How to Prolong Lithium-based Batteries - Battery University
The short list of takeaways about consumer Lithium-based battery use (from this and other resources I've read about lithium battery technology):
- Avoid overheating batteries
- Avoid deep discharging as much as possible (don't let the device run all the way down)
- Frequent partial charging is desirable (charge whenever you can)
However, this document is not presented as a how-to guide for consumers using electronic devices powered by lithium-based battery technology. Indeed, in their own self-interest, many consumer goods contain sophisticated battery maintenance and management mechanisms, which apply many of the specific principles discussed in the above article.
In EV applications particularly, any significant procedural or parametric choices regarding battery charging are strictly controlled and limited by the charging systems which are built into the cars themselves (with the exception of "DC Fast Charging" solutions, which typically involve a combination of systems internal and external to the car). In order to be able to offer a product with acceptable performance and to have acceptable warranty risk, EV batteries are aggressively underutilized, masking their actual capacities. So the "empty" to "full" battery capacity available to the car and displayed to the user as "0 - 100%" may in actuality be, say, 22 - 90% (or whatever range the manufacturer has decided presents optimum cost/performance/risk for the given application). This ensures that their entire life cycles fall within a statistical sweet spot for longevity, never deeply discharging nor fully charging the pack. A user's deliberately preventing their car from achieving an indicated "100%" charge (which is actually something far less) is therefore unnecessary, as that has already been protected by this artificial capacity range. While there might still be battery longevity benefits to further conservatism, I wouldn't concern myself with trying to stop charging my EV short of an indicated 100% every day.
Likewise, battery temperature control is such a critical issue that most EVs both cool and heat their battery packs during charging and operation, actually using not-insignificant power (and therefore affecting energy budget) to protect the battery pack and optimize its range performance. There's little that a user can do to improve upon this thermal management - indeed, many EVs will defend themselves with warnings and shutdowns when faced with conditions which threaten damage to their expensive propulsion battery pack.
That said, experts appear to agree that frequent "topping off" of a lithium-based battery pack whenever practical does contribute to prolonged capacity performance. Because we attempt to exclusively use our EV for transportation, my choice is to charge it whenever I'm home, regardless of its state of charge when I return. My sensibility is that in the event of an unexpected transportation need, I want as much range as possible as soon as possible.
When we got our first EV four years ago, I made spreadsheets to compare and evaluate several charging options, and determined that choosing to charge our EV on an automated schedule during off-peak utility hours when rates were lower yielded minimal benefits - on the order of $200/year if we drove 10,000 miles. This was my financial justification for Always Plugging In. Likewise, the cost of installing a separate utility meter so that we could qualify for an EV discount from our utility was so high that we wouldn't break even from the $1,100 electrical contracting work for 6-7 years of discounted electricity - and that EV was a 36-month lease.
These same principles generally apply to portable consumer products - their internal charging mechanisms attempt to shield the battery from deep discharge and overcharge. It is still true that consumers can benefit from charging their phones or laptops as much as possible. And unlike cars, it's very easy to leave a phone or laptop in the sun in a window or car and irreparably damage or severely cripple the battery in one event.