CellChart
Silver-oxide watch (SR)

SR521SW battery: equivalents & specs

The SR521SW, universally known as the 379, is a 1.55 volt silver-oxide watch battery measuring 5.8 mm across and 2.1 mm tall, rated around 16 mAh. It is the standard cell for small ladies' watches and one of the top ten watch sizes worldwide.

SR521SW battery drawing: 5.8 mm across, 2.1 mm thick SR521SW 1.55V + ⌀ 5.8 mm 2.1 mm
SR521SW, face and side profile, drawn to scale: 5.8 mm across, 2.1 mm thick.
Voltage
1.55 V
Size (D×H)
5.8 × 2.1 mm
Chemistry
Silver oxide (Ag2O)
Capacity
16 mAh

Same battery, other names

Every code below is the exact same cell as SR521SW. Any of them will fit and work identically.

379SR63V379D379

Compatible replacements

  • LR521 / AG0: Alkaline same-size cell; works but with lower capacity and sagging voltage. The 379 is the proper watch fit.

Buying and replacement notes

The 379 is a stock item at every watch counter and among the most reliable small cells to buy. Its alkaline twin, the AG0 or LR521, fits the same recess but is a downgrade in every respect except price. If a seller offers you a bargain multi-pack of AG0 for your watch, decline and buy a single Murata, Renata or Energizer 379 instead.

Commonly used in

small ladies' watchescompact quartz movements

Need a replacement?

Shop SR521SW batteries on Amazon →

⚠️ 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 can replace a 379?
The 379 is the SR521SW; any brand's cell carrying either code is identical at 5.8 x 2.1 mm. The alkaline AG0 (LR521) physically fits but gives shorter life and sagging voltage, so it is a stopgap rather than a true replacement in a watch.
How long does a 379 battery last?
Two to three years in a typical small analog watch. Its 16 mAh capacity and low self-discharge suit low-drain movements well. If replacements keep dying within months, the movement is drawing excess current and needs servicing rather than another battery.
Is there a difference between brands of 379?
Quality varies less than with alkaline cells, but stick to Murata (formerly Sony), Renata, Seizaiken or Energizer for watches. Their cells meet tighter leakage and self-discharge specs than unbranded imports, which matters for a battery that sits inside a movement for years.

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