Being an engineer, this got way longer than I intended, so here's the
executive summary/tl;dr version:
Based on real-world tests and how a car works, you will probably best extend the lifespan if you:
- Charge your car every time you drive it, even if the pack is still half full (most important)
- Go easy on the accelerator (probably minor)
- Leave your car plugged in all the time, particularly if it's hot outside.
Also: Your Clarity will probably lose about 1% of its capacity per 10,000-20,000 EV miles you drive it. You probably won't start to see the effects of this until you've driven at least 150K EV miles and/or owned the car for 5-10 years.
Showing my work, hard numbers, and offering some citations to back it up:
Finally found the article I was looking for. Battery University has an
good write-up of experimental Lithium-ion battery capacity maintenance regarding a number of factors.
This page has a bit more on discharge rates.
Figure 6 on that first article is the most interesting (capacity loss at different max charge and max discharge levels), with figure 7 showing an extrapolation of those numbers. Table 2 provides some less specific information on capacity loss as a factor of just depth of discharge. Table 3 shows the permanent impact of storage temperature on battery capacity, and figure 4 in the second article shows the impact of discharge rate on capacity.
The data is mostly for either small pouch-style Li-ion batteries or single-battery canister styles (which are the building block of most large battery packs), so the results aren't 100% applicable to high-quality automotive battery packs, but the fundamental chemistry is the same so the general conclusions apply. Likewise, the data is all from lab tests, and real-world batteries will not perform as well (more erratic charge/discharge patterns, less climate control, aging that isn't factored in, and whatever else), but the basic conclusions aren't affected.
Takeaways:
Doing a bit of math on the simplified table 3,
the deeper you discharge a Li-ion battery, the greater the capacity loss. No big surprise. Discharging to only 80% gets you about a 20% increase in total energy you can extract from a Li-ion battery over its lifespan versus 100% discharge. 60% discharge gets you 50% more use than 100%. 40% depth of discharge (that is, charging when the battery is well past half full) will double the usable energy compared to 100% discharge.
So, in vehicle terms,
charge your car every time you drive it. This would
not necessarily be true if the vehicle were being charged to 100% capacity, but indications are that the Clarity reserves 15% at the top, so that's not an issue.
Figure 4 in the second article shows what happens at C1 discharge rate (that is, drawing enough energy out that the battery will be empty in 1 hour), versus C5 (that is, it'll be empty in 12 minutes), C7.5, and C10 (empty in 6 minutes). After 500 cycles discharged at a C1 rate, the battery has lost about 5% of its capacity. At a C10 rate, it's lost a whopping 30% after the same number of cycles. So,
the faster a Li-ion battery is discharged, the faster it loses total capacity.
Now, a vehicle has active cooling on the battery pack, so this actually won't be as big of an issue as it is with a free-air-cooled cell. While the Clarity doesn't report numbers, the older Volts do, so I can say that at around 60mph on a flat highway those use around 15kW. The Clarity is probably somewhat more, but that says that you're only drawing more than around a C1 or C2 rate when accelerating or driving up a hill.
So, in in bottom-line vehicle terms,
don't lead foot it. It probably won't make a big difference thanks to active cooling and the relatively short periods of time one is accelerating, but it's certainly not helping.
Table 3 shows what happens at various storage temperatures. In short, the hotter the battery is, the faster it loses capacity. I think (though have not confirmed in the manual) that the Clarity will cool the battery if it's left plugged in and the ambient temperature is high. This would be a
very good thing if you live in a hot climate. So with the caveat I haven't confirmed Clarity behavior, leave your car plugged in all the time if you live in a hot climate.
Figure 6 and Figure 7 in the first article are the most interesting. Those measure and then extrapolate the capacity of a battery at various max charge and max discharge points. Skipping over the initial spike in capacity loss (which I'm guessing might be much less noticeable on a high-quality vehicle battery, if it exists at all), figure 6 shows:
Used at 100% to 25% SoC, a Li-ion battery will lose about 1% of its capacity every 250 cycles. This is probably the approximate pattern that the aforementioned 200K mile Tesla charged to 100% every time was run at. If it was a 100kWh battery pack model, that would put it at around 900 charge cycles, so a 6% capacity loss is well within the predicted range (worse, actually, but some of that may be due to an initial dip, some is probably environmental factors, and some may be simple age).
Used at 85% to 25% SoC, a Li-ion battery will lose about 1% of its capacity every 450 cycles. This is probably representative of a Clarity with daily driving of 30-35 miles total. This is probably around what a Clarity is allowing the user to use, based on Viking79's note on the total available capacity.
(Showing my work, if 14kWh goes in at the wall, and we assume battery has a 88% charge efficiency and the AC/DC converter is about 97% efficient, that means there's about 12kWh usable, which is 70% of the total capacity, meaning around 15% at the top and 15% at the bottom are reserved. That very roughly agrees with the two bars left at which it kicks in the ICE. That would put full cycle capacity in the 85% to 15% band, if you fully emptied it every time you drove it.)
Used at 75% to 45% SoC, a Li-ion battery will lose about 1% of its capacity every 830 cycles. This isn't realistic for a vehicle, but isn't all that far off from what you'd see if you didn't bother to plug your Clarity in if you only drove a couple of miles, and the rest of your driving was in the 20-30 mile-per-day range.
So, let's that 85% to 25% SoC as a baseline, keeping in mind that the pack has to lose 15% of its capacity before it will actually impact usable range (I'm assuming here the Clarity does the same thing as the Volt and "hides" the capacity loss in the non-user-accessible top portion of available charge). The reports of Volts that haven't lost any capacity after 100K EV miles are no surprise. That's about 3000 charge cycles, which would only be 7% capacity loss under ideal conditions, and even assuming it's twice as bad in reality you still would only be down 14%, which doesn't yet hit the "visible" portion of the capacity.
In the case of a Clarity, assuming real-world you get around 40 miles to a charge, most of the time the ICE hasn't kicked in when you plug in to charge, you're theoretically looking at 270K EV miles before you start to see decrease in battery capacity. If real-world is twice as harsh as a lab, that's still 130K EV miles. Not bad.
Not shown in any of those charts is age-related battery decay. I've seen industry numbers in the range of 1% decay in total capacity per year based on aging. This would start to become significant after 5-10 years of ownership, depending on how much you drive. Nothing you can do to help with that, though.
