Key Takeaways
- SR626SW and 377 are two names for essentially the same silver-oxide button cell.
- Both measure 6.8mm × 2.6mm and deliver a stable 1.55V.
- They’re interchangeable in nearly all watches and small electronics.
- Buyers should double-check thickness tolerance across brands, and avoid confusing 377 with the physically different SR621SW (364).
If you’ve popped open a watch case back and found “SR626SW” printed on the old cell, then gone to a store and only seen “377” on the shelf, you’re not alone. Button cell naming is one of the most confusing corners of the battery world, and the SR626SW/377 pair is the single most common source of that confusion — mostly because it’s also the single most common watch battery on the planet.
Watch Battery SR626SW Guide
The SR626SW is a silver-oxide button cell built around the IEC 60086-3 coin-cell naming standard. Breaking down the code tells you almost everything about the battery:
- S – silver-oxide chemistry
- R – round cell
- 626 – dimensions, referencing the 6.8mm diameter and a nominal height code
- SW – “SW” grade, indicating a low-drain cell optimized for steady, continuous discharge
That last part matters. Watches draw a small, constant current to run the quartz oscillator and stepper motor. They need a cell that holds a flat voltage curve for months or years, not one built for short power bursts. The SR626SW does exactly that. According to Renata’s technical datasheet, it delivers a stable 1.55V across nearly its entire service life on a light, continuous load, then drops off sharply near end-of-life — a predictable failure pattern that’s easy to plan around.
Physically, the SR626SW measures 6.8mm in diameter and 2.6mm in height, weighs roughly 0.4g, and carries a typical capacity in the 24–28mAh range depending on manufacturer and test method — Renata, Maxell, Seiko Instruments, Energizer, Duracell, and Panasonic all produce versions of this cell. It’s the standard battery in analog quartz watches from brands like Fossil, Timex, Casio, and Swatch, and it also shows up in calculators, glucose meters, laser pointers, and small remote controls.
What Is a 377 Battery?
“377” isn’t a different battery — it’s a different naming convention for the same cell. While the SR-prefix system comes from the IEC standard, “377” comes from the older ANSI/NEDA numbering scheme that many battery brands, especially in North America, still print on their packaging. Energizer’s own datasheet lists its 377/376 silver-oxide cell under the designation “ANSI/NEDA-1176SO, IEC-SR66” — confirming, straight from the manufacturer, that 377 and SR626SW (SR66) describe the same physical part under two standards bodies.
This is why retail packaging often lists both: “377/SR626SW” on the same blister pack. It’s the same part, just marketed under two labels for two different audiences — industrial and technical buyers who look for SR-codes, and retail consumers who grew up seeing three-digit ANSI codes on watch battery cards.
SR626SW vs. 377: Are They the Same?
In practical terms, yes. Both codes describe a 1.55V silver-oxide button cell with the same footprint, built to the same physical and electrical specification. The table below lays out the comparison directly.
| Spec | SR626SW | 377 |
| Chemistry | Silver-oxide | Silver-oxide |
| Nominal voltage | 1.55V | 1.55V |
| Diameter | 6.8mm | 6.8mm |
| Height | 2.6mm | 2.6mm |
| Typical capacity | ~24–28mAh | ~24–28mAh |
| Discharge type | Low-drain, flat curve | Low-drain, flat curve |
| Naming standard | IEC 60086-3 | ANSI/NEDA |
| Standard designation | SR66 | 1176SO |
| Typical use | Watches, small electronics | Watches, small electronics |
Because the specs line up across the board, a 377 battery will drop into an SR626SW-rated device without any electrical or functional issue, and vice versa. Individual manufacturers hold slightly different tolerances on the 2.6mm height — a difference of a few tenths of a millimeter between brands. That can occasionally affect fit in watches with a very shallow battery compartment. It’s a manufacturing-tolerance issue between brands, though, not a difference between “SR626SW” and “377” as designations.
377 Types Calculator
Because so many equivalent codes exist for this one physical cell, it helps to think of equivalence as a quick lookup rather than a set of names to memorize. Here’s the cross-reference for the 6.8mm × 2.6mm silver-oxide family:
| Also known as | Chemistry | Notes |
| SR626SW | Silver-oxide | IEC standard code; low-drain “SW” grade |
| 377 | Silver-oxide | ANSI/NEDA code (1176SO); same cell as SR626SW |
| SR626 / SR66 | Silver-oxide | Same cell family; SR66 is the IEC size designation |
| AG4 | Alkaline | Same physical size, different chemistry — 1.5V, declining curve |
| LR626 / LR66 | Alkaline | Same size, alkaline equivalent of AG4 |
| 376 | Silver-oxide | Higher-drain variant for backlit digital watches |
| D377, V377, S626E, G6, SW66 | Silver-oxide | Additional brand-specific labels for the same SR626SW cell |
To find your battery’s exact match, take whichever code is printed on your old cell and check it against this table rather than assuming by name alone. If the code starts with “SR” or ends in “377,” it’s almost certainly the silver-oxide 6.8mm × 2.6mm cell. If it starts with “LR” or “AG,” it’s the alkaline version of the same footprint — physically interchangeable, but electrically different (see below).
