Charge to 80% of original or current capacity?

[genevb]

I apologize if this was discussed before, but I have not been successful at finding the answer through searches...

Does anyone know if the guidance on charging EV batteries to 80% (or generally keeping the charge near 50% and away from 0 ad 100%) apply as a percentage of the original battery capacity, or the actual (slowly diminishing as it ages) battery capacity at any given time in its life?

 

[genevb]

To highlight why I'm uncertain, and perhaps prompt others to comment on this thread...

I've heard that a contributor to capacity loss is storing the battery at high voltage because that increases reactivity of the positive cathode with the electrolyte. If I understand correctly, capacity loss doesn't really affect the voltage vs. SOC relation, so 80% SOC is roughly the same voltage regardless of a new (full capacity) or old (degraded capacity) battery. In which case, charging up to 80% SOC for the _current_ (not original) capacity is the appropriate guidance.

However, I've also head about the volumetric changes when the positive cathode's crystals are above ~75% of their Li ion site occupancy, which is why one should minimize going back and forth across this to reduce stresses and micro-cracking on the crystals. But if the current capacity has degraded to 80% of original, then can this region of volumetric change even be crossed any more? Are there still enough free Li ions to do so? If not, then I think the guidance should be referring to 80% of original capacity.

 

[cornbinder89]

It is a hard question to answer with any accuracy.
There have been people here that report long life, charging to 100% every night, and getting a life span of close to 200K miles,
We have plenty of those that have batteries fail prematurely regardless of how "easy" they were on them.
Both sides can come up with reasons why what they did was best.
I think one can lose their sanity looking for the "holy grail" of how to treat a battery.
What I can say with some certainty is that two people can charge the exact same way and get differing results.

 

[Drew21]

Note that if you use the charge percentage indicator in the dash display to define where 80% is, you will be charging to 80% of current battery capacity.

 

[Jeffstumpf]

It is a hard question to answer with any accuracy.
There have been people here that report long life, charging to 100% every night, and getting a life span of close to 200K miles,
We have plenty of those that have batteries fail prematurely regardless of how "easy" they were on them.
Both sides can come up with reasons why what they did was best.
I think one can lose their sanity looking for the "holy grail" of how to treat a battery.
What I can say with some certainty is that two people can charge the exact same way and get differing results.

Good summary