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This is a curiosity question. The batteries tested were Duracell coppertop alkaline batteries.

Recently, my bass guitar was acting weird and it was caused by a weak 9V battery. I checked the voltage and it was like 6.5V or something crazy low. Now, if I check a C 1.5V battery when it's weak, its voltage is relatively high. I think I measured the weak C at 1.42V.

I forgot to check the 9V while under load and I wasn't able to measure the voltages of the C battery under load b/c all the devices I could find needed the cover closed for it to work.

In any case, why does a 9V drop that much when it's weak but a 1.5V doesn't?

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    \$\begingroup\$ Alkaline cells aren't "weak" until they hit 0.9V. If your electronics starts dropping out at 1.42V then it's not decent quality. \$\endgroup\$ – Ignacio Vazquez-Abrams Jan 22 '14 at 0:38
  • \$\begingroup\$ @IgnacioVazquez-Abrams, thx for the reply. It's possible they dropped to 0.9V or below while under load but I can't tell since I couldn't/didn't measure with my meter while under load. With that said, I think the C battery was weak b/c when I stuck new batteries in my turbo groomer, you could tell the blades were whirring a lot faster than with the old batteries. Either that, or you're right and it's time for me to switch to a new brand :) \$\endgroup\$ – Classified Jan 22 '14 at 0:43
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    \$\begingroup\$ You want commentary on your poorly collected observations? \$\endgroup\$ – Samuel Jan 22 '14 at 0:47
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    \$\begingroup\$ I think @IgnacioVazquez-Abrams makes an important point: you are comparing different electronics devices that will very likely have different drop out voltages due to their design. The cause of drop out is not just the battery voltage, it is also the power requirement of the circuit. \$\endgroup\$ – jippie Jan 22 '14 at 6:14
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A ""typical"" ""1.5v"" Alkaline C ""Battery"" is a single Cell (so not really a battery), ranging from ~1.6v at full charge, averaging 1.5v to 1.3v for most of it's life, eventually rapidly dropping to ~0.9/0.8v.

A ""typical"" ""9v"" Alkaline Battery (notice, no quotes) is composed of multiple 1.5v cells in series. So even in a perfect scenario, where all 6 cells discharge at an equal rate, the lower voltage of each cell is multiplied. The same drop from 1.5v to 1.4v (under load) for a C battery would mean 0.1v * 6 or 0.6v for a 9v Battery.

THEN we have the problem that a 9v battery is designed for higher voltage and a long life via low current draw. A C battery, on the other hand, is designed for high current draw and higher capacity. For comparison, a typical 9v is ~400mAh while a C is ~8000mAh. That is 20 times the capacity. The tradeoff? Lower voltage, higher weight and size.

For clarification, electrical semantics dictate that A Battery is a device consisting of multiple cells, while a cell is a device that converts chemical to electrical energy.

And this ALL varies based on manufacturer and quality of the battery and battery chemistry. Some Alkaline 9Vs are composed of AAAA batteries, while other's have fat stack types inside. Carbon Zinc are weaker, Lithium stronger, etc etc.

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  • \$\begingroup\$ double double quotes?!?! \$\endgroup\$ – Phil Frost Jan 22 '14 at 3:35
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    \$\begingroup\$ @PhilFrost single double quotes mean verbatim quoting. Double double quotes is highly condescending air quotes with a side of sarcasm :P Kidding aside, double quotes are for highlighting a skeptical quotation. Like when a company says "Unlimited Internet", they mean ""Unlimited"" in exactly the opposite of what the average person would expect it to mean. \$\endgroup\$ – Passerby Jan 22 '14 at 3:58
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    \$\begingroup\$ I always thought it was an empty quote, followed by some unquoted words, followed by another empty quote. :) \$\endgroup\$ – Phil Frost Jan 22 '14 at 4:03
  • \$\begingroup\$ @PhilFrost are you sure your name isn't regex? \$\endgroup\$ – Passerby Jan 22 '14 at 4:19
  • \$\begingroup\$ @Phil Frost I actually thought so as well! I never would have guessed what double quotes mean. \$\endgroup\$ – AndrejaKo Jan 22 '14 at 13:01
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Here is a graph that shows the voltage of a cell, depending on chemistry used, as it discharges

enter image description here

Your 1.5v alkaline battery is one cell and the 9v battery a 6 cell, if both were at the same level of discharge then you would have observed a similar percentage of voltage drop.
Since you get totally different results it can only be explained with a different level of discharge, probably because different devices can work with different minimum voltage levels.

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Some years ago I tested a few AAA batteries in pairs, here's the results

graph of volts over time

After an initial fall, the drop in voltage is gradual but finally plummets.

You just have not fully discharged your C-sized 1.5V battery.

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