As in many other aspects of life, for watch batteries size matters. They
come in about 60 different sizes, but are all button cells and designed
to pack as much energy as possible into the smallest space possible.
Current watch batteries nearly all use silver oxide battery technology,
because that is most efficient when squeezing the last bit of electricity
into a confined space for storage. This also means that watch batteries
are all rated at 1.55V, and differ primarily in their physical dimensions.
Of course a large watch battery will also be able to store more electricity,
and will thus be rated at a higher capacity.
Over time, the watch industry has added new features such as background
lighting or audible alarms to watches. Some even feature calculators, digital
diaries or other functions. This has lead to the development of additional
'high drain' versions of watch batteries to feed the extra requirement for
power. In the IEC 'standard' battery designation convention, the low drain
or normal version of the watch battery will end with the letter 'W', while
the high drain version would end in 'SW'. A common watch battery is the
SR44W (or the SR44SW for the high drain version). Because both versions
have the same voltage and same physical dimensions, it is perfectly safe
to use the high drain version instead of the regular version. Using the
regular battery in a high drain watch will however mean that you'll have
to change the battery much earlier. This interchangeability has actually
led a number of watch battery manufacturers to drop the low drain version,
and only produce the high drain version. When replacing your watch battery,
it is useful to know that there isn't a reason for alarm when you're fitting
a battery that has a different designation, just because you are upgrading
to the high drain version.
Until a few years ago, several watch batteries used mercury technology.
This has now been phased out for environmental reasons. Mercury is highly
toxic, and there is no reasonable way to safely extract and recycle the
mercury in watch batteries. For most watches, there will be a suitable replacement
battery that uses silver oxide technology with the same physical dimensions.
Mercury button cells did however operate with a nominal 1.35V, rather than
the 1.55V of a silver oxide watch battery.
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