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This is something I ran into when testing a small heating coil with 9V batteries.

I tried an Energizer battery that was previously used so it only had 9V remaining. The coil heated up a weak red, but still noticeable.

I next tried a freshly opened Sunbeam 9V battery from the dollar store. Nothing. No heating at all. The battery measures 10V from my multimeter.

Even though it's at a higher voltage, its seems to provide less power?

I have two hypotheses on this:

  1. Higher internal resistance. Perhaps cheaper batteries have higher resistance. But I don't think it should be that big of a difference.
  2. It can't reach the same level of current. P = VI, so if less power given more voltage should mean much less current. I'm not sure how batteries work, but does the battery have a limit on much current it can produce? So the cheaper battery might have a shallow limit.

Which is correct? Or am I completely wrong?

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    \$\begingroup\$ I honestly don't know why they keep producing zinc-carbon batteries, other than the fact that for some reason people keep buying them. \$\endgroup\$ Apr 18, 2018 at 3:51
  • \$\begingroup\$ Measure the voltage when the heater is connected, and you will have your answer. \$\endgroup\$
    – gbarry
    Apr 18, 2018 at 4:00
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    \$\begingroup\$ I know nothing of battery chemistry, but arent point 1 and 2 the same? Lower current delivery == higher internal equivalent resistance? \$\endgroup\$
    – Wesley Lee
    Apr 18, 2018 at 4:05
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    \$\begingroup\$ The cheaper battery drops to 4V while other is closer to 6V. Wow that's a difference under load. \$\endgroup\$
    – DTOAL
    Apr 18, 2018 at 4:16
  • \$\begingroup\$ Because energy costs money. \$\endgroup\$
    – Chu
    Apr 18, 2018 at 11:26

1 Answer 1

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Your hypothesis #1 is the correct one. 9-V batteries are not designed to deliver high current and heat some heating coils. Given the battery's small physical size and necessity to have 6 primary elements inside (6S configuration), they just can't possibly deliver high current, and are intended to use in long-lasting low-current consumer devices as smoke detectors and DMMs, with consumption in a fraction of milli-amp. Their capacity is 300-400 mAh, and devices are expected to last years. So the internal impedance is of least concern as long as the battery serves its main purpose. The actual difference in internal impedance between your two batteries is not that much, according to your measurements, no more than a factor of two.

And the hypothesis #2 is essentially the same as the first one. Cheaper battery is likely using less sophisticated and less fine-grained materials that result in higher internal impedance, which results in less output voltage under the same load, and therefore "less power".

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  • \$\begingroup\$ Using "optimized" is quite a stretch. Their performance is crap so they only get used where it doesn't matter. Before the advent of good rechargeable batteries there have been tons of devices that needed 6x1.5V batteries \$\endgroup\$
    – PlasmaHH
    Apr 18, 2018 at 6:45
  • \$\begingroup\$ @PlasmaHH, agree, "optimized" maybe not the best sentiment, see my edit. However, I suspect that the ability of 9-V battery to have very low self-discharge rate is taking some toll on the value of internal impedance, so there is likely some truth in "optimization". \$\endgroup\$ Apr 18, 2018 at 16:01
  • \$\begingroup\$ There are some that claim low self discharge, usually sold for smoke detectors, however a big standard Energizer 9V has a shelf life of 5 years, whereas a AA has 10 years... \$\endgroup\$
    – PlasmaHH
    Apr 18, 2018 at 16:12

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