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Comparison

LR721 vs SR721SW

The LR721 and SR721 occupy the same case, 7.9 mm across and 2.1 mm tall, and swap with no fitting concern. The LR721 is 1.5 volt alkaline with 18 to 25 mAh. The SR721SW, sold at watch counters as the 362, is 1.55 volt silver oxide with 24 to 27 and a flat discharge.

LR721 and SR721SW drawn side by side at the same scale: LR721 is 7.9 mm across, 2.1 mm thick; SR721SW is 7.9 mm across, 2.1 mm thickLR721SR721SW LR721 1.5V + ⌀ 7.9 mm 2.1 mm SR721SW 1.55V + ⌀ 7.9 mm 2.1 mm
LR721 vs SR721SW at the same scale: both 7.9 mm across, 2.1 mm thick.
LR721SR721SW
ChemistryAlkaline (Zn/MnO2)Silver oxide (Ag2O)
Voltage1.5 V1.55 V
Diameter7.9 mm7.9 mm
Height2.1 mm2.1 mm
Capacity18-25 mAh24-27 mAh
Also sold asAG11, LR58, L721362, SR58, V362, SR721W (361, high drain)

Availability decides this one more often than performance does. The AG11 is among the less common alkaline sizes on retail pegs, while the 362 is a staple that watch counters keep in depth, alongside the 364, 371 and 377. If a device takes this size you may well find the silver cell easier to buy, and it is the better battery anyway.

The performance gap follows the usual pattern. A 362 runs a simple analog quartz watch two to three years at a steady 1.55 volts, so the movement keeps accurate time right up to the day it stops. An alkaline AG11 manages roughly a year, drifting as its 1.5 volts falls away. Midsize quartz movements are the main habitat for this size, which is why repairers reach for silver.

Codes to recognise: LR721, AG11, LR58 and L721 for the alkaline; SR721SW, 362, SR58 and V362 for the silver, with SR721W, the 361, as the high-drain version for watches driving a light or alarm. One habit worth keeping either way: do not leave a dead alkaline button cell sitting in a device, because spent alkaline cells leak more readily than silver oxide.

All about LR721 All about SR721SW

Frequently asked questions

Is the 362 a drop-in for an AG11?
Yes. Both cells measure 7.9 by 2.1 mm, and no watch movement objects to the small step up to 1.55 volts. The 362 gives 24 to 27 mAh against 18 to 25, and holds its voltage flat, so you get two to three years of service instead of roughly one.
Why can I not find AG11 batteries in shops?
Because almost everyone buying this size is servicing a watch, and watches want silver oxide. Counters stock the 362 and 361 instead. If your gadget calls for an AG11, a 362 fills the same 7.9 by 2.1 mm space and lasts longer, so nothing at all is lost in the substitution.
What does the 361 do differently?
The 361, or SR721W, is the high-drain version of the same silver cell in the same case. It is formulated for watches that drive a light, an alarm or chronograph hands. For a plain two-hand or three-hand watch, the low-drain 362 is the correct and cheaper pick.

⚠️ 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.