General > General Technical Chat
Lithium-Ion battery degeneration
gueston:
I know that Li-Ion batteries degenerate over time, even when not used. But is there a general rate for it at room temperature (20-25 degrees Celsius)?
And does it mean that after a while they can't be used at all anymore, or they just wont hold as much charge?
Nauris:
Gradually lose capacity and become higher resistance. I don't think there is any general rule, depends on chemistry and manufacturer and is not that well known. Samsung rates their 94 Ah cells for 70 % capacity after 26 years stored fully charged at 25 C for example.
gueston:
Ah ok. And is it a problem to leave a battery fully charged? For instance if an adapter is connected to a charger circuit constantly, then the battery would be fully charged all the time.
Siwastaja:
Both capacity loss and ESR rise depend heavily on state-of-charge, and temperature. When stored below about 50% SoC at room temperature or lower, aging is very slow (say, below 1%/year of capacity loss, or <5%/year ESR rise). On the other hand, stored between 80-100% SoC at elevated temperatures (say 40-50 degC), aging can be quite fast, say, 20% capacity loss per year or 50% ESR rise per year.
Exact figures depend on the actual product chemistry. For example, Sony cells seem to perform quite well regarding shelf life.
End-Of-Life is often defined as capacity loss of 30% or ESR rise of +100%, but this is obviously completely arbitrary and nothing prevents you from using End-Of-Life cells further.
Storing fully charged isn't a huge problem if you can avoid high temperatures; surprisingly storing at 80% or 90% may not offer any lifetime benefit. Going below 70% starts increasing the lifetime significantly, though, and is recommendable whenever you can do it.
gueston:
Thanks for your answer.
The reason I ask this is, because I am developing a product with a li-ion battery. The user can charge the battery and operate the device on the battery. But some users may keep the charger plugged in all the time. In that case the battery will never be discharged and keep charged maxium. But as you say, this wouldn't cause any real problems right? Also not safety wise?
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