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What Enginer has done is develop a method of tapping into the Leaf's high voltage power system, to add additional battery capacity. Kit installation does not cut any wires, and the car can be returned to stock state at any time. The company claims it does not void the Leaf's warranty.
The kit consists of a large box containing a 48 volt lithium-iron-phosphate battery pack, and a DC-DC converter to step the voltage up to what's necessary for the Nissan Leaf. The box does take up room in the trunk area. The 4 kilowatt-hour model weighs 160 lbs, extends the driving range by 20 miles, and costs $3,495. The 8 kilowatt-hour model weighs 260 lbs, extends the driving range by 40 miles, and costs $5,495. The 12 kilowatt-hour model weighs 360 lbs, and costs $7,495. Enginer's battery pack is rated for 2,000 charge cycles. We should note that the miles of driving range Enginer quotes is similar to the "100 miles of driving range" that Nissan claims, when in fact the EPA certified range of the Leaf is 73 miles. That is to say we should expect the actual range extension to be less than the figure Enginer quotes, and the actual range extension will depend on driving habits.