Watch batteries used in quartz watches, specifically Silver Oxide cells, often leak. (click here to learn more about watch batteries)
The electrolyte used in a Silver Oxide watch battery is highly alkaline and corrosive and so when a battery leaks inside a watch it can be catastrophic for the movement.
In a combined survey of over 3000 watches where the battery had expired, the percentage of those expended batteries that had leaked was more than 40%. (The full conclusions of the survey are towards the end of this article).
Of those leaked batteries, a third, or between 12-15% of the total number of watches examined, were sufficiently damaged to warrant either a full-service or total replacement of the watch movement.
In the remaining two thirds of the cases where the battery had leaked, the damage done was not severe enough to warrant a service of the watch or an exchange of movement, but rather a simple clean-up, which will be demonstrated later where I will show you how to clean the battery terminals in a watch.
As you can see, this under-reported problem can be the cause of great expense and inconvenience to the owner of a quartz watch. There are however certain steps you can take to limit your exposure to any potential problems, which this article hopes to introduce.
Why do Watch Batteries Leak?
To learn why Silver Oxide batteries leak so frequently, it is important to understand how the battery is assembled.
The watch battery is split into two distinct parts, the anode can (negative) and the cathode can (positive). The anode is a gel containing a mixture of the negative electrode (Silver Oxide) and the electrolyte (Sodium Hydroxide). The cathode contains the positive electrode (Zinc). Each part is surrounded by a metal shell, coloured grey in the above image, which forms the outside of the cell as well as acting as the corresponding positive and negative contact points.
A separator, which is usually a mesh, prevents the electrodes from mixing but allows ions to flow between each half of the cell.
Although the electrodes do not touch one another, they are electrically connected by the electrolyte.
The insulator ensures that the two parts of the casing are held in place and do not touch one another. It also acts as a seal preventing the chemical mixtures from escaping. It is usually made from nylon. Sometimes it is referred to as the gasket.
As a battery cell ages the separator can degrade, this can allow some of the electrolyte in the anode to come into direct contact with the zinc in the cathode. When this happens electrolysis occurs.
Because electrolysis only occurs once the separator has degraded, the possibility of a battery leaking increases as the battery approaches its shelf life expiry. Other factors such as temperature and remaining capacity also contribute to the likelihood of the electrolyte escaping.
Electrolysis is a chemical reaction where the sodium hydroxide electrolyte splits and passes its oxygen onto the zinc.
In the below formula Na is Sodium, O is Oxygen, H is Hydrogen and Zn is Zinc.
NaOH + Zn = Na + ZnO + H
Sodium Hydroxide plus Zinc becomes Sodium plus Zinc Oxide plus Hydrogen
Hydrogen is the least dense element, in fact once separated from the electrolyte it takes up over 20 times more volume inside the battery case. As the cell is sealed the hydrogen cannot escape which causes a build up of pressure.
This pressure, as it tries to equalise itself with the outside air, will try and push its way out. As we can see from the diagram above, the insulator (coloured red) is the weak point, and this is where the gas will try and escape through. If the build up is significant enough, the insulator will rupture allowing the hydrogen gas to escape.
Once this seal is broken, the highly alkaline and corrosive electrolyte is then free to leak out of the battery. Any additional Hydrogen gas being produced will help to expel the electrolyte at a faster rate.
Mercury Content in Silver Oxide Cells
Watch Battery Electrolytes
There are two types of electrolyte used in Silver Oxide watch batteries. We have already discussed Sodium Hydroxide, which is the most common type, and is in use in the general low drain batteries for standard quartz watches.
Sodium Hydroxide can also be known as either Caustic Soda or Lye
In what are termed high drain batteries, Potassium Hydroxide (KOH) is used.
Potassium Hydroxide is sometimes called Caustic Potash or Potash Lye
The function of the electrolytes is almost identical in both low and high drain batteries, however in a low drain cell the internal resistance is higher, which limits the amount of energy that can be drained at any one time. High drain cells have a much lower internal resistance, which means that if a watch demands a large amount of energy, such as for sounding an alarm, then the battery will provide it.
Both electrolytes have a similar white crystalline appearance, have a comparable corrosive nature and both are used in the form of a slightly liquid gel which helps aid their conductivity. The fact that it is a liquid is what allows the electrolyte to flow and therefore leak.
In high drain batteries the separator is thinner, which allows the cell to provide larger pulses of energy if required. The downside to this is that it is more susceptible to degrade.
As a result: high drain batteries are significantly more likely to leak than low drain cells.
Safety Notice: care should be taken when touching a leaked battery, or any chemical residue, as both types of electrolyte will decompose proteins and lipids in the skin. These chemicals have a strong exothermic reaction as they dissolve in water; the heat from which can cause a chemical burn. Latex gloves or finger cots are therefore recommended when handling expended batteries. If your skin does come into direct contact with any leak from a battery you should dry brush any trace of chemical from your skin before you attempt to wash the area with water
Coin Cell Watch Batteries (Lithium)
Identifying Watch Battery Leaks
How to Clean a Watch Battery Leak
Watch Battery Button Cell Survey Results
A combined survey was undertaken to assess the issue of watch battery leaks. The outcome of the survey comes from the evaluation of over 3000 watches where the battery had expired. The results derive from a mix of sources from both North America and Europe including independent watch repair centres, official branded workshops and solo enterprises. The variety of watches tested covers the full-range of prices and brands that occupy the majority of the market segments.
In these results the use of the phrases such as “Best” or “Better than” relates only to the cell’s relative potential to leak, and should not be taken as a recommendation of which type or brand of cell to use. When selecting a battery the overall characteristics must be considered. Quality indicating factors such as the relative flatness of the discharge rate over the cell’s lifespan, or the total capacity of the battery, were not examined.
The conclusions will be presented as a comparison to the parts considered. This is because although there was some variance in the results based mainly on geographical area, the results were all in agreement when viewed in the form of a general list as exhibited below.
Many thanks to those individuals and businesses that participated in this survey.
As discussed earlier, the need for the separator to be thinner in a high drain battery is one of the major contributory factors that lead this battery type to leak more frequently than a low drain cell.
The height of the cell appears to be the main contributory factor in the likelihood of a battery leaking.
Examples; Wide and Tall 394/SR936SW, Wide and Short 371/SR920SW
Taken in isolation these results can be misleading. However when you note that most quartz Chronograph watches use Wide and Tall cells and quartz Gents movements generally use Wide and Short battery sizes ,the relationship between the type of movement and the chance of a leak occurring is more apparent
Only brands where more than 100 examples of drained batteries were examined are included. “Origin” was noted due to the clustering of results that became evident. This could be due to the regional exchange of technology and resources where the Button cells are manufactured. The difference in each of the “Rating” is based on the derived rate for the likelihood of a battery leaking, and are separated by a difference in the potential leaking rate of at least 3%.
Although not included in the results, unbranded batteries if grouped together were by far the most likely type to leak.
How to Reduce the Risk of your Watch Battery Leaking
You will never be able to completely exclude the risk of a Silver Oxide battery leaking. The following tips however can be followed to reduce the risk of the watch needing a full-service or exchange of movement:
- Do not leave a battery in a watch unless you intend to use it
- Do not wait until a battery has died before replacing it
- When fitting a battery, if possible use one that is less than 6 months old
- Any stocks of watch batteries should be refreshed every year
- Always use a branded battery
Always remember to recycle your old Silver Oxide button cells – even leaked ones. Not only does it stop harmful chemicals contaminating the environment, but the cells retain some value and so can be sold for scrap to an appropriate recycling company.