Hello wade11a,
There are many factors at play and many variables. So the question doesn't have any one exactly correct answer.
Lithium Cobalt chemistry cells have become better in recent years, such that their total life-span has grown from somewhere around 2-5 years to something more like 3-8+ years. (ballpark). Lithium Cobalt rechargeable cells age whether you use them or not, but the rate of their decline has a great deal to do with average state of charge, and temperature. The irreversible wear and tear from aging will come in the form of reduced total capacity and voltage suppression.
I've read many different ideas about the self-discharge rate of li-ion cells, and I think that in many ways, all of the reports are somewhat correct, because each scenario is true if the variables change. (Like temperature and average state of charge).
Charging to 4.20V and always topping the cell off to 4.20V every few months will cause the cell to denigrate in total capacity and performance more quickly. So while the cell would always be topped off, a loss of total cell capacity that is irreversible would be accelerated.
Charging to ~4.0V instead, and maybe also storing the cell in a cool place, would give you a ~80% state of charge on tap, and a much lower rate of irreversible aging.
Under the best possible state of charge conditions, temperature, etc, after about a decade most LiCo cells are going to have less than 80% of their original total capacity, at which point they should be considered no longer fit for duty and properly recycled for safety reasons.
So far I have talked a bit about actual cell life not in relation to holding a charge. Now lets consider "holding" a charge long term...
*Most* Lithium Cobalt RCR123 size cells are going to be sold with a protection circuit. Protection circuits have a parasitic drain effect. On smaller size cells, that drain rate becomes more noticeable. I would estimate 7-20% of the cells capacity will be drained off due to the PCB (protection circuit board) itself. (depends on the specific board).
Plan on maybe 3-9% per year of capacity degradation, (depending on conditions), and that 10-20% drained off by parasitic drain from the PCB.
Or, 13-29% per year of total loss.
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Rechargeable li-ion cells have no real business being used for long term energy storage. They are best utilized if charged and discharged frequently enough that the wear and tear from simple aging does not over-take cycle related aging. Stick to CR123s for emergency cells that you intend to have stored away for long periods of time.
Eric
