Currently own a Tesla Model S 85kWhr model. Been eyeing a 2nd electric car for awhile and used Leafs are pretty cheap. But battery degradation is obviously a concern. I'm in engineering and an avid DIYer on auto maintenance so have been learning about EVs.
First, I'd like to explain a little of what I learned about battery degradation. There are major 2 factors (capacity and heat) but most people only talk about heat which is probably the secondary problem.
A rechargeable battery life chart shows lifecycle is a lot longer if only cycling with low amount of charge and discharge. Large charge and discharge cycles degrades battery life in any technology including our trusty old 12V lead acid batteries. Here is some info
http://batteryuniversity.com/learn/arti ... _batteries
So a 24kWhr battery used frequently in say 40-50 mile range charge/discharge cycles will loose capacity really fast regardless of thermal control. Using the LiPO4 discharge table from the above link, we can see 80% discharge/charge cycles provides about 1000 cycles before losing 30% of the original capacity. So 50 miles x 1000 cycles = 50k miles. And 4/12 bars loss = 33%. This pretty much matches the data we see from <= 24kWhr battery electric cars.
Cooling is not as much of an issue at this capacity level because high depth of discharge will pretty much guarantee high capacity loss at 50k miles. Other ~24kWhr EVs like Focus Electric also have active battery thermal heating+cooling and all show the same range loss pattern at 50k miles. I think Nissan recognized this and decided to just leave out the expense of active thermal control for a 24kWhr design.
Compared to a Tesla 85kWhr driven 50 miles per day. 20% DOD (50mi/250 range) 2 yields 500,000 miles before 30% degradation. So I hear about high mileage 85kWhr Teslas lose about 10% at 200k miles.
2. Thermal Control
The reason rechargeable battery degrades faster in heat is due to basic chemistry. From high school chem class, we learned every chemical reaction has parasitic reaction. Parasitic reaction basically produces unwanted products and use up some of the reagents. With increased heat (say a bunsen burner in the chem class), reaction (and parasitic reaction) goes faster.
Thermal control also isn't just cool the battery cell. It is about how to cool every corner and interior of the chemical reaction surface. Large cells like Nissan and LG Chem surely have difficulty to cool the center interior.
So what would be an ideal aftermarket LEAF battery look like?
1. Leverage energy density increase to get say 2X capacity for the same space. An aftermarket design using newer battery chemistry should gain the energy density improvements since 2001. Not sure if 2X has been reached but certainly soak up the improvements. This helps with DOD discussed above. If replacement capacity stays at 24kWhr, it will suffer the same fate at 50k miles.
2. Cooling. Tesla battery design is very different from Nissan and LG Chem and I'm guessing there is huge benefit. Teslas uses small cells (little bigger than AAs) and submerge them in coolant for thermal control. Nissan and LG Chem makes big battery cells (size of a shirt box). Bigger cells have better density but smaller cells have better cooling (can't cool the center of the big cells). The key is probably to find the good balance between these 2 factors. Tesla's cell design is evolving from the original Panasonic battery cell to now slightly bigger to provide 2X capacity. Likely a design trade off after gathering statistics on the original design. Cell size can grow a little while maintain thorough cooling to the cell center.
Self contained thermal control would need air scoops on the bottom and internal heat exchangers + pumps + coolant bath) to provide a plug+play solution. Tesla's a battery have 2 coolant ports because they mount the heat exchangers mounted in the front of the car to collect cool air. Leaf have no such coolant port so would need to self source air and internal thermal control components.
Last, I can see Tesla also does 3 other things
1. Limit charging rate when battery is cold or full. Regen is limited under these conditions
2. Limit discharge rate when battery is cold. Can't do rocket take offs.
3. Cooling system goes into overdrive at the Superchargers (basically faster version of Nissan Quick Charge). DC fast chargig is the fastest chemical reaction so it generate the most heat so the cooling system needs to to offer max performance here.
With a few sensors accelerometer and temp sensors, maybe its possible to build in these controls directly into the battery pack.
In summary, following Tesla's world class battery design, a good aftermarket pack would need to
1 Source a good cell design. This is probably the most challenging. Tesla+Panasonic factories probably produce the best cell right now at the right size for density+cooling. They are probably not available for anyone else.
2. Build a good thermal+smart architecture with the good cells. Incorporating active thermal control (heater for cold and cool air for heat). Limit charge/discharge when cold etc..
Probably not going to get this level of replacement battery design but I thought I share with what I learned from Tesla's design.