Battery Capacity and Range

Quest for an Electric Porsche Boxster.

I went to the Chevy dealer in Austin yesterday, lucky to be in one of the early release markets. The sales lady said all the Volts are accounted for thru April or June of 2011, and they are selling them at $55K! She said there is a waiting list 39 deep, which I suppose means beyond the ones already alloted to the dealership. So I won't hold my breath waiting for a Volt.

As for battery management on the Porsche, we are top balancing K2 Energy, 26650EV blades. I am finding lower than expected range from the pack so far, presumably due to weak or shorted cells. There are 1920 cells arranged in spot welded arrays: 10 cells in parallel columns, with 4 rows in series per blade. We have column level battery management shunting up to 0.5A while top balancing.

Top balancing takes a long time and I don't think we have completely top balanced yet, with about 0.1-0.2V difference between cells at the top of charging. So far, I haven't burned down the house, and my wife and I (and two chihuahuas) sleep right above our gargage. I don't charge at night while asleep after following Jack's EVTV blog. http://web.me.com/mjrickard/

We are still getting acquainted with measuring pack capacity. We carefully monitor the voltage toward the end of pack capacity using a laptop, and still don't have the eVision set up properly. It seems using eVision for measuring Ah consumed while driving works well. We are seeing about 320 whr/mile with conservative driving, mostly freeway, at 60-65mph. So we should get close to 60 miles range with a 20KWH pack, similar to Tim's BMW Z3. http://www.evalbum.com/3189  However I think we are about 10 miles and 20 Ahrs shy.

When initially testing max range, there was a column that was consistently about a 0.3-0.4V below the rest of the columns, markedly limiting capacity of the pack. We replaced that blade with a backup, and range improved about 5+ miles. Now, there is another column that is about 0.3V below the rest of the columns toward end of pack capacity, and we are only getting about 75Ah out of what should be a 96Ah pack. The batteries did sit for 2 years while we experienced delays building the car. So I was expecting some loss of capacity, but not this much.

I have driven the car about 250 miles so far, and it is great fun! I intend to use it as my daily commuter car once it is more reliable. Unfortunately, the left rear CV joint suddenly unscrewed itself while driving 2 days ago in heavy traffic, so we are in the shop for awhile waiting for parts. :)