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a few days ago I had a asked a question regarding longer lasting batteries for a walking robot built with a Tamiya gearbox. Since the AA batteries were being depleted too fast, it was suggested that I use D cells. I would like to know would using C or D batteries instead of AA spoil the motor? Even though the voltage is same, the current is different. Thanks.

Sangita
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3 Answers3

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The larger C and D cells can supply more current than the smaller AAs, but the motors and other loads will only draw the current they require. AA, C and D cells all produce the same voltage. The motors will not be damaged by the greater current capacity of the larger cells.

Peter Bennett
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The motor will consume only as much current that it requires at that point in time.

That said - if the motor is under-sized for the load, the extra current available from the battery can lead to motor damage.

Edit:

There appears to be some confusion regarding my statement about how using a larger-capacity battery can lead to motor damage if the motor is too small to properly drive the load.

The damage that occurs is strictly thermal and happens when the motor tries to drive the load but gets too hot while doing so. With small cells (a small battery), the cells deplete fairly quickly and the motor temperature spikes to some high value, then decays as the cells decay. The motor often survives the abuse.

When you have much larger capacity cells driving the motor that is too small to properly drive the load, the motor gets hotter and hotter until something fails.

Something that I discovered when I was a kid. <Grin>

Note that this is only a problem when driving a motor that is too small for the load.

Dwayne Reid
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    Or to put it more clearly, "weak" cells end up being a sort of overload-limiter, which a "stronger" cell would not be, meaning that the motor rather than the battery pack might end up the weakest link. – Chris Stratton Apr 29 '16 at 03:20
  • D size possibly 5 times more capable than the AA. Assuming it could deliver 5 times stronger current during dead short, still the power it generates not much different than the AA because the voltage anyhow will drop around a mere one volt. Nothing much to worry. – soosai steven Apr 29 '16 at 03:55
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    According to datasheets, internal resistance (IR) of D cells is approximately equal to IR of AA cells. For example, compare http://data.energizer.com/PDFs/E91.pdf and http://data.energizer.com/PDFs/e95.pdf . 150 to 300 milliohms in the both cases. So a maximum current that a fresh cell can deliver is the same. Of course, a capacity (in mAh) of D is much greater, but that does not imply higher current capability. It means that a cell will last longer. – dmitryvm Apr 29 '16 at 05:40
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    @dmitryvm: You should write that up as an answer – slebetman Apr 29 '16 at 06:25
  • I have added the answer, including the text of my comment above. – dmitryvm Apr 29 '16 at 08:55
  • An AA Nickel Cadmium cell is capable of supplying more short-term current than a D zinc-carbon cell. Designers should not be relying on battery resistance to limit current to a load. – Level River St Apr 29 '16 at 10:14
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A current consumption of the motor is determined by two factors:

  1. The mechanical load on the shaft
  2. The internal resistance of the power supply

Even if the internal resistance of the power supply is near zero, the current will not exceed the maximum rating, given that the mechanical load is within specified limits. Nevertheless, the internal resistance of the power supply may limit the current in the case of mechanical overload of the motor, e.g. if the shaft becomes locked.

According to datasheets, internal resistance (IR) of D cells is approximately equal to IR of AA cells. For example, compare http://data.energizer.com/PDFs/E91.pdf and http://data.energizer.com/PDFs/e95.pdf . 150 to 300 milliohms in the both cases. So a maximum current that a fresh cell can deliver is the same. Of course, a capacity (in mAh) of D is much greater, but that does not imply higher current capability. It means that a cell will last longer.

Therefