Quartz Watch Batteries and Capacitors

Quartz Watch Cells

Written By Alex Hamilton

Alex Hamilton is a watchmaker, collector of fine watches and writer of all topics in the study of Horology.

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A Brief History on Quartz Watch Batteries

Battery powered watches were experimented with during the 20th century. They first became popular in the late 1950’s but were not quartz at the time. They used a small button cell (so called because of their size and shape).

There were three basic technologies explored.

  • Electro-mechanically impulse balance
  • Electronically impulse balance
  • Tuning Fork

Suffice to say they performed well, but not sufficiently better than the mechanical watches being sold alongside them to warrant their higher cost. Thus, battery powered watches only held a minor share of the market for about 20 years, until the development of the affordable quartz watch in the early 1980’s.

The improvement in timekeeping was exceptional as quartz watches would keep time within several seconds per week. Many of these early quartz watches managed to keep time within a few seconds per month.

As sales volumes increased, this led to a further reduction in cost and before long a vast range of quart watches became available at ever price range.

An Overview of the Quartz Watch Battery

Cut away view of quartz watch battery
All quartz watches need a supply of electricity to operate the electronic circuit, the crystal oscillator, and the time display. Most quartz watches use a battery, or more correctly, a cell.

All cells consist of electrodes, which are connected to the positive and negative terminals on the outside of the cell. Between the electrodes is an electrolyte. Electrical energy is produced by chemical reactions between the two terminals and the electrolyte.

There are two fundamental types of cells that you need to know about: the primary cell and the secondary cell. There is also the capacitor, which uses an entirely different technology and is not actually a cell but is used in the same role.

Primary Cell

The primary cell generates electricity as the chemical reactions take place inside it. The original electrode materials are chemically transformed into non-reactive materials. when all the original material has been transformed, the chemical reaction stops, and the cell no longer generates electricity: it is exhausted or considered “flat”.

The primary cell will provide electricity until it is exhausted, at which point it must be discarded.

Optional Secondary Cell

The secondary cell is like the primary cell in its construction, but different materials are used, and the chemical reactions are reversible. That is, if electrical current is forced backwards through the cell when it is exhausted, the original electrode materials are reconstituted. Eventually all the electrode material is restored to its original form, at which point the cell will produce electricity as if it were new.

This process is called recharging the cell. Secondary cells can be discharged and recharged a few 100 times before degradation starts to become a problem.

Thus, the secondary cell acts like a store for electricity. From fully charged, it produces electrical current until it becomes fully discharged, or “flat”. At this point it must be recharged.

The fact that it stores electricity means it can be used with a system which generates electricity. Examples include solar powered watches, and those powered by the movement of the wearer’s wrist.

The Quartz Capacitor

A capacitor stores electricity and can be charged and discharged repeatedly, just like a secondary cell. Indeed, capacitors and secondary cells are-in some roles-interchangeable, though a capacitor can only store a tiny amount of electricity compared to a cell of comparable size. A capacitor has one particularly useful feature which we will discuss later.

 Cell Characteristics

There are five important characteristics of a cell:

  •  voltage
  •  capacity
  •  size
  •  internal resistance
  •  self-discharge

Voltage

The voltage produced by the cell is determined by the material it is made from.

Mercury

The Mercury cell was the first type to be widely used. It produced around 1.35 volts. It is no longer made, due to environmental problems with the use of Mercury.

Silver Oxide

Most watch cells in use today are silver oxide. They were introduced after Mercury cells, and now have replaced them entirely. The silver oxide cell produces around 1.55 volts.

Lithium

The lithium cell generates 3 volts. It is used in applications which require a lot of power, such as the backlight for a digital display, or multifunction chronographs.

Cell Capacity

For both primary and secondary cells, the rate of the chemical reactions is determined by the amount of electrical current drawn from the cell. That is, if we draw more current from the cell, the chemical reactions occur more quickly, and the cell becomes exhausted more quickly. Therefore, the cell has a certain electrical capacity.

Electrical current is measured in amps, as you know. The current a watch cell could provide is exceedingly small, so we normally use milliamps instead (thousands of an amp), when giving the current draw from a watch cell. The capacity of a cell is measured in milliamp-hours (mAh). For example, a cell with a capacity of 100 mAh can provide 100 mA for one hour (in theory-more details later). If you draw only 10 mA from it, it will last for 10 hours. With a one mA current draw, it will last for 100 hours. In each case the current (in mA), multiplied by the time (in hours) always comes to 100.

Obviously, a cell with a capacity of 200 mAh will provide twice the current for a given period or will last twice as long for the same current. Therefore, it has twice the capacity.

Note that if you draw high currents from a cell, it will exhaust it more quickly than the calculation we have just described would suggest. For instance, if you draw 100 mA from a 100 mAh cell it will probably not last the full one hour we would expect. Button cells are customarily rated at their 100-hour capacity: that is, a 100 mAh cell will provide 1 mA for 100 hours.

Battery Life Calculation

Battery life can easily be calculated using the following formula.

Battery life in months = capacity of the battery in mAh X 1000 (to convert mAh into µAh) divided by the consumption of the watch X 730 (the average number of hours in a month).

Example: Watch caliber ETA 255.485 with a consumption of 1.06 µA.
Life in Months= 25 mAh x 1000 = 32.3 months

1.06 µA x 730

Size

Button cells are sized by their height and diameter. Both sizes are given in millimeters. The general rule is, the larger the cell, the larger its capacity. Watch cells vary between 0.9 mm and 5.4 mm, and between 4 mm and 11.6 mm in diameter.

Internal Resistance

There are limits to how quickly the chemical reactions in a cell can take place. This limits the maximum current that can be drawn from the cell. The effect is rather like having a resistor inside the cell. As current is drawn from the cell, the internal resistance causes the terminal voltage to fall. The higher the internal resistance, the less current can be drawn from the cell.

Different types of cell have different internal resistances. We will look at this again a little later.

Self-Discharge

All cells gradually discharged themselves, even when they are in storage. In the case of a primary cell, the effect is to reduce the capacity of the cell. A primary cell loses around 5% of its capacity every year. Clearly, a cell which has been in stock for a few years will have lost a significant amount of its charge and will have a shorter service life.

Secondary cells also discharged themselves overtime, and at a higher rate than primary cells. Obviously, secondary cells can be recharged, so it does not mean the cell must be discarded.

Capacitors also lose their charge, at a similar rate to secondary cells.

Self-discharge is undesirable, but sometimes it must be balanced against other cell characteristics.

Low drain and High drain cells

Silver oxide primary cells come in two types: low drain and high drain.

Low drain cells have a high internal resistance, and therefore cannot provide much current. This means they are no good in applications which require a high current drain. Examples would include any watch with a backlight, or with an alarm.

However, low drain cells have an extremely low rate of self-discharge and are very resistant to leakage which would damage the movement. Therefore, they should always be used except in cases where a high drain cell is clearly required. Low drain cells are suitable for all normal analog watches, or digital watches with no special functions. Most digital watches today are multifunction and require a high drain silver oxide cell, or more commonly a lithium cell (3 V).

Low drain cells lose up to 5% of their charge per year, whereas high drain cells lose up to 10% per year. in addition, high drain cells are more prone to leakage when discharged.

Low drain cells use sodium hydroxide for the electrolyte. High drain cells used potassium hydroxide.

Discharge Curve

Battery discharge chart
Note that all three cell chemistries maintain an almost constant voltage as they discharge, with the voltage dropping off steeply right at the end of the cell’s life. This constant voltage is regarded as a good feature, but it has one significant disadvantage: it is not possible to predict the remaining charge in the cell by measuring its voltage (except at the point where it falls steeply right at the end, which we can use to detect end-of-life or EOL).

This might be important when we are using a secondary cell, if we knew the level of charge in it, we could decide whether it needs recharging.

Now observe the discharge curve of the capacitor. The voltage from the capacitor is directly proportional to the charge remaining in it. Therefore, simply by measuring the voltage we can determine the exact level of charge and indicate to the user how much longer the watch is likely to run. The wider voltage range from the capacitor means we need more sophisticated electronic circuitry to handle it, but this is worth doing for some watches.

The Seiko Kinetic

The popular Seiko kinetic range was the first watch in large scale production to use a capacitor. Electricity is generated by an alternator in the watch; this alternator turns because of the movement of the wearer’s wrist. The electricity is stored in the capacitor and runs the watch when it is not being worn. By measuring the voltage of the capacitor, the anticipated run-time before exhaustion can be determined and communicated to the user.

In the more recent years of production, Seiko replaced the capacitor with a rechargeable cell. This new cell has a much greater electrical capacity than the capacitor it replaces, and thus provides a longer runtime off the wrist. As we mentioned earlier, most rechargeable cells have an almost flat discharge curve, making it impossible to determine the remaining charge. Seiko developed a special cell chemistry which provides a falling voltage as the battery becomes discharged. This means that the rechargeable cell can be used in place of the capacitor with no changes to the electronic circuitry and will still provide an indication of the remaining charge.

Comparison of Cell Types

Cell Chemistry and Voltage

Cell Main Characteristics

High and Low Drain Silver Oxide Cells

End-of-Life or EOL Indication

It would be inconvenient if a watch simply stopped when the cell became exhausted, without warning the user in advance. Quartz watches with a second hand invariably have an end of life, or EOL indication. Most watches will make the second hand go faster, (this is called fast stepping) for 2, 3, four or even five seconds at a time, pausing for the appropriate time before fast stepping again. Thus, the watch keeps the correct time, but the unusual motion of the second hand alerts the user that a new cell is required.

Just digital watches usually have a flashing element on the display to indicate EOL.

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