4 that were more than 5/100's of a volt off, with the spread from the lowest to the highest cell at 0.253 volts.  Three things come to mind looking at this data.  First, they aren't balanced.  Second, the amount by which the are out of balance is quite small.  At that end of the discharge curve, the difference between 2.684 volts and 2.937 volts is a fraction of an amp hour.  The third thing is that I think I simply wasn't patient enough when I performed the bottom balance.  You may have heard this elsewhere, or experienced it yourself if you've ever bottom balanced a pack of batteries, but it is an extremely boring, tedious, and lengthy endeavor.  Or to put it another way, it sucks big time.

On that first attempt at bottom balancing, it wasn't until after I was sick of the whole process and charged the batteries back up that I realized the proper thing to do would have been to let them rest for several hours to be sure they remained balanced.  I had them all within 1/200ths of a volt when I charged them, but I now know that if I'd waited, I would have seen them settle, and found they were likely a bit further off.  I think that inaccuracy is reflected in the variations in this data. 

In spite of my ineptness demonstrated here, I must have balanced them well enough to be, what I consider, successful.  The car would not have moved another 10 feet if I needed it to, yet no cell went below 2.00 volts let alone reversed itself and died a horrible death.  Something a top balanced pack simply can't do.

Now, I know what you're thinking.  "But Tim, you brought on this situation yourself.  This was completely and utterly self inflicted!  There was no need what-so-ever to discharge the pack this much.  I never intend to take my pack that low and expose them to this peril.  Consequently I'll never face the jagged, rocky bottom of the discharge curve, risking one or any cells in the process."   In part, you're right.   But consider this.  These cells, like any other, lose capacity over time.  How much and how fast is determined by how you treat them.  The problem is, the dangerous, jagged bottom of the curve sneaks up with every charge.  In other words, a pack that started out as a 120 amp/hour pack eventually becomes a 110 amp/hour pack, and then 100 amp/hour pack.  If you don't know where the bottom is, you risk hitting it and running a cell or 10 into reversal.  Since mine are bottom balanced, I "see" that imbalance at the top.  The charger cuts off at a preset voltage and the batteries will eventually reach that voltage regardless of how many amp hours they can actually hold.  The difference is, if I hit bottom the car stops moving and the batteries are fine.

So what's a fella to do at this point?  Best try to balance them again and do it properly.  This was monumentally difficult.  Not because the job is hard, and not because the batteries put up a fight or anything.  Rather because it's miserably hot and humid in AZ at the moment and spending 3 days in the garage balancing the batteries was not my idea of a good time.  One of those days was 18 hours long!  Suffice it to say, I got them all between 2.757 and 2.761, 4/1000ths of a volt, and that was with letting them rest for 4 hours at the end, before I put the charger on them.

One of the key pieces of information I wanted to get and was eager to share with you was the total number of amp/hours that went back into the pack.  That really is a measure of how the batteries have held up to the 545 cycles they've seen.  Sadly because of another, yet different stupid mistake, I'm not able to share that with you.  In may haste to get the pack balanced and the car back on the road, and my zeal to get it done right, I forgot something very important.  When ever you disconnect the main battery pack from an e-xpert pro meter, you MUST remove power from the meter.  I forgot to do this.  I opened the car and saw the following...

Notice the haze in the lower corners of the display?  The sharp eyed ones among you may also notice that the meter is not actually displaying any data.  I opened the door and immediately smelled the distinct aroma of a fried circuit board.  NOOOOOOOOOOO!   It gets better.  I took it out this morning, hoping to send it back to Evolve Electrics for repair, and this is what I discovered:

I'm no doctor, but that does not look good.  This meter was a few oxygen atoms away from catching fire.  I'm not sure if the deformation of the cylinder is clear in the photo, but it is not healthy looking.  Don't be like Tim.  Disconnect power from your meter before you work on the battery pack.  Incidentally, it fried the 1:10 prescaler as well.  *Sigh*

So, the end result of that is that I don't have a precise number to give you regarding how much power the batteries were able to accept when I finally charged them.   I can tell you that I turned the charger's dial to what I believe to be the position where it delivers 20 amps, and it took almost exactly 6 hours to charge.  That works out to 120 amp/hours, but it's really no better than a guess at this point. 

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.