Open up the battery tray on a red dot sight and you will find one of a handful of small cells doing the work. Which one your optic takes is not a throwaway detail. It sets how long the sight runs between swaps, how easy that swap is when you are away from home, and even how compact the optic can be in the first place.
This guide covers the four batteries you will run into most: the CR2032, the CR1632, the CR123A, and the AAA. We will keep the chemistry simple, show which optics use which cell, and explain why the choice matters when you are picking a sight.
The four batteries at a glance
| Cell | Voltage | Approx. capacity | Typical optics | Availability |
|---|---|---|---|---|
| CR2032 | 3V lithium | ~210-240 mAh | Aimpoint Micro T-2, Trijicon RMR / MRO, SIG Romeo5, Holosun AEMS, Aimpoint ACRO P-2 | Everywhere (the standard coin cell) |
| CR1632 | 3V lithium | ~120-140 mAh | Holosun K-series (507K, 407K), Holosun 508T, Steiner MPS | Common, but check a larger store |
| CR123A | 3V lithium | ~1300-1500 mAh | EOTech XPS/EXPS holographic, larger tube optics | Sporting goods / online; less common in a junk drawer |
| AAA | 1.5V alkaline or lithium | ~1000-1200 mAh | Vortex SPARC AR Gen II, Aimpoint CompM5 | Universal (any store) |
A note on the numbers: coin-cell capacity varies by brand and by how the manufacturer rates it, so the figures above are typical ranges rather than exact specs. The four digits in a cell’s name encode its size, not its capacity. A “CR2032” is 20mm across and 3.2mm thick; a “CR1632” is 16mm across and 3.2mm thick. That is why a thicker or wider cell will not drop into a tray built for a smaller one.
CR2032: the default coin cell
The CR2032 is the battery most red dots ship with, and for good reason. At 3 volts and roughly 210 to 240 mAh in a 20mm coin, it packs the most energy of the common coin cells while staying small enough for a compact optic. You can buy one at a hardware store, a pharmacy, or a grocery checkout, which makes a field swap painless.
Optics that run a CR2032 include the Aimpoint Micro T-2, the Trijicon RMR and MRO, the SIG Romeo5, the Holosun AEMS, and the Aimpoint ACRO P-2. Manufacturers commonly quote tens of thousands of hours on one cell at a low or medium setting. Aimpoint, for instance, claims around 50,000 hours of constant-on use at a moderate setting for the T-2 on a single CR2032. Run any of these at maximum brightness and the real number drops well below the headline, but at everyday settings a CR2032 optic can sit powered on for years.
CR1632: the micro-optic cell
The CR1632 is the same 16mm diameter as the thinner CR1620 but a little taller, and it is the cell of choice for many micro and subcompact pistol optics. It holds roughly 120 to 140 mAh, a little over half a CR2032’s energy, because the smaller footprint of a carry optic simply does not leave room for a bigger battery.
You will find a CR1632 in the Holosun K-series (the 507K and 407K), the Holosun 508T, and the Steiner MPS. Some of the smallest carry optics drop even further: the Holosun EPS Carry uses a CR1620, which is thinner still and holds less (the full-size EPS steps up to a CR1632). The takeaway is consistent. The more an optic shrinks to ride a subcompact slide, the smaller its cell, and the shorter its runtime compared with a full-size sight on a CR2032.
CR123A: the high-capacity option
Step up to a holographic sight or a larger tube optic and you often step up to the CR123A. This is a 3-volt lithium cell, but physically much larger than a coin: around 1300 to 1500 mAh, several times the energy of a CR2032. That extra capacity is how these optics deliver long runtimes from a single battery despite power-hungry emitters.
EOTech’s XPS and EXPS holographic sights are the classic CR123A example (some EOTech holographic models, like the 512 and 558, run on AA cells instead). A holographic reticle draws more current than a simple LED red dot, so the larger cell does real work here. The trade-offs are size and stocking: a CR123A is bulkier, costs a bit more per cell, and is more of a camera-store or sporting-goods purchase than something in the kitchen drawer. Most shooters who run a CR123A optic keep a spare or two with the gun.
AAA: the familiar tube-optic cell
Some tube-style optics skip coin cells entirely and run a single AAA, the same 1.5-volt battery as a TV remote. The Vortex SPARC AR Gen II and the Aimpoint CompM5 both take one. The appeal is availability: a AAA is genuinely universal, and you can find one anywhere, in alkaline or longer-lasting lithium form. The optic’s circuit steps the 1.5 volts up to what the emitter needs, so runtime stays competitive with the coin-cell sights.
One close relative is worth a mention. The Aimpoint PRO uses a DL1/3N (also written 1/3N or CR1/3N), a 3-volt lithium cell that is essentially the height of two stacked coin cells. It is less common than a CR2032, so if you run a PRO it is worth keeping a spare, even though Aimpoint rates it for tens of thousands of hours.
Why the cell choice matters
When you are comparing two sights, the battery they take tells you three useful things before you ever read a runtime spec.
Smaller optic, smaller cell, shorter runtime. A subcompact carry optic on a CR1632 or CR1620 cannot match a full-size sight on a CR2032, and neither matches a holographic sight on a CR123A. If long unattended runtime matters more than a tiny footprint, that points you toward a larger cell.
Shake-awake and solar stretch any cell. Motion-activated sleep (often called shake-awake) powers the emitter down when the optic sits still, and a solar panel can run the dot in daylight without touching the battery at all. Both features meaningfully extend runtime, and they matter most on the small-cell optics that have the least energy to spare.
A common cell is easier to live with. A CR2032 or a AAA can be replaced almost anywhere, which is reassuring on a defensive optic. A CR1632, a CR123A, or a 1/3N is easy enough to buy ahead of time but less likely to be on a shelf at the corner store, so plan to keep spares. The smartest habit, whatever cell your optic takes, is to change the battery on a calendar (many makers suggest once a year) rather than waiting for the dot to dim.