michael said:
...
1. Keep the batteries cool ( everybody knows this, no surprise to anyone)
...
3. Charge them only to the level that allows you to complete your trip and arrive at a charging station with a reasonable ( maybe 20-30%) state of charge. Try to keep centered around half charge.
Thanks for your recommendations. How do you reconcile the fact that 1 and 3 work against each other? It becomes a matter of priorities as to which is worse - heat or state of charge (SOC). Based on everything we've both read, heat is worse, especially if you never charge above 90% SOC to begin with (like the FFE). I've read a lot of well funded studies and virtually all of them simulate conditions that are different than we have in a Li-ion powered automobile. There are 2 primary differences in how we use batteries.
While it's true that storing a Li-ion cell at a lower SOC is better for it, the effect is minimal below 85 to 90% SOC, depending on chemistry and other variables. Since the FFE (and virtually all EVs) only charge up to about 90% to begin with, this problem is almost eliminated. This is especially true, as you mentioned, if the battery is kept cool.
The other major difference between most of the studies and an EV application is how energy is drawn from the cells. In your commute to work you will use the same amount of energy regardless of whether you start at a high SOC or a medium SOC. Drawing the same amount of power from a battery at a lower SOC requires, by definition, more amps. More amps equals more heat. But most lab and field studies looking at SOC vs longevity didn't consume power in the same fashion. Instead, many of them tried to keep temperatures the same to be more scientific and eliminate heat as a factor that might influence results. The difference can be dramatic. Driving the same speed at 3v/cell requires 30% more current than driving at 4v/cell. In my other EV, 2 clicks on a touch screen will display battery temp, motor temp, and inverter temp (another feature Ford should have). It's easy to observe the motor and battery temps climbing much faster at lower SOC.
Some people have made a good argument that Ford's thermal management is good enough that it prevents any heating effect at lower SOC. Unfortunately it's not completely true. It takes time to conduct heat out of the large prismatic cells that Ford is using. Your coolant temps rise while you are driving and the cells are always warmer. Worse, when you arrive home your cells are usually at their warmest. If you don't plug in immediately, you end up soaking your cells hot for many hours until you charge.
[edit: just noticed paw160 already addressed content of this paragraph.] There's another potential problem with your practices that you are ignoring and is not generally addressed in any of the studies. That is the subject of balancing cell voltage. I think the motor operates on about 375 volts? A lot of cells are stacked in series and the curtain gets closed when the weakest group of cells in series reaches the lowest allowable voltage. Most EVs balance their cells when they are resting near their highest SOC because it's difficult to do it effectively while you're charging, driving, or at a low SOC. Driving without well-balanced cells reduces longevity because the weakest cells are taxed the hardest, ultimately shortening their life faster. I called this a "potential" problem because we don't know much about Ford's electronics to know when and how they balance but it's likely that your cells are less balanced than somebody who charges full every day.
I've been advocating the best practice for longevity (not necessarily practical) would be to cool your battery as soon as you stop driving by starting a slow charge (say 12-14A 240v). Slow enough not to heat the pack or raise it to a high SOC, but enough so it activates the A/C cooling system until the pack is cool. Then sit at a still low SOC, then do a Value Charge so it's full about 15 min before you leave. That allows a little time to balance, and minimizes the time at the FFE's highest SOC while also minimizing the heat generated while driving.
michael said:
7. ...The takeaway is that just-in-time charging is better for the battery than end-of-day charging.
What if your car ends up sitting hot for several hours as a result? Do you think your advice is only good as long as you plug-in to cool it off before it sits until just-in-time starts?
