Why are alkalines "required" for metal detectors?
&quot;Heavy Duty&quot; batteries are what we engineers would call a zinc-carbon battery. This type of battery is cheap to make but has a lower capacity and a higher series resistance than an Alkaline battery.
Probably the metal detector draws more current than a zinc-carbon battery can handle properly and/or the battery's voltage will drop too much during extended use for the device to work properly. All of this relates to that series resistance.
So to prevent users from having issues, the manufacturer recommends using Alkaline batteries. That might save them from users complaining that the metal detector doesn't work well / for very long while that's actually caused by using an unsuitable battery.