Duracell MN21 Security Batteries. Alkaline battery. 12v N Size. Duracell Specialist Battery suitable for car alarms and security devices. Duracell MN21 can fit many electronic key fobs, such as garage door openers. The Duracell MN21 battery is a highly flexible battery that can be used in a large amount of applications. Extensively used in security applications. Use for applications such as alarm systems, motion detectors, wireless PIRs, and magnetic reed switches. Users that have used the Duracell MN21 key fob battery in garage door openers have reported that it keeps a larger range, for a longer amount of time when compared to other batteries. For our full range of batteries, please click here.