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.