Button and coin cells must be kept away from young children who might swallow them. See this article for more information about the hazards.
Shopping for button and coin cells
These are available inexpensively online, if
you don't mind waiting for international shipping. Silver oxide cells
(SR...) have more capacity than alkaline cells (LR...), have a nominal voltage slightly higher (1.55V instead of 1.5V) and are also more expensive. Zinc air cells are most often used for hearing aids.
Watch battery notes
The W on the end of an SR number (eg, SR626SW) means it's specifically designed for watch use – it meets the international IEC 60086-3 standard for watch batteries.
An S in the suffix (eg, ...SW) means it uses sodium hydroxide for the electrolyte, which Energizer says is suitable for continuous low drain devices, such as watches (source – Energizer silver oxide application manual).
Less common is a P suffix (eg, ...PW), which means potassium hydroxide electrolyte, suitable for continuous low drain with periodic high drain pulse demands, or "high drain" (eg, watches with alarms, backlights, calculators, etc).
The potassium hydroxide cells are harder to seal and thus have less salt resistance.
Just a W suffix does not necessarily indicate either low drain or high drain, but for at least some companies including Maxell and Renata it means high drain. Sadly, resellers are very likely to get mixed up, so if you want a particular type, be careful.
Low drain/high drain does not mean low capacity/high capacity. A Renata 395 (SR927SW, low drain) is 55 mAh, a Renata 399 (SR927W, high drain) is 53 mAh. (Source – Renata datasheet.)
For most uses it probably doesn't matter.
cells in this table are listed by size, diameter first. Often a thicker or thinner
cell will fit.
Remember to check how many you need – some watches take two cells.