I realize this topic is almost a year old, but there's still a few weeks to get in under the wire. ;-)
Here's what Citizen says about it's Eco-drive solar cells and batteries (found in the
FAQ):
Citizen Watch Co said:
According to the experimental data, both will last more than 10 years.It is also considered to lose up to around 20 % of their initial capacities in 20 years, however, we speculate, that won't cause trouble for the use of the watch.
If this is true, then Citizen's earlier claim that the Eco-drive "never needs a new battery for a lifetime" is not so horribly outlandish. Half a lifetime is still a significant time period.
From what I understand, this is not based on "charge cycles" as would typically be the case with high powered devices like cell phones. In essence, the Eco-drive battery is not run down all the way and then recharged from nearly dead. The idea is to get it up to a full charge and keep it there whenever feasible. If you give the watch a reasonable amount of light exposure on a regular basis (daily or at least every few days), then the battery level should be near full capacity most of the time. The life of the battery will be diminished if allowed to drop down to the low power reserve end and then charged back to full a number of times. However, what appears to normally diminish the battery life most is simply age. Exposure to extreme heat can help shorten the battery lifespan as well.
Up until recently I didn't know about the Eco-drive battery draining to near empty as being unhealthy for it, as I ran one of mine down to near exhaustion as a test to see if the claimed 80 days power reserve was accurate. This was at the point when the watch had been owned and used for nearly 12 years. At the 77th day, I observed that the second hand was moving in 2 second increments but that the displayed time was still correct. It had maybe a day or two left of power. So, in just over 10 years the capacity was shown as being nearly 99%. Citizen says the battery will slip to about 80% of maximum capacity after 20 years. Perhaps there's an acceleration in decay at a certain point, rather than a gradual linear decay. Unfortunately the only real way to determine this is with empirical evidence (i.e. wait it out!).
The other consideration is the life of the solar cells. From what I've read on-line, the general consensus seems to be that "all solar panels lose about half a percent a year in efficiency." This is with regard to industrial solar panels, which refers to a much larger scale. I suspect that with the extremely small solar panels installed in a solar rechargeable watch, the lifespan would be greater. Not only because the current involved is so minimal, but because they aren't on duty most of the daytime in direct sun exposure. So even if they were to degrade to 80% efficiency at the milestone of 40 years, that's still well within a usable range.
In comparison, take a solar recharged CASIO G-SHOCK with a CTL-1616 battery and you'll see that it's not expected to last more than 15-20 years. I suspect the same may be true of the older ML-2016. But CASIO has been known to understate battery life (e.g. some models supposedly lasting 2 years on a battery ending up going for 7 years or more). Already I've seen more than a handful of people post on WUS about failed batteries in their Tough Solar G-Shock watches, probably due to letting them run down to exhaustion a few times. The Raysman and G-2300 are two of the earliest solar G-Shocks. I have the G-2310 with original battery, but I have no idea of the manufacturing date as I got it used. It was first released in 2001 then produced for at least several years before the succeeding GW-2310 came out in 2009. Hopefully someone on WUS had bought an early tough solar G-Shock brand new in 2000/2001 and can eventually report on how long the original battery lasts them.
