How do those batteries fare in terms of capacity loss over time ? My li-ion rechargeable sanyo eneloops AAA batteries seem useless after a couple of years.

It depends on the depth of discharge and heat management.

Laptops, phones, etc have a poor lifespan because they are frequently discharged from 100% to almost 0%. It is better to discharge them from 100% to 50% and then recharge.

Batteries generate heat during charging and smart phones can get hot during heavy use. Without good enough cooling the lifespan of the battery drops.

It also helps to charge them more slowly, so in case of solar farms, you can increase the lifetime of a battery by using more of them and charging them more slowly. Discharge is slower as well, which also benefits lifetime.

Usually the best case scenario is 80 - 20 as far as I know.

I've heard the opposite! Any piece of information that you can share to back it up? Highly interested.

Sorry I have studied this in other language and I not even sure how these things called in English. There is one graph that I remember called Woehler curve and there is another one that shows how much is the life expectancy operating between different charging levels (which I cannot find the name for).

https://www.researchgate.net/figure/Woehler-curve-for-lithiu...

Update:

It is called SN curve:

https://www.researchgate.net/figure/SN-curve-applied-to-batt...

It shows a bit different picture for Li-ION that I remembered.

The reading I did years ago stressed that you should definitely cycle batteries to as close as Zero as possible, but if you want to do long-term storage keep them at 40%. Seems like there is a lot of myth around how batteries work and proper practice. A unified resource on this would be great!

Depends on the battery chemistry. NiCd and NiMh batteries which were very popular 10+ years ago do have a memory effect: https://en.wikipedia.org/wiki/Memory_effect

LiPo batteries do not have a memory effect, and you should avoid deep discharges.

I think the linked study gives you some insight. The problem is that we need all the capacity we get using a mobile phone for example. You can't really have a 2x capacity battery just for the ideal life time unfortunately.