Now Let's Review Briefly How a Watch Battery Works and What Causes It to Supply Electrical

Batteries and Cells

Need to add Lithium, Rechargeable, 377 tech guide

Batteries vs. Cells

The term “battery” has become the ubiquitous term for the power source in an electric / electronic watch. It is however, a misnomer and should be corrected.

 Cell – An individual unit  Battery – A group of similar units

Therefore, the power source in an electric / electronic watch is most always a cell. On rare occasions, more than one cell may be required (still requiring cells) or even a rechargeable unit containing many cells (a true battery).

Cell construction

A cell contains chemicals within 2 metal cans. These chemicals react to each other across the membrane, creating electric potential.

The cathode is (+), the anode (-). Into the bottom cathode can of a silver-oxide cell is inserted a pellet of divalent silver-oxide (Ag2O). On top of this metallic oxide is placed the membrane, a micro-porous fiber cellulose disc. Over the disc is placed an absorbent material not unlike a cotton pad containing a very precisely metered amount of electrolyte (sodium hydroxide).The anode can, together with its zinc pellet and gasket, are then pressed onto the bottom cathode can and crimped to mechanically seal the unit. Inside, a chemical reactor is formed which can convert the stored chemical energy into electric potential when a load is applied across its (+) and (-) surfaces. Chemical Reaction

When the (+) and (–) are "loaded" by the demands of the watch, electrons begin to flow from the (–) to the (+), creating zinc oxide. The electrolyte carries and releases ions to the anode and the zinc to give up electrons that are needed to power the watch. After powering the watch, these electrons return to the polarizer (cathode) where silver oxide is reduced to pure silver (quicksilver). The time it takes to deplete the cell completely depends on the severity of the load. A properly balanced cell, which provides the proper volume of silver as compared to the zinc anode in precisely metered proportions, will avoid gassing (hydrogen) and thus prevent causing internal pressures.

Current, Voltage and Capacity

Watch cells are rated with both a voltage and a capacity. The standard silver-oxide cell provides an initial voltage of ~1.55v which stabilizes to around 1.5v for quite some while. See the chart below for details.

(Chart from Energizer 377 tech guide) Batteries and Cells

The total energy capacity of the cell is also given in the form of mAh. For example, the same 377 is rated at XXmAh. This is the total amount of electric current which is possible to be had from this cell under perfect conditions. If a cell is drained too quickly or drained too slowly (significant time on the shelf) the total power output will not be realized.

The current flow from the cell is also a consideration when choosing cells. Examples: Watch #1 LCD Watch #2 Electroluminescent LCD with alarm The plain LCD watch will require fairly little current. When the electroluminescence and alarm are not functioning, watch #2 will also require about the same current. When the light is triggered or the alarm goes off, the current consumption is highly increased. This maximum level of current output necessitates different cell chemistry / design in order to operate correctly and efficiently. This brings us to what are called “High” and “Low” drain cells. A high drain cell is designed for conditions which require a high current drain while low drain cells are designed to take advantage of the lower drain requirements and will not work properly in high drain situations.

How it’s all related

A cell is a chemical storage device, a chemical to electrical reactor and it has a slight capacitive nature. Understanding how these work together to create electric potential is critical to understanding how the cell works.

The chemical storage is related to the mAh rating. More chemical storage, more mAh possible. The voltage of the cell is a function of the reactor’s ability to create electrical potential AND the slight capacitive nature of the cell. See the following diagram:

The ability to get coffee out of our system is analogous to get working potential out of a cell. We have our chemical supply (Coffee Beans) in large quantity. We also have the chemical to electrical reaction (Coffeemaker) which converts the chemical potential (Coffee Beans) into voltage (hot coffee). Now let’s put this system into action and do some work with our electrical potential (get a hot cup of coffee). If I put a very small drain on the system (1 cup), the total voltage drops a little but is quickly restored (my 1 cup of coffee drains the carafe to 11 cups, which is immediately replenished by the percolator). Now a large load is placed upon the system (entire office goes on coffee break). The small capacity of the system is drained and voltage measurement is no longer from the reserve, but from the chemical reaction itself. (Carafe runs out of coffee and the potential of the system to do work is defined by the percolator which converts the coffee beans into coffee). In other words, the true voltage of the cell is not simply a reading of potential across the (+) and (-) terminals, but the reading when the system is under load.

Testing the Voltage of a Cell

From our previous analogy, we learned that the voltage reading from a cell can vary 2 Batteries and Cells depending upon the condition it is in when it is checked. For a cell in a watch, we need to check its ability to create electric potential, for this is the true measurement of how much reserve the cell still has, thus, how “good” the cell really is. To do this we have 2 options.

1. Check the cell while it is under operating conditions (in the movement) 2. Apply a load to the cell during checking to simulate the drain of the watch

Shelf Life/ Service Life:

The amount of time that a watch cell spends in stock or inventory is termed "shelf life". Cells will lose a certain amount of their mAh capacity during storage and should be considered a perishable item. Inventory control and "first in, last out" stock rotation routines will ensure that maximum electrical capacity is maintained in replacement cells.

A standard shelf life estimate for watch cells stored under cool, dry conditions is retention of 90 to 95 per cent of total capacity after one year of storage. Cell Type Height (mm) Storage Time (mo)

Silver oxide Ag2O/Zn < 2.15 6

Silver oxide Ag2O/Zn >2.15 12

Lithium Li/MnO2 18

"Service life" is defined as the amount of time that the cell spends in a particular electronic timekeeping device. This time period will vary from watch to watch and from cell type to cell type. A normal, generic expected service life for today's watch cells are usually not less than one year and not more than three years for silver-oxide systems.

Handling of cells

Batteries should he handled only with plastic or fiber tweezers, and never with metal ones, which would cause short-circuiting. Oil from fingers will cause a slight connection between (+) and (-), quickly draining the cell and corroding the gasket. Batteries should show no signs of deformation; their gaskets should be clean and free from white crystals or liquid. If a cell becomes dirty, wipe it with a clean cloth. Never use a solvent for this purpose.

Cell storage conditions

Cells must be stored in their original packaging in a cool and dust-free environment under the following conditions:

1. Temperature: 50°F to 75°F (never above 85°F) 2. Humidity: <50%

If the cells are packaged incorrectly, they may short-circuit, causing dead cells, leaking acid and heat. It is therefore important to ensure that the cells cannot be mixed within their Batteries and Cells packaging. In the event of an overall short-circuit, the cells will heat considerably, reaching a dangerous level and thus leading to the combustion of any surrounding flammable material. There is also a high risk of explosion if the batteries are recharged outside the watch or thrown into the fire. Please follow all manufacturer recommendations.