Micah Toll wrote a great article at Electrek on two factors that threaten the longevity of lithium-ion batteries. While most people understand that high temperatures can shorten a battery’s life, probably a less recognized factor is high voltage. Much of Micah’s article sources a discussion by Professor Jeff Dahn of Dalhousie University where it’s claimed that “high charge levels result in extra performance for a few cycles, followed by a crash in performance and much faster deterioration of the cell”.
In the article Toll writes:
Next, you should aim to charge to lower levels when possible, especially if the car will be resting for a long period of time. While it may be comforting to see your battery meter read “100%”, your battery will be anything but comfortable.
It is important to note that the most damage from high charge levels comes from when the battery rests at such high levels for long periods of time. I’ve heard of many people who freak out after learning about the effect of high charge levels, with some swearing off 100% charging forever.
But 100% charging isn’t a big deal in small doses. If you are planning a long trip and will be heading out shortly after you finish charging, a 100% charge will have very little impact on your battery’s lifespan. However, if you will be leaving your battery unused for many days or weeks, a charge level of between 30-60% is much healthier for the batteries over the long-term.
So in summary, avoid heat and a long-term full charge on your lithium batteries. While some batteries will discharge on their own after so many days of storage, others you may need to discharge to a lower level manually if you have no plans to use the battery in the near future. I’ve seen some recommendations that for long-term storage a charge level of 30-60% is recommended, but some manufactures have suggested a charge level of 80% is acceptable for long-term storage of your lithium batteries.
I’ve included below Jeff Dahn’s lecture entitled, “”Why do Li-ion batteries die and can they be immortal?”