Duracell Specialty alkaline MN21 batteries - long lasting power guaranteed for your devices. Duracell Specialty batteries have up to 5 years of in-storage guarantee so you can be confident these alkaline batteries will be ready when you need them. They are suitable for use in wireless remote controls, wireless doorbells and security systems, alarms, locators. MN21 size battery. Also known as A23. Pack of 2.