Wednesday, December 07, 2016

Batt-decided!

A multi-pronged pack design strategy paid off, and in the end we didn't get any finished CAD drawings, but a simple re-stacking of blocks to test all options produced the simplest (and hopefully best) approach.  The 88 buddy-paired LEAF modules will fit fine in 'pancake stacks' with none of the more complex 'library' orientation that requires heavier custom bracing.  The flat stack is easiest to build, but poses some thermal risk, as the topmost module will experience the most heat while the bottom one could get cold.  We believe this risk to be insignificant because of several factors.

  1. The vehicle's duty cycle.   The drive in a Ranger is of a very similar power level (84 kW vs 80 kW in a LEAF), but since our modules are doubled up, they will be discharging at roughly half the amperage as their original application, generating less heat.  Likewise, they'll also go through a slower Level 2 charge, generating less heat.  The Ranger EV will not be subject to DC Fast Charging, which poses the greatest heating concern.  A side benefit of slower-rate charging is that the truck will only be able to do one full charge-discharge cycle per day, giving plenty of time for cooldown.  
  2. Our mild climate.  The Puget Sound Region is ideal EV territory, with moderate summers and mild winters that in general keep battery packs happy without supplemental heating or cooling.   As the LEAF modules are particularly vulnerable to heat, there is extra insurance in that mileage will be lower during summer when school is not in session. 
  3. The insulated box.  What looked like a simple fiberglass battery box is actually a wood-cored composite on the bottom and sides, providing strength and insulation from the cold, while allowing excess heat to radiate through the lid.  The lid has two passive vents that could be upgraded with forced air if temperature issues arise. 
  4.  Testimony from other LEAF module users shows that in an unventilated Library stack (or row of modules), it is the center module that gets hottest, though not by a troublesome margin. That's info from the most similar truck conversion that's been built; http://www.evalbum.com/2898
Here's the current status.  The original base plates for four stacks were cut in half for use with two stacks, which we're through-bolting to the battery box, and then the stacks of modules bolt onto their factory base plates with factory top plates.  We've laid out all electrical interconnects with tape, and are bolting the stacks together for proper compression to 1.333"/module before adding busbars.  The multiple stacks allow student work at low voltages on the individual stacks, while I'll make the higher voltage interconnections and final high voltage pack connections.

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