Have you ever wanted to own an electric car? or have you ever wanted to drive a Porsche 911? I wanted both, so I decided to build one. This blog is here to document my journey as I convert a 2002 Porsche 911/996 Carrera 2 from a gas guzzler to a completely electric vehicle - not a hybrid!

Thursday, March 1, 2012

Battery planning

The best battery technology available today for an electric car is a Lithium pack of some sort.  Designers have many options -- there are LiPo cells which are typically found in consumer devices -  LiCoO2 used by A123 battery packs, LiMn2O4, LiFePO4 used by many popular EV battery suppliers like CALB and Sinopoly, LiFeMnPO4 used by GBS, and other options out there.  The goal is high capacity at very low weight.  The capacity directly translates to vehicle range.  The thing thats frustrating is these batteries are very expensive, luckily, the local Electric Auto Association Minnesota chapter organized a group buy so we could get a discount and save some money on shipping.  We ordered the LiFeMnPO4 100AH cells from GBS in packs of 4.  Here is a picture for reference of what a pack of 4 looks like:

Now, to figure out how many batteries I needed, I had to do some calculations as well as determine how much volume the car could support.  The basic premise is to estimate how much power it takes to move the car a mile or km.  Typical conversions for my size, shape, and weight car end up with 300-400Wh/mi.  I also need to account that I can't discharge this type of chemistry below 30% without causing damage to the cell.  My goal is a range of 60 miles.  Each cell is nominal 3.2V with 100Ah capacity.  So each cell is 320Wh, derated down to 224 usable Wh per cell.  So to go 60 miles x 400Wh/mi = 24,000Wh.  24,000Wh/224Wh per cell = 107.1 cells. (I rounded up to 108 cells).  So this is a 34.5kWh battery pack, with only 24.2kWh usable for driving.  How does that compare to other electric vehicles you might ask?  The Toyota Prius uses a 1.8kWh pack, Chevy Volt uses a 16kWh pack, Fisker Karma uses a 22kWh Pack, Nissan Leaf uses a 24kWh pack, and the Tesla Roadster uses a 53kWh pack.  Regarding range, if my car performs closer to the 300Wh/mi rate, then I could feasibly reach 80 miles on a charge.  108 cells x 3.2 Volts (3.7Volts during charging) results in a very high voltage, too high for most controllers and motors, so I'm running two parallel strings of 54 cells for a nominal pack voltage of 173V, peak of 200V during charge.

The second problem was to find where to put all the cells without disrupting the weight distribution of the car.  I ordered some shipping boxes that represented the exact size of the 4cell packs and used those to test fit where I could squeeze these packs into the car.  Once I figure that out, I drew a picture so I could map out the pack to pack jumpers.  Turns out each jumper is 0.5mm thick and only supports 50Amps continuous current, but I need to support 300Amps continuous between cells (total continuous current of 600Amps with the two strings in parrallel), so I need to bunch up 6 jumpers between each pack.  The pack orientation also dictates the type of jumper I need to buy.  Trying to figure it all out in my head was getting messy, so I drew some pictures to map it out.  The arrows below point to cells that I run cables to, the little hoops between packs are the jumpers.  There are 3 types of jumpers I needed to order, type B, D, and E configurations, math is included on the side for each cavity.




2 comments:

  1. Any issues with battery orientation on the GBS cells? I'm going through similar calculations on mounting 56 CALB 180 aHr cells in a 1983 911. I'm trying to keep my cells upright, and that constrains the possibilities some. After looking at your drawings, I'm going to see if I can fit any cells behind my rear tires.

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    1. Yuan Dao, Ph.D from ElitePowerSolutions told me: yes you can mount them horizontal/sideways.

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