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A handheld metal detector specifies only "Alkaline" batteries and not "heavy duty" or "rechargeable". Since there are li-ion 9v batteries available not at low cost I was wondering what is different that requires these in a metal-detector specifically? Is there something about the metals or magnetism reacting within the device or is it an issue of power draw?

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    \$\begingroup\$ I think that the concern is more about having the expected (planned) operating voltage available and expected decline in voltage as the batteries discharge over time. Rechargeable batteries are likely to have slightly different operating voltages and different discharge curves and it is just safer for the manufacturer to specify batteries they have well-tested. I don't think it is a lot more complex than that. (Magnetism, etc.) \$\endgroup\$
    – jonk
    Mar 24 at 8:12
  • \$\begingroup\$ probably it does some pulsing and requires a battery with low internal resistance or it need the whole nominal voltage (alkalines are a little higher on voltage per cell) \$\endgroup\$ Mar 24 at 12:18
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"Heavy Duty" 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.

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  • \$\begingroup\$ The common 9V rechargeables could have been NiMH which are not 9V, but nominally 6x1.2V = 7.2V. \$\endgroup\$
    – Justme
    Mar 24 at 10:26
  • \$\begingroup\$ @Justme Some NiMH 9Vs use seven cells, for 8.4 V nominal. \$\endgroup\$
    – Hearth
    Mar 24 at 13:20
  • \$\begingroup\$ "Heavy duty" or "high power" usually refers to Zinc Chloride, and improved version of zinc carbon. But still not as good as alkaline. \$\endgroup\$
    – Simon B
    Mar 24 at 13:56
  • \$\begingroup\$ @SimonB but wouldli-ion batteries be better? eg newegg.com/ebl-2-pack-9v-lithium-batteries/p/14R-03UH-00056 \$\endgroup\$
    – NoBugs
    Mar 24 at 14:33
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    \$\begingroup\$ @NoBugs I'm perplexed by those batteries because lithium-ion chemistry has a typical cell voltage of 3.7 V. Two in series would give 7.4 V, and most devices that take a 9 V alkaline battery would probably work OK off that, but others maybe wouldn't. You can put three in series but then you get 11.1 V which would be too much. You could theoretically make a three-cell Li-ion battery in 6F22 format using circuitry to regulate the output to 9 V, but if those batteries have that I'd expect the description to highlight it. I'd take the claim to be 'wildly compatible' with a pinch of salt. \$\endgroup\$
    – nekomatic
    Mar 24 at 16:40

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