What is the same
Both are disposable zinc-air cells (~1.4 V). Peel the tab when ready, wait about a minute for air activation, then insert. Pack brand (Rayovac, Duracell, Power One, etc.) does not change the size code.
What is different
Retail size 312 (brown tab / IEC PR41 / ZA312) is about 7.9 × 3.6 mm. Size 13 (orange tab / IEC PR48 / ZA13) is the same diameter class but taller (~5.4 mm).
A taller 13 will not fit most 312 trays; a shorter 312 will sit loose or lose contact in a 13 compartment.
Examples on this site
Phonak Audéo L-312 and Bolero M-M map to size 312. Phonak Audéo P-13T and Naída L-SP map to size 13. Always match the empty pack or the manufacturer’s non-rechargeable size table.
Do not use list
Do not swap 312 and 13 by looking at the face of the cell alone.
Do not use these sizes in sealed rechargeable hearing aids — that is a different power system.
This is a size guide, not clinical fitting advice — see the hearing-aid size hub for 10 / 312 / 13 / 675 / rare size 5.