What is the longest lasting (number of years or cycles) commercially available battery technology?
This answer is to some extent a work in progress. Check back if you care.
As a first step it outlines the requirement and overviews the extreme cycle life capabilities in selected situations.

What is the longest lasting (number of years or cycles) commercially available battery technology?


I asked this question because I felt that an answer would be useful to myself, and anyone else who might design a portable or other battery powered system that is meant to last several decades without requiring a new battery.

Stated specification:
Capacity - at least 80 watt-hours.
Load - 10 W max
May take up to 8 hours to charge.
Weight - 10 pounds or less.
Operating temperature range - 5°C to 60°C.
Once per day, the battery will be charged to 100% of its current capacity, and then discharged at up to 10 W for 8 hours.
The battery voltage is not important. Assume we can place a converter at the battery output to make whatever voltage we need, and that the converter efficiency is factored into the 10 W load draw.
I have long considered a battery with many similarities to this specification.
In my case the application is a street light / refugee camp latrine light / home light for use in developing country role in a more or less &quot;install and forget&quot; mode.
Contenders in roughly descending order of goodness are:
LTO, LiFePO4, LiIon,  NiCd, NimH, NiFe, Lead Acid, 'Flow' batteries.
Cycle life:
The &quot;several decades&quot; requirement suggests 365 days x 20 - 30 years  = 7300 - 10950
Say 7000 - 10,000 cycles lifetime. That is VERY demanding by any usual standards.
I'll add a 10 year variant of about 4000 cycles.
So 4000 / 7000 / 11000 cycles
LTO (Lithium Titanate) chemistry often claims around 7000 cycles.
This will be examined in more detail below.
LiFePO4 (Lithium Ferro Phosphate) is typically claimed to yield 2000-3000 cycles with substantially larger being possible under very specific conditions.
LiIon / LiPo (Lithium Ion) - under very controlled conditions of limited Vmax &amp; Vmin (resulting in much reduced capacity) cycle lives of about 8000 have been reported. Examples include &quot;Mars planetary Rovers&quot;. This is achieved by charging to a maximum of about 3.9V and also limiting lower discharge voltage. This reduces available mAh capacity per cycle to well under 50% but gives much greater whole of life mAh capacity.
This is exceptional performance and well over an order of magnitude better than in typical consumer applications.
NimH  (Nickel Metal Hydride) - Low in normal use. 1000+ with care. NimH can achieve 2000+ with well controlled Vmax &amp; Vmin. (2000 cycles with NimH are claimed by the &quot;One laptop per child&quot; organisation - by setting upper and lower voltage limits with some reduction in capacity. Of likely interest is that the open source C code for the charging algorithms is available for inspection. (I've looked at it a few years back).
NiCd (Nickel Cadmium) - now unlikely to be viable due to concerns re Cadmium toxicity. Very long cycle lives if treated well. In use in communications satellites !!!
Lead Acid - Thousands of cycle with extremely limited depth of discharge (maybe 20% - 30% DOD). Resultant reduction of capacity is liable to make total weight unacceptable.
Just maybe:
NiFe  (Nickel Iron)  A very old technology with low energy density but immensely long lifetimes. Many many decades of operation are not unknown.
'Flow' batteries - Vanadium and other. Active ingredient is a liquid. Potential for very long lifetimes. Not without issues.
WIP ...