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I am testing the RPM values of a 1.5-3V motor with AA and AAA batteries. When I attached a dual battery holder with two AAA batteries to the motor, the motor spins almost twice as fast as when it is connected to the dual battery holder with two AA batteries.

Since both types of batteries have the same voltage, why does the motor spin faster? I know that AA batteries typically have more capacity in terms of mAh, however I'm not sure how this translates into motor RPM.

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    \$\begingroup\$ Higher current capability/less voltage drop under load. Read the datasheets and compare. \$\endgroup\$ – winny Nov 12 '17 at 18:41
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    \$\begingroup\$ Measure the voltage across the motor in each case and see if that explains it. \$\endgroup\$ – Transistor Nov 12 '17 at 19:17
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The obvious guess would be that your AAA battery holder has the batteries wired in series, so you're getting an output of ~3V, where the AA battery holder has the batteries wired in parallel, so it's producing ~1.5V output (with higher current capability, which probably doesn't make much difference in this case).

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    \$\begingroup\$ Or the AAA cells could be new, while the AA cells are old, and nearly dead. \$\endgroup\$ – Peter Bennett Nov 12 '17 at 18:51
  • \$\begingroup\$ Or bad connections with high resistance. \$\endgroup\$ – Passerby Nov 12 '17 at 19:23
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Assuming perfect conditions, two brand new Alkaline AA cells and two brand new Alkaline AAA cells will provide the same characteristics, except for the AA cells lasting longer. If we include maxing out their current sourcing capabilities, the AA will perform better due to their higher capacity and slightly lower ESR. In your case, you should not see a difference.

That said the issues that could be happening is:

  1. Battery age
  2. Quality of brand
  3. Not wired the same (parallel vs series)
  4. Bad connections or holders or soldering or wire
  5. You are use Alkaline or better AAA while use heavy duty, carbon zinc or zinc chloride AA batteries. Those suck.
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