When They’re NOT Interchangeable
The SR626SW/377 pair is safe to swap. Three related mix-ups are worth flagging separately, since these are the ones that actually cause problems:
- Alkaline vs. silver-oxide (LR626/AG4 vs. SR626SW/377). Both fit the same slot. Alkaline cells start around 1.5V and sag toward 0.8–1.0V over their life. Silver-oxide cells hold a flat 1.55V until near end-of-life. Precision electronics like quartz watches are calibrated for the steady silver-oxide voltage curve, so an alkaline substitute can cause timing drift over the life of the cell.
- 376 vs. 377. The 376 is a higher-drain variant meant for devices with occasional power spikes, like a digital watch backlight. The 377 (SR626SW) is low-drain, built for continuous, steady current. Some manufacturers sell a dual-rated “376/377” cell, which is Energizer’s approach, covering both use cases in a single product.
- SR626SW vs. SR621SW (364). This is the mix-up that actually causes fit problems. The SR621SW is only 2.1mm tall versus the SR626SW’s 2.6mm. That 0.5mm difference is enough to prevent reliable contact in one direction, or prevent the case back from closing in the other. Always confirm height, not just diameter, before ordering a replacement.
How to Replace Your Watch Battery
- Open the case back with a case knife or case-back tool (screw-down backs need a wrench-style opener).
- Note the exact code printed on the old cell — this is more reliable than guessing from the watch brand or model.
- Cross-reference that code against the table above if it isn’t already labeled SR626SW or 377.
- Insert the new cell with correct polarity, matching the old cell’s orientation in the holder.
- Reseal the case back and confirm the watch is running before fully closing any water-resistant gasket seal.
Storage, Shelf Life, and Handling
Unopened SR626SW/377 cells store well — manufacturer data shows self-discharge under 5% per year at room temperature, so a sealed cell bought today performs close to spec years later if kept cool and dry. A few practical notes for anyone stocking these as spares or sourcing for production:
- Store below 25°C where possible; heat accelerates self-discharge even in uninstalled cells.
- Don’t mix old and new cells in devices that use more than one — a partially discharged cell can drag down a fresh one.
- Dispose of used cells through a battery recycling program, not general trash; silver-oxide cells are regulated in many regions.
- Keep loose cells away from children and pets. Button cells this size are a recognized swallowing hazard — seek emergency medical attention immediately if one is ingested.
FAQ: SR626SW and 377 Watch Batteries
Q: Is a 377 battery the same as an SR626SW?
Yes. Both codes describe the same 6.8mm × 2.6mm, 1.55V silver-oxide cell — Energizer’s own datasheet lists its 377/376 product under the designation “IEC-SR66,” the same size family as SR626SW. “SR626SW” comes from the IEC standard; “377” comes from the older ANSI/NEDA system. Same physical part, two labels.
Q: Can I use a 377 battery instead of SR626SW in my watch?
Yes, in virtually all cases. The diameter, height, voltage, and discharge behavior all match, so a 377 will function identically to an SR626SW once installed. The only edge case worth a second look is a watch with an unusually shallow battery compartment, where brand-to-brand tolerance on the 2.6mm height could matter — if the old cell fit snugly, stick with a well-known brand for the replacement.
Q: Is AG4 the same as SR626SW?
They’re the same physical size but different chemistry. AG4 is an alkaline cell rated at 1.5V with a voltage that declines steadily over its life, while SR626SW is silver-oxide, rated at 1.55V with a flat voltage curve until near end-of-life. AG4 will physically fit and often works fine in low-precision devices like toys or flashlights, but silver-oxide is the better choice for anything timing-sensitive, like a watch.
Q: Why does my watch battery have so many different names?
Manufacturers and standards bodies use different numbering systems for the same physical cell. The IEC assigns codes like SR626SW, the ANSI/NEDA system assigns codes like 377, and individual manufacturers sometimes add their own part numbers on top of both. That’s how one physical battery ends up with five or six equivalent labels on different packaging.
Q: Is SR626SW the same as SR621SW?
No, and this is the pairing to watch for. They share the same 6.8mm diameter but differ in height — 2.6mm for the SR626SW versus 2.1mm for the SR621SW (also sold as 364). That 0.5mm gap is enough to cause a poor connection or a fit problem, so treat these as separate, non-interchangeable batteries despite the similar names.
Conclusion
SR626SW and 377 are two labels for the same 1.55V silver-oxide button cell, and swapping between them is safe for the vast majority of watches and small electronics. The real things to watch for are chemistry (silver-oxide vs. alkaline), drain type (376 vs. 377), and height (SR626SW vs. SR621SW) — not the SR626SW-vs-377 naming itself.
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