This is my opinion AND common advice:
Keep battery between 20% and 80%
Do not use fast DC charge CHAdeMO routinely**
Do charge when temps are cooler (just before sunrise)
Do charge to 100% as needed or on occasion, but don't let it sit day or days at 100%, use a full charge fairly soon after full charge
** My Nissan dealer has CHAdeMo and my 2015 has the option. I used it a few times when I first got the car. I bought car used in late 2017, 3 years old. I have had it 6 years. So it now 9 yrs old. .
This is totally anecdotal. My 2015 I bought used late 2017 (3 yrs old at time). It had 12 bars the day it rolled off the car trailer. It was a month or two after I got it, it went to 11 bars right after a fast DC LEVEL 3 charge. Hey new car (to m) with a CHAdeMo port and free at dealer. I don't know if this hastened the loss of that one bar. It is now 10 bars after 9 yrs (6 years of my ownership). I used "LEAFSPY"* to see that the car had many fast DC charges before I bought it. Is this a factor? May be..
Second anecdotal observations. Early LEAF's battery packs were not so great, but had 5 yr warranty (I think warranty is longer on 2019). If you got below a(8 bars I recall, before warranty expired, Nissan would replace it under warranty. So some folks would abuse the pack in year 4, by fast charging and fully depleting, then fast charging again and again to damage battery to get to 8 bars. THIS IS WHAT I READ... I never did this, I still have my one and only LEAF now 9 years and 10 bars. At this rate my LEAF will serve my modest range needs for another decades. To get there I baby the battery. So I avoid LEVEL 3. I don't need it, but nice to have option. If I need it I will use it.
HEAT kills the battery pack. It makes sense the battery pack will get hotter with faster charge. However the charger has sensors and protects the battery. So back to my bullet points above, Again this is coincidental or anecdotal. I am agnostic, no fanatical opinion on fast DC LEVEL 3. This is a second knock around car to go from A to B to C back to A.... no big deal to not fast charge. You rely on you LEAF for long distance (over full battery pack range), yes fast charge may be normal operations. The new Nissan AYRA would be a better choice, with active (forced liquid) thermal management of battery.
Keep in mind charging is not linear. The first part of the charge, say first 50% is faster. The last 20% takes more time. So if you only need to go from 30% to 60% for your trip LEVEL 2 will be pretty short, an hour or two? If you want to go cross country in your Nissan LEAF, say 700 miles a day, for example, you likely want fast charging. Otherwise what should be a fairly easy one day will be a very long day or two day trip. So having fast charge option is nice. For local driving most people don't go over 40 miles peer day. You also do not need to always fully charge, just adding say 40 or 80 miles be all you need to run your errands. Again 80% rule is kind of tribal knowledge. I also find that the first 10% of the battery charge drops off much faster than the rest of the battery %. It is how batteries work. That last 20% or 10% of charge does not add as much as the first 80%.
The LEAF is a fantastic car, low maintenance, fun to drive. The bonus is not going to the mad house gas stations. Congratulations.
. I have a small 24kWh battery pack that is about 20kWh capacity now after 9 years.... It would take +9 hours to charge my small pack at Level 1. A 40kWh pack? Too long for most practical daily use. However LEVEL 1 works and if all you have use it.
LEVEL 1 = 2 to 5 miles per hour. 1 to 2.5 days
LEVEL 2 = 10 to 25 miles per hour, 7.5 to 11 hours
LEVEL 3 = 80% of your battery’s charge in just 30 minutes, fully charging your LEAF in 40-60 minutes depending on your battery capacity. The time varies because the charge may be throttled due to temperature. However it is much much faster than LEVEL 2.
Note the battery charge controller is ON BOARD your car for LEVEL 1 and 2. It monitors temps and will protect the battery as best it can. Fast DC charging bypasses the on board charger and will control the charge rate, again it has battery protection. This is why it is hard to say fast charge is bad for battery life. In the short term, no. Long term in my opinion, a definite may be.
My personal observations Fast DC LEVEL 3 could shorten battery life in a LEAF. I recommend it should be used as needed and sparingly. Does fast charging damage other EV brands? May be? But most EV's have forced (pumped) active (liquid) cooling and heat exchanger (radiator). Nissan LEAF uses air cooling for the battery. So it is more prone to head damage.
The new Nissan ARIYA uses active forced battery cooling. Very nice SUV. Is LEAF air cooling of battery bad. No, just be aware it can't shed heat as well. High outside air temps, plus charging may not bee great for the battery as it pushes its max thermal limits. In my case my 9 year old LEAF I predict will serve me another 9 years. However I only expect to drive 3K-4K miles / year and min range to not be too limiting would be about 50 miles. I suspect at that point I could drive it more but battery degradation will increase (I assume). Who knows. If I baby it it might last 15 years or more? It could puke up one of the many cells in the pack, kill it tomorrow? This is the joy of EV ownership. To replace my pack today about $9000 which is the value of the car. I'll cross that bridge when I get to it. I hope to find a used pack and replace it my self? Who knows Elon says new battery technology will increase range x2 in the next year... It would be lovely if new battery Tech could be retrofitted in all EV's. The cost in resources, energy, position, waste to make new car frames is awful waste.
I don't and never have charged at commercial for fee charging station to day. To do a typical top off charge (80%) at home is about $1.50 to $2. At a commercial charge station might be $10 to $20 or more. I would have to be desperate. I charge at a nice public Park 1.5 miles from my house. I drive over to ride bike, go for walk. So while there I plug in and add 15-20 miles. Weeeee. Freee. Ha ha. Every time I entertained a commercial charger it did not exist (but was listed), stupid expensive, or in use. BTW Tesla is about to open up their chargers to ALL Makes of EV's. I suspect it will have limits and not be cheap.
With my experience your LEAF with a little care should have great usable range for the next two decades. This is why Nissan went with air cooling. It works and will last longer than most people keep cars (if you don't abuse it). I only need about 60-80 mile range which my old 2015 can do summer or winter and for the foreseeable distant future. I
NOTE driving style effects range. Coast to stops, accelerate slowly, avoid sustained high speed driving. If you drive on the freeway doing speed limit not 20 over will improve range, quite a bit. Better range by driving more economically means less charge cycles and longer battery life.