The problem I see with this
The problem I see with this is the math doesn't exactly check out in my humble opinion. You have a 2015 leaf and I suspect that if you added the price of the battery and installation you would be pushing close to the value of a new vehicle,
How much more mileage would this give you over a brand new 2018? And for that matter for a little more you could get a 2019 e plus when it hits the market and get even more range for around 35k ish usd. Once you factor incentives, and natural depreciation over time for you 2015 i think you would be better off just paying for the new vheicle.