Adventure EV

Archive for February, 2010

Fuel tanks.

by on Feb.12, 2010, under Batteries, EV Land Rover

Just before I had to leave the southwestern Rockies for California’s warmer wetter climes a set of large crates arrived containing my fuel tanks, Lithium Iron Phosphate (LiFePO4) cells manufactured by Thunder Sky Energy Group in China.

Crates of LiFePO4 cells

In total, the battery system is capable of containing about 33 kWh of energy which should be able to give my Land Rover an approximate range of between 55 and 75 miles, when conservatively discharged to 80% capacity.  Or with the amount of electricity I’m currently using, enough electricity storage to power my house for two days.

Each cell is 3.2 volts-nominal and stores 160Ah of energy.  I’ve got 64 of them.  The calculation to determine how much energy a group of cells can store is:

Total Capacity = cell voltage x cell capacity x number of cells = 3.2V x 160Ah x 64 = 32,768 watt-hours.

And the calculation of how much range achievable is:

Range = pack capacity / vehicle watt-hours per mile = 32,768 /  500 = 65.536 miles

This last range calculation is tricky and highly variable.  It’s only the roughest estimate based on the average efficiency of my vehicle based off of parameters like weight, aerodynamic drag, average speed, rolling resistance, and drivetrain drag.  Driving slower will decrease the watt-hr/mile usage, while driving faster will increase it.

Based off of the real-world experiences of others, however, an EV conversion will range between 200 and 500 watt-hrs/mile, the former figure being with a conversion of something light, small, and aerodynamically efficient like a Geo Metro, the latter being a heavier, bulkier conversion of… say, a 40 year old Land Rover.

Each crate contains 16 cells.

Crate of 16 cells

Each cell is about the size of a large, hardbacked, Tolstoy novel and weighs about 5.6kg (12.32 lbs) for a total pack weight of just over 350 kgs (or about 760 lbs).  That’s about the same amount I took out of the Land Rover in ICE components.

Cell detail

The cells come grouped in sets of four which just about matches the size of a conventional 12v Lead-Acid car battery, but you can specify alternate groupings if desired.  You can see the aluminum end-plates and strapping hardware below.  It’s used to prevent the cells from swelling when taking a charge.  Swelling increases internal resistance, which reduces power output.  This behavior is really only prevalent in prismatic lithium cells.  The trade-off is a reduced cost and a cell that is capable of storing a large amount of energy.  Having to strap the cells together becomes a minor inconvenience.

Grouped cells

In designing my battery boxes, I added a bit of space for cell end-plates, but I had no idea that the stock hardware from Thunder Sky would be as robust and thick as it is.  Unfortunately, these cell blocks don’t fit my boxes,  but all is not lost.  I will probably discard the provided plates and use the structure of the battery box itself to accomplish the same task.

Also included in the shipment was a box full of harware for hooking up the electrical side of things.  Here we have aluminum bolts and laminated, copper interconnect bars wrapped with heat shrink tubing.  The copper bars are used to connect the individual cells together, while larger and longer runs of 2/0-sized welding cable will connected the battery boxes together and to the motor controller and charger.

Included hardware

The next task is installing all these cells in the battery boxes, mounting them to the Land Rover’s frame, and wiring it all up.  Oh, and sort out some kind of battery management/monitoring system.  More on that later… when I’ve sorted it.

None of this will happen until the spring, however, as other work demands attention.

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