Products > Dodgy Technology

Solar cell watches like Citizen ECO-Drive, Seiko SOLAR etc

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I got this solar charged Citizen watch for my wife about 5 years ago. She used it actively for 2 years and then tossed it at a dark corner of the drawer. Now it won't charge. I left it under sun for over two months, but no.

Some people mentioned that the "capacitor" in this watch is actually a lithium battery that gets damaged if left uncharged for long time. That battery (a repackaged Panasonic MT516F) costs about 20USD. I normally replace the batteries in my watches myself and I hardly pay more than 1USD for high quality swiss batteries. They last for a minimum of 2 years, sometimes 3-4.

This solar watch thing is definitely a good example for dodgy technology. I probably will replace the movement with a regular one.


My Citizen is going strong after fifteen years.  Includes some months long intervals in very dark (but not drawer dark) conditions.  I'd be very happy if this was the dodgiest consumer good I came across.

You may have encountered a dud.  Happens to even the best products.

Likewise, my Citizen Eco-drive has been running happily for years.

Yes, it is a rechargeable lithium cell, not a capacitor. If it has been left in the dark for 3 years then it will have discharged to zero volts (even when the battery has discharged to the point where the watch stops, it still has leakage current). No rechargeable Lithium cell, regardless of it's chemistry, will tolerate that. If she had stopped the watch before putting it away (as the manufacturers and jewellers do), it would have survived.

This is a time proven technology, no dodgyness involved.

I was just thinking about this the other day. At what point should I declare the battery dead and shut the product down?

On one hand, I want the battery to last as long as possible so I don't want to shut down until it is really low.

On the other hand, it should shut down early enough to retain enough energy to survive sitting in a drawer for years.  The battery keeps draining even after shutdown and if it gets too low, it can't be charged.

Also, I think shutting down early can increase the cycle life of the battery.

I'll probably set it plenty early and then my boss will demand a smaller battery with more capacity and then I'll turn it down a bit.

Yes, that's a tricky compromise. Just in the case of the OP's wife's watch, it goes into a low battery warning mode, where it alternates one second advances of the second hand with a gap and 2 second advance. This probably drops the power consumption of the stepper a little but there's a limit to what you can do with just a second hand (the other hands being geared from it). I think more expensive models go into hibernation, where the on-chip counters continue to run, but the hand stepper is disabled until more power is received, at that point it re-syncs, clever stuff. With any solar (or battery) powered device, I guess you ultimately have to come to the conclusion that if somebody throws it into the corner of a dark drawer for 3 years, they no longer want it.

In the case of your product, based roughly on the thoughts above, I guess the questions you need to ask yourself are...

1. How critical is it's application? The more critical, the longer you want it to run before shutting down - effectively sacrificing battery life for performing its critical function for as long possible.

2. Can you provide any meaningful warning of low battery that can attract attention? That may allow you to delay shutdown for longer - assuming that the warning doesn't massively increase the current drain!). The watch and smoke alarm technique.

3. Can you implement any sort of hibernate mode as an interim step between operation and shutdown? - retaining timekeeping, volatile data etc. for instance.

4. How long, and how likely is it to get put away for a long time (presumably that makes it non critical)? If it's a long time then you obviously want to shut down earlier.

5. What battery chemistry (you're presumably talking Lithium)? Some are a little better at withstanding deeper discharge than others. Typically the ideal long term storage capacity is 40%. If you want absolute resistance against long periods of disuse this might be a guide, leaving a reasonable margin before battery minimum capacity is reached.

Without knowing something about the product, it's difficult to say more. You might want to start a thread on it in the Products and Design section (or Power and Renewable) as I suspect that this one is destined to sink into the mire of forgotten ones.


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