This page attempts to explain the construction and power source of popular Eco-Drive movements from the Citizen Watch Company.
All of the findings and comments here are the results of my observations as an owner of six various models of these watches and do not necessarily represent the opinions of the Citizen Watch Company.
For more exact information contact: Citizen U.S.A.
Popular lore suggests that the Eco-Drive series of watches will run by solar power alone and "will never need a battery".
Our observations show that these watches contain a rechargeable battery to keep the watch running when there is no light falling on the solar cell. A simplified schematic operational diagram may be found on our movements page.
Primary Power Source
In the Eco-Drive design the primary power source is a solar cell located behind the translucent face of the watch. This can be seen in certain models at certain viewing angles as a cross with the intersection at the center of the watch face.
The solar cell creates electricity whenever light strikes it. Unfortunately, the amount of current which such a small solar cell produces is not enough to power the functions of a watch.
To provide the power to run the watch a rechargeable battery is installed, and the solar cell recharges the battery. Since the battery, when installed already has a charge the solar cell is able to keep up with the power demand of the watch movement.
When the rechargeable battery is fully discharged the watch stops functioning until the solar cell has charged the battery enough to run the watch.
Battery
The choice of a Panasonic Manganese Titanium Lithium rechargeable battery to power these watches is a good one since this type of battery was designed specially for this type of service. This type of battery has a capacity of more than 10 times that of capacitors of the same size.
Full specifications for this line of batteries can be downloaded HERE (Note: this is an Adobe Acrobat .PDF file).
The MT920 battery can operate for 2500 hours at a 1.2uA load with a flat operating voltage of between 1.5 and 1.2 volts.
Charging efficiency is nearly 100% with very small charging loss.
The MT920 can withstand temperatures as high as 60 Deg. C (140 Deg. F) and charge voltages as high as 2.6 volts.
The life of the battery is as much as 500 full discharge cycles to a discharge low limit of 1.0 volt. A discharge depth of 100% is possible.Battery Life
The replacement life of these batteries is hard to determine because of the many factors involved. Most of our computations suggest that, in an ideal operating environment, a battery should need replacement somewhere between 15 and 28 years.A simplistic example:
A Citizen Skyhawk can run for 4 years on a fully charged battery.
The battery may be fully charged/discharged up to 500 times.
A simplistic calculation may imply that (4 Years per charge * 500 maximum charges) = 2000 year battery life !
This, of course is wrong, because it does not take into account aging of the components of the watch, battery electrolyte, temperature and voltage aberrations and other factors.
This demonstrates why it is so difficult to determine the replacement life of one of these batteries.Movement Views - Click to enlarge picture
Citizen SkyHawk Citizen 200m Eco-Drive Chrono
A - Shows the battery holder in place
C - Shows the battery under the Stainless Steel cover plate.
D - Shows what appears to be a trimmer (may be used to adjust the frequency of the oscillator with the correct equipment).
E - Shows the 32.767kHz crystal.
B - Shows the battery cover removed revealing the MT 920 battery.Movement Construction
NOTE: the description below is based upon my observation of the movement of these watches. I have not actually disassembled these watches since both are still working.The Eco-Drive movement is typical of today's quartz watches and utilizes the most modern materials and manufacturing methods to achieve a rugged, long lasting, reliable watch.
Going from the back of the movement forward to the face we find:
1 - A stainless steel rear plate which serves as electrical grounding, electronic circuit shielding, pusher leaf springs, detent for the crown and acts as a stiffener for the movement itself. This component is made out of one precision sheet stainless stamping.
2 - Under the rear plate is located the main circuit board containing the control integrated circuit, oscillator, power conservation circuitry, and interconnects to the movement itself.
3 - Under the circuit board is the movement, an all plastic, no jewels quartz movement.
NOTE: the use of plastic for the movement plates does not indicate a cheap movement, rather a suitable material for the intended purpose. No jewels are required because the power train of quartz movements, unlike mechanical movements is under very light, intermittent load.
The use of self-lubricating plastic elements eliminates the need for cleaning and lubrication.
Gears in the movement may be made of either metal or plastic depending on model.
The movement also houses the stepper motor(s) which provide power to the gear train.4 - In models with digital display (like the SkyHawk) the display components are mounted on the front of the analog movement.
5 - The next component is the solar cell which is mounted on the front of the movement. The solar cell has a connector which goes through a space in the movement and attaches to the circuit board.
6 - The last component(s) are the face and hands.
