Lithium batteries

[Dieter Britz]

I have been hearing for years that lithium batteries, unlike
Ni/Cd ones, can - or even should - be charged long before they
are discharged. They were said not to have the memory effect of
NiCads. Now my wife tells me she heard that they do, after all,
and should only be charged when they are very low.

What is the truth?

[John Henderson]

Post by Dieter Britz
I have been hearing for years that lithium batteries, unlike
Ni/Cd ones, can - or even should - be charged long before they
are discharged. They were said not to have the memory effect of
NiCads. Now my wife tells me she heard that they do, after all,
and should only be charged when they are very low.
What is the truth?

The consensus of the experts here over many years has been that lithium-
chemistry batteries last longest when maintained at around half-charge.
Both very high and very low states of charge increase degradation.

Most of those experts have likely long departed from usenet.

My own experience in following that advice and keeping my lithium batteries
in that happy medium zone seem to to have been very successful. That's seen
batteries being still in good condition after more than 5 years' use.

John

[Yevgen Barsukov]

Post by Dieter Britz
I have been hearing for years that lithium batteries, unlike
Ni/Cd ones, can - or even should - be charged long before they
are discharged. They were said not to have the memory effect of
NiCads. Now my wife tells me she heard that they do, after all,
and should only be charged when they are very low.
What is the truth?
--
Dieter Britz

For Li-ion battery degradation comes down to reaction of intercalated
Li with solvent, resulting in insoluble reaction products which take Li out of the system, while at the same time blocking the pores and increasing cell impedance.

Since this reaction is happening only after Li is intercalated,
this reaction slows down at low voltage and SOC (e.g. when there is little Li available)

More detailed view of this degradation is that it has 2 main modes of operation:

1) slow leakage of solvent through SEI layer, than reacting with intercalated Li (happens with storage and cycling, strongly increased at high temperature).
This degradation will slow down with time since SEI layer grows thicker.

2) expansion/shrinkage of particles when battery is recharged. This is caused by 10% increase in graphite volume when Li is intercalated. This causes SEI to crack, and healing the cracks takes out fixed amount of Li each cycle. This mechanism is exponentially dependent on delta SOC in this particular cycle.
For 100% delta SOC cycle this mechanism is about 5 times stronger capacity killer compared to first mechanism for the same time duration.

Given 1 and 2, it is not easy to answer if it is better to immediately recharge the battery, as it has opposite effect on 1 vs 2. Immediate recharge
reduces delta SOC (good for 2) but increases average voltage (bad for 1). There is some optimal recharge level that gives best compromise but to find it we need exact parameters of the exponential dependency on delta SOC, as well as know the time you will store the battery.

Given that effect 2 is ~5 times stronger, I would just recharge when you have a chance in normal usage, but if you know battery will be stored for long time (where effect 1 will dominate) discharge it to 50% soc first.

Regards,
Yevgen