Maxell SR521SW Battery (379) – Silver‑Oxide Watch Cell

The Maxell SR521SW is a 1.55V silver‑oxide button cell sized 5.8 × 2.1 mm (~11 mAh). It is optimized for low‑drain devices that require stable voltage and long service life—most commonly analog quartz watches and compact measuring instruments.

At a Glance

• Chemistry: Silver‑oxide (0% mercury added)
• Nominal voltage: 1.55 V
• Capacity: ~11 mAh (typical)
• Dimensions/size code: 5.8 mm (D) × 2.1 mm (H); SR521SW
• Operating temp: −10°C to +60°C
• Common code equivalents: 379, 521, SR521SW, SR63, SB‑AC

When to Choose This Cell

• Use if your device (or old battery) is marked 379, 521, SR521SW, or SB‑AC.
• Best for low‑drain, steady‑load devices such as analog watches, dial indicators, and compact quartz instruments.
• Choose silver‑oxide (SR) over alkaline (LR521/AG0) when you need longer runtime and a flatter discharge curve for accurate timekeeping.

Avoid If

• Your device specifies a different size (e.g., SR626SW, SR920SW) or a 3V lithium coin (CR series).
• The device explicitly requires a high‑drain “W” variant—follow the device marking and manual.
• You are powering hearing aids—most use zinc‑air sizes (e.g., PR70/10), not SR521SW.

Performance and Runtime

Silver‑oxide chemistry provides a flat discharge curve, keeping voltage close to 1.55V for most of the cell’s life—important for accurate watches.

• Typical analog watch: 1–2 years depending on movement and features.
• Quick estimate: runtime (hours) ≈ capacity (mAh) ÷ device draw (mA). Example: at 1 µA (0.001 mA), ~11,000 hours (~1.25 years).

Actual life varies with movement age, ambient temperature, and complication load (e.g., alarms, date change).

Compatibility and Cross‑References

This cell is widely interchangeable across brands. Match size and chemistry:

Maxell	IEC/JIS	Common Retail	Seiko/Citizen	Notes
SR521SW	SR63 / SR521SW	379 / 521	SB‑AC	Low‑drain silver‑oxide

Alkaline look‑alikes: LR521/AG0 fit physically but deliver shorter life and less stable voltage—use only if specified by the device.

Replacement Steps

1. Open the case with the proper tool; note battery orientation (+ facing up in most watches).
2. Remove the old cell; avoid touching contacts with fingers—use plastic tweezers.
3. Insert the new SR521SW matching polarity; ensure the retaining clip sits correctly.
4. Check timekeeping and reseal the case. For water‑resistant watches, replace gaskets as needed and pressure‑test.

Storage and Handling

• Store in original packaging at 20–25°C, low humidity; avoid heat and direct sun.
• Do not carry loose with metal objects—risk of short circuit.
• Shelf life: typically up to 3–5 years when properly stored.

Safety and Recycling

• Keep out of reach of children; button batteries are a choking and ingestion hazard.
• If swallowed, seek immediate medical attention or contact Poison Control.
• Do not recharge, crush, or incinerate.
• Recycle at designated battery collection points; do not dispose of in household trash.

FAQ

Is SR521SW the same as 379?

Yes. 379 is a common retail code for the SR521SW silver‑oxide cell.

Can I use SR521SW to replace LR521?

Yes—SR521SW is preferred for better voltage stability and longer life. The reverse (LR521 replacing SR521SW) may reduce accuracy and runtime.

Will this work in digital watches with alarms or lights?

Many digital watches use different sizes or higher‑drain variants. Check your watch back or manual for the exact code; use the specified “SW” or “W” type.

Is it mercury‑free?

Modern Maxell silver‑oxide cells are produced with 0% added mercury.

What if my device lists 521?

521 is a legacy/catalog number that corresponds to SR521SW/379. Match dimensions and chemistry to confirm.