Another source of angst is battery life. And why not? For those not driving an EV yet, their only experience with batteries in cars is with lead-acid units that last five to seven years before crapping out. Even worse, their only experience with Li-ion batteries is probably with cell phone and laptop power sources that suffer noticeably degraded performance after a couple of years of continuous use. Not to mention the fact that there are stories of LEAF owners out there that lost 20% or more of their range after two or three years. Why should we believe that the batteries in EVs will fare any better than cell phone and laptop batteries?
Fortunately, my Focus Electric came with an 8-year/100K mile warranty on its thermally-managed Li-ion battery, so initially I didn’t worry too much about battery life, figuring that I can just ditch the car after three years if the battery turns into a relatively useless brick. But in short order I fell in love with the car, and since it might be a keeper, I started surfing the net for battery info and started paying attention to articles offering tips on prolonging Li-ion battery life in a plug-in EV.
There’s lots of stuff out there, but this is one that I like to refer folks to, from insideevs.com:
Understanding Battery Capacity Loss From A Four Year BMW Electric Trial Veteran
In short, five tips offered in this article are (in case you don’t want to read through the whole thing):
- Avoid deep discharge.
- Don’t leave a fully charged EV sitting unused for long periods of time.
- Avoid excessive fast charging.
- Don’t leave the car parked in a hot parking lot in direct sunlight if possible.
- If you don’t need all the range the car can offer on a daily basis, then don’t fully charge it every night.
Eight Tips to Extend Battery Life of Your Electric Car
This article has more tips than the previous one, but they center around the same three mantras as the first article: (1) avoid deep discharge/full charge, (2) avoid excessive fast charging, and (3) keep the battery cool.
As an Enginerd, I yearned for technical information that these tips are based on, as well as some specific data and values that can provide better guidance (i.e. how cool is cool?). Fortunately, batteryuniversity.com is a great reference for such information:
How to Prolong Lithium-based Batteries
Interesting stuff in this article includes a comparison of cycle life as a function of depth of discharge, which illuminates why we should avoid full charging and deep discharging. It also illustrates the effects of elevated charge voltages on cycle life, which backs up the the tip to avoid excessive DC fast charging. It talks about capacity loss as a function of temperature and state of charge:
“Lithium-ion suffers from stress when exposed to heat, so does keeping a cell at a high charge voltage. A battery dwelling above 30°C (86°F) is considered elevated temperature and for most Li-ion, a voltage above 4.10V/cell is deemed as high voltage. Exposing the battery to high temperature and dwelling in a full state-of-charge for an extended time can be more stressful than cycling.”
With 86F being a considered an elevated temperature, it’s no wonder that a thermal management system should be considered a necessity for an EV. It also explains why I’m constantly frying laptop and cell phone batteries, which have no thermal management system.
Although not published anywhere, owners of Focus Electrics have figured out that only about 85% of the battery’s advertised capacity is used, leaving a 15% reserve. One owner on myfocuselectric.com did a pretty good analysis (in my opinion), and surmised that 10% of that reserve, about 2kWh, is used to avoid full discharge, and 5% (about 1kWh) is used as a buffer to avoid full charge. I would think other EVs also implement similar reserves and buffers to prolong battery life.
What about when we need to leave our EV unplugged at some parking facility, like at an airport, for a long period of time? Well, here is another batteryuniversity.com article that might help:
How to Store Batteries
This article addresses how to store several types of batteries, including Li-ion. In essence it says to avoid storing at elevated temperatures, and that leaving it at a partial state-of-charge, ideally 40%, is recommended.
Oh, there’s one more article on this subject of prolonging life that I like. It's from DesignNews, one of my favorite trade rags:
Can EV Batteries Last 20 Years?
This article states, “Lithium-ion battery packs need to stay as close as possible to a 50 percent charge”… “usually going no higher than 80 percent and no lower than 20 percent.” It’s a good read, explains why we should expect thermally managed Li-ion batteries to last 20 years before losing 20% of its capacity… as long as we treat them right.