The SR731SW, sold as the 329, is a 1.55 volt silver-oxide watch battery measuring 7.9 mm across and 3.1 mm tall, with about 37 mAh of capacity. It slots between the 362 and 392 in the crowded 7.9 mm diameter family of watch cells.
SR731SW, face and side profile, drawn to scale: 7.9 mm across, 3.1 mm thick.
Voltage
1.55 V
Size (D×H)
7.9 × 3.1 mm
Chemistry
Silver oxide (Ag2O)
Capacity
36-37 mAh
Same battery, other names
Every code below is the exact same cell as SR731SW. Any of them will fit and work identically.
329
Buying and replacement notes
The 329 is a somewhat less common size, seen in older and mid-century-style quartz movements, so smaller shops may need to order it. Do not accept a 362 or 392 as a substitute: at 2.1 and 3.6 mm tall respectively they bracket the 329's 3.1 mm but neither seats correctly. Renata and Murata both keep this size in production.
⚠️ Keep away from children. Swallowed button batteries, especially 20 mm lithium coins, cause severe injury within hours. If a child swallows one, call the 24/7 National Battery Ingestion Hotline (US) at 800-498-8666 or poison control at 800-222-1222 immediately. What every parent should know.
Frequently asked questions
What battery is a 329 equivalent to?
Only the SR731SW; both codes mark the same 7.9 x 3.1 mm silver-oxide cell at 1.55 V. No alkaline version is commonly sold, and neighboring watch sizes such as the 362 or 392 differ in height and do not seat properly.
My jeweler doesn't stock the 329. What should I do?
Order it online from a watch-parts supplier or direct-from-manufacturer listing; Renata and Murata both make it. Avoid the temptation to shim a shorter cell with foil, which creates unreliable contact and can short against the case back.
How long does a 329 last in a watch?
Two to three years is typical. Its 37 mAh is generous for a low-drain analog movement. If the watch is a vintage piece, have the movement's current draw checked when the battery is changed, as older calibers can drift out of spec and eat cells.