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I am trying to replace two AAA batteries with a capacitor in a wireless mouse. The mouse pulls at 1.2V and .023 A and I was trying to find what kind/size capacitor there was a similar question posted here (How to calculate equivalent capacitance to a battery?)

But I couldn't quite makes heads or tails of the variable s he had used in his formula otherwise I would not be posting. So hopefully I can find some help.

Marcus Müller
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josh b
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    repeat after me: A capacitor is not a battery. They serve different purposes, and can't be arbitrarily interchanged without considering the consumer of electrical energy. So, simply put, it's very unlikely your whole thing makes sense. – Marcus Müller May 23 '17 at 23:22
  • "The mouse pulls at 1.2V" - Huh? Do you mean that this is the lowest voltage that the mouse will run on? – Oskar Skog May 23 '17 at 23:23
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    The voltage of a capacitor drops smoothly as it is discharged, but a battery maintains a pretty stable voltage until it is completely empty. = The mouse is designed to run on about three volts, but the voltage on a capacitor is dropping and fast. The mouse will stop working long before the capacitor has been completely discharged. – Oskar Skog May 23 '17 at 23:28
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    Why on earth would you want to do such nonsense? – Andrey Pro May 23 '17 at 23:35
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    I think you can replace the batteries with a big-ass super capacitor charged to 3.3 volts (you can get that voltage from a computer PSU). Unfortunately, you'll have to recharge the capacitor very often, but it should work. – Oskar Skog May 23 '17 at 23:35
  • I am aiming to have the capacitor receiving constant flow of electrons but I misspoke on the voltage I guess that number I said was a mistake and I am back to the drawing board to find the volts but the mAh would be roughly 2500 I am thinking I could plug that into an equation and find the voltage somehow from that. – josh b May 24 '17 at 21:59
  • And once again sorry I am such a novice – josh b May 24 '17 at 21:59

2 Answers2

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A capacitor can economically replace a battery for only short period of time due to its limited energy storage. Using your numbers, a 0.023 amp load at 1.2 volts is equivalent to a 487 ohm load (1.2 volts / 0.023 amps). If we assume that the capacitor must keep the voltage within 5% of the 1.2 volts (i.e. no less than 1.14 volts) for say a 10 day period, we can compute the required value of capacitor.

The formula to use is:

Capacitance = -t/(R*ln(Vend/Vstart))

Where capacitance is in Farads, t is in seconds, and R is in Ohms. So if we plug in your numbers, we find that we need a capacitor > 34,000 Farads. Note that is Farads, not microFarads. A capacitor bank this size would cost several thousand dollars and occupy a volume greater than a server class PC. Not at all practical for the stated conditions.

But now you have the formula so you can plug in your own values to find the required capacitance.

Glenn W9IQ
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    A part of me wanted to let him do it and then realize the sizes involved. – StainlessSteelRat May 24 '17 at 00:47
  • A single NiMH cell is 1.2 V, but Josh says he has two AAA batteries. I'm not aware of any common chemistry for AAA batteries that has a nominal voltage of 0.6 V. // This is why I asked him what he means with the statement regarding 1.2 V, but he has not yet answered. // Whatever is the case, the answer is still in kF though. – Oskar Skog May 24 '17 at 08:20
  • Just a note: you could relax the 5% voltage threshold withtter a step-up converter, though that wouldn't make the whole idea much be – clabacchio May 24 '17 at 09:54
  • The 1.2V is what the devices is rated at if that makes sense, and it takes two AAA batteries to power. I am trying to get into this even though I am a novice. And once again I appreciate all the attention for my question. – josh b May 24 '17 at 21:41
  • @joshb: Could it be that each battery is supposed to be 1.2 V? That would make perfect sense since there are such batteries (NiMH and NiCd). – Oskar Skog May 25 '17 at 09:36
  • The batteries may be connected in parallel. My microsoft mouse is wired this way. – ricardomenzer Feb 27 '18 at 14:47
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To answer your question, s stands for seconds. That 3600 value means the poster wanted to run their load for 1 hour.

Dmitry Grigoryev
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