Friday, December 31, 2010

What I've Learned This Year

You may have noticed that I haven't posted anything in a while.  That's largely due to the fact that nothing interesting has happened in a while.  I'm pleased to report the Z3 continues to zip along trouble free and in relative quiet (still can't find that pesky rattle).  People occasionally ask me about the car, but I haven't had any interactions that are worth mentioning lately.  I've had a number of people tell me I should drive it down to the BMW dealership nearby and show it to them.  I'm kind of torn on this, I can see it going one of two ways.  One, they are interested and a few people come out to look at it.  Or two, they don't give a damn and are irritated that I'm taking up their time and not there to buy a new car from them.  I'm just not sure it's worth my time.

The batteries have been a complete non-issue.  Meaning that I charge them, I discharge them, and they do nothing out of the ordinary or unexpected.  I still need to bottom balance the pack, but I simply haven't had the time, what with the holidays and all.  However, I have had time to think about it and draw my own conclusions on the topic.

As I see it, bottom balancing means I'm not going to run the risk of ruining one or more cells by driving them to reversal if I draw the pack down too far.  So that's one argument for the case.  I may not be able to charge all the cells all the way up if they have slightly different capacity, but that difference is so minimal, it has almost no impact on range; less than a mile, probably 1/2 a mile at worst.  I may have to tinker with the charger to make sure it cuts off at the 3.45 volts I've been using, but so be it.  Having to set the charger for the proper cut-off is necessary regardless where you balance the pack, top or bottom, or even if it's not balanced at all.  So that is not a factor.  If we tally that up, we have one positive and two non-issues.

If I top balance the pack, I can be sure all the batteries are fully charged at the end of a charging cycle, which gets me nothing really.  I may be able to squeeze out that extra 1/2 mile I mentioned before, but there is always the ever present danger of reversing a cell or two if I discharge too far because some hit bottom first.  Setting the charger for the proper cutoff still has to be done, so that's not any different from bottom balancing.  So, that's one strike against top balancing, and two non-issues.  If you weigh it up, you see where the balance falls (no pun intended).

I learned something about Ohm's law in September.  I had no idea why turning down the current I was charging at would push the batteries to too high a voltage.  To be clear, I had no idea it would happen, and after it did, no idea why!  I tinkered around with the charger settings for a month before I understood what was happening, but still had no idea why.  Jack Rickard was kind enough to explain it to me and mentioned that even he'd fallen victim to it.  I suggested to him that if we both had been bitten, others might as well, and that he may want to mention it on one of his weekly shows.  Boy did he!  Fully half of one of his shows was dedicated to showing how I messed up and explaining why it happened.  To me, this may be one of the best shows he's published.  If we can point out the land mines involved with these batteries, hopefully people won't step on them.  They really are the best solution for enthusiast, home built EVs at this time.  Fortunately, the batteries are robust enough to withstand even my clumsiness. Dr. Jay Whitacre pointed out that as long as the cells are not charged above 4.3 volts, they are more or less unaffected.  Mine never came close to that kind of voltage.

From the time the car rolled out of the garage until roughly two months ago, I watched the Link-10 meter like a hawk, and range anxiety was a real issue for me.  I suppose that watching the meter over that time was necessary for learning purposes.  But eventually what I learned is that there really isn't much reason to watch it at all.  I know how far the car will go, I know how far my trips are, so range anxiety is really not part of my reality anymore.  This year I've come across only a handful of instances where I needed to use my ICE car instead of the Z3. Most of those times it was because I needed to carry more than one person.  One of those was because I needed to carry some 2x4's home from the hardware store (tough to do in a Z3).  I can recall only one occasion where I needed the ICE because I needed to drive across town and range was an issue.

Summer was tough.  It was too hot to drive the car because there was no AC.  The Zilla kept flashing at me warning that it was getting too hot.  I'd back off the throttle, or coast,a bit and the Zilla would stop complaining.  I can fix the AC problem, but it will involve removing the batteries up front and hiring someone to come out and fabricate hoses for me.  I could throw a bigger radiator at the Zilla to dissipate heat better, but I'm not too sure that would be very effective when the air is 120 °F or hotter on the street.  I may try, but it may be a mute point as I'm about to explain.

My real concern is the batteries.  They typically warm up to between 15° to 25° over the ambient air temperature, and that's the temperature taken at the terminals, internally it's bound to be warmer. I have no idea how much warmer, but it's probably on a few degrees.  They all have plenty of airflow around them, but there is only so much cooling 120° air can do.  The problem is the electrolyte in the cells starts to degrade between 135° and 140 °F.  It's easy to see how that temperature can be achieved in the Arizona summer if the batteries run 25° hotter than the ambient air.  Keep in mind that when you hear that it's 110° in Phoenix, that's at the airport.  If you actually measure the temperature of the air over any surface street in the valley, it's going to be North of 118 °F.  Heat is the second biggest threat to these batteries, and unfortunately summertime in Phoenix has that in spades.  I may very well end up using the car only 9 months out of the year, which is too bad.

There you have it; the good and the bad, the ups and the downs.  Nothing really earth shattering on either side of the spectrum, and I suppose that's good.  I was shooting for a car that I could just charge and drive, relatively worry free, and that's pretty much what I got, now that everything has settled down from the motor problems in May and the charging problems in September.  I've put over 3150 miles on the car.  It has become my daily driver.  It's rare that I have to drive my ICE car.  I've said this many, many times before and I think that manufacturers need to use this logic when selling EVs, If you have more than one car, the chances are very good that one of them could be an EV and you wouldn't notice a difference in your life except that it would save you money in the long run.  If you have more than 2 cars, then it's an almost certainty that one of them could be an EV and you'd never feel any inconvenience apart from a heavier wallet.

When we get to March 2nd, the anniversary of the Z3's road debut, I'll publish every sordid detail and piece of information I can find on the car.  I've been keeping tabs on quite a lot of data, and I'll share it all then.  In the mean time have a wonderful new year, and thanks for reading!


Michael said...

Hi Tim,
Dr. Jay has spoken about the Thundersky cells, when he said 4.3V max. Not our blue SkyEnergys.
With 3.5V you are 0.1V under the top, which is perfect I think.

Congratulations to your smart vehicle and may the force be with you ;)

Tim Catellier said...

Good point Michael, thanks.