I decided to try and answer my own question above. My knowledge of electrochemistry is rudimentary, but I found a good recent paper that I could understand about half of. It's available open access at http://jes.ecsdl.org/content/163/9/A1872 , and I consider Fig 2b the key graph, which I can't seem to paste here.JonathanT wrote:Hieronymous, I've followed your elegant data-rich posts with interest, and thank you for them. Right now, I'm a little confused on two counts. First, you seem to be postulating that if SOC is kept chronically below 40% the BMS algorithm considers the vehicle unsold and doesn't register any loss of capacity. But if that's the case, doesn't that complicate the interpretation of a lot of your carefully collected data, and if so, what is your rationale for maintaining your conclusion that 30-40% is the optimal resting charge level?hieronymous wrote:Hi Jim - thanks for the FtF contact re above. Thought any discussion may possibly benefit others here.jim0266 wrote:Ran across this article of a gentleman in NZ doing the same thing with his 2012 Leaf by keeping the SOC below 40% as he also drives short distances.
Early adopters as teachers: a case of extraordinary care of a Leaf battery
https://flipthefleet.org/2018/early-ado ... f-battery/
"The goal was to minimise battery degradation using optimum charging practices, identified as shallow depth-of-discharge, frequent part-charging, and as far as possible to keep charge level at or below a level considered best practice for storage (30-40%). Web discussion papers suggest that li-ion batteries are most stable at 3.92 cell voltage; a SoC level of 39.0% gives a maximum cell voltage of 3.925v for the Leaf under discussion."
My thinking now is that the BMS is designed not to function below 40% SoC, a Nissan mechanism to allow batteries stockpiled for the production line, and transported subsequently, to remain at 100% until sale time perhaps many months ahead. My Leaf was stockpiled for 26 months before I bought it as a new car, with SOH at 96%. No doubt calendar ageing is still occurring, but the stable numbers have a "feel good" factor. The main benefit is the stability offered by the 3.9v cell level.
On your longer trips I think the 100% charge is the key to your better metrics; the battery becomes optimally balanced for a while, returning the best range and boosting range related data. A spell under 40% tends to do the opposite, with an increasing cell voltage delta (not that it matters for short trips).
So, for long battery life: choose a temperate climate to live in (my annual battery temperature range is about 13-25C), keep the battery below 40% as much as possible (2 charge bars or less), charge only as much as required, and only when needed (the car timers can make this easy), and build in one regular 100% charge/fairly full discharge cycle every month or two, to maintain reasonable pack balance.
Just my 2c worth
Second, you say that the optimal maximum cell voltage for preserving the capacity of lithium iron batteries is 3.92 V, and I've seen that stated elsewhere. But according to Leaf Spy measurements of my 2019 40 kWh Leaf, to achieve that the SOC needs to be 70-75%. As I write this, the vehicle is at 60% SOC (according to Leaf Spy, 55% according to the dash), has been sitting undriven in the garage for over 20 hours, and (with charger unplgged and climate control off) the maximum cell voltage is 3.77 volts.
I'm really enjoying my new Leaf and want to keep it as long as possible. I've been following your advice for the most part, but it would certainly be more convenient to charge to 75% instead of 40%!
I invite hieronymous and other forum members to weigh in on this, as I know your advice will be worth a lot more than $0.02!
One of its strengths is that the measurement period is 10 months, with the whole range of SOC at 5-10% intervals. The authors consider SOC a more relevant variable than cell voltage. The bottom line, for me at least, is that keeping SOC below 60% as much as possible minimizes calendar degradation, so going down to lower resting SOC appears unnecessary unless you live in a very hot climate. As we all know, heat is bad for batteries.
The Fig 2b 25°C plot for the NMC chemistry that's used in the Leaf, I believe, showing only a very small difference between 100% and 80%, may be why Nissan dropped the long life 80% charging option in the 2014 and later model years. But it makes a big difference at 40 and 50°C!