---
title: "MR9 equivalent: PX13, MR9, RM625"
description: "The MR9 is a 1.35 V mercuric oxide (banned) cell, also sold as PX13, MR9, RM625. Full equivalents, what does not substitute, and where it is used."
url: https://atoneplace.net/battery/px625/
site: "At One Place"
---

# MR9 battery

**What replaces a MR9: PX13, MR9, RM625, EPX625, V625PX, H-D**

1.35 V mercuric oxide (banned), 15.6 × 5.95 mm (diameter × height)

The MR9 is a single-use 1.35 V mercuric oxide (banned) button cell. The codes below are the same physical cell sold under different manufacturer names, and any of them will fit.

## Specification

- **IEC designation:** MR9
- **Nominal voltage:** 1.35 V
- **Chemistry:** Mercuric oxide (banned)
- **Size:** 15.6 × 5.95 mm (diameter × height)
- **Rechargeable:** No — never charge it

## Codes that mean the same battery

*Different manufacturers, one physical battery. Voltage, chemistry and dimensions all match.*

| Code | Fits? |
| --- | --- |
| PX13 | Yes — same cell |
| MR9 | Yes — same cell |
| RM625 | Yes — same cell |
| EPX625 | Yes — same cell |
| V625PX | Yes — same cell |
| H-D | Yes — same cell |
| M20 | Yes — same cell |
| MR-9 | Yes — same cell |

## Codes that look close but are not

| Code | Why it is not a substitute |
| --- | --- |
| LR9 / PX625A / V625U | The alkaline cell sold in the same shell, and the reason a lot of vintage light meters read wrong. It starts at 1.5 V and slides downwards through the whole of its life, so the meter is over-exposing by a varying amount that no fixed compensation can correct. It fits perfectly, which is what makes it a trap. |
| S625PX silver oxide | A steady 1.55 V, which at least is constant: you can dial in a fixed exposure compensation of up to about two stops and leave it. Better than alkaline, still not 1.35 V. |
| MRB625 zinc-air | Actually 1.35 V and stable, so no compensation is needed — but a zinc-air cell breathes, and once the tab is off it is finished within a few months whether used or not. |
| MR-9 adapter with an SR43 / 386 | A machined shell the size of a PX625 with a voltage-dropping circuit inside and a cheap silver oxide cell in the middle. It gives a true regulated 1.35 V, costs more up front and then costs almost nothing. This is the correct answer for a meter you care about. |

> The MR9 is a primary cell and must never be put in a charger. Charging a sealed primary battery builds gas pressure inside a can with no vent designed for it, which is how they rupture or catch fire. Rechargeable look-alikes exist for several sizes and carry different codes — check the code, not the shape.

## Where it turns up

- Gossen and Weston handheld light meters
- Nikon F, Nikkormat and Canon FT camera meters
- Olympus Trip 35 and Rollei 35 meters
- Vintage hearing aids and instruments

> Banned. Mercuric oxide cells were prohibited for consumer sale in the United States by the Mercury-Containing and Rechargeable Battery Management Act of 1996 and across the EU by the batteries directives, on toxicity grounds. What made them ideal for light meters is exactly what cannot be replaced casually: a mercury cell holds 1.35 V almost perfectly flat until it dies, and a meter calibrated against that reference gives a wrong reading on anything else. Dimensions here are for the PX625; the PX13 is the same cell under Mallory's other number.

## Common questions

### What is equivalent to a MR9?

PX13, MR9, RM625, EPX625, V625PX, H-D, M20, MR-9 — all the same 1.35 V cell under different manufacturer codes.

### Can I use a LR9 / PX625A / V625U instead of a MR9?

The alkaline cell sold in the same shell, and the reason a lot of vintage light meters read wrong. It starts at 1.5 V and slides downwards through the whole of its life, so the meter is over-exposing by a varying amount that no fixed compensation can correct. It fits perfectly, which is what makes it a trap.

### Is the MR9 rechargeable?

No. It is a primary cell — charging one risks rupture or fire. Rechargeable look-alikes exist in the same size under different codes.

## Related pages

- [LR44 battery](https://atoneplace.net/battery/lr44/)
- [SR626SW battery](https://atoneplace.net/battery/sr626sw/)
- [SR44 battery](https://atoneplace.net/battery/sr44/)
- [PR41 battery](https://atoneplace.net/battery/za312/)
- [PR48 battery](https://atoneplace.net/battery/za13/)
- [SR621SW battery](https://atoneplace.net/battery/sr621sw/)
- [PR44 battery](https://atoneplace.net/battery/za675/)
- [SR920SW battery](https://atoneplace.net/battery/sr920sw/)
- [LR41 battery](https://atoneplace.net/battery/lr41/)
- [PR70 battery](https://atoneplace.net/battery/za10/)
- [SR927SW battery](https://atoneplace.net/battery/sr927sw/)
- [LR1130 battery](https://atoneplace.net/battery/lr1130/)

## Sources

- Typical range compiled from manufacturer specifications — At One Place (/about/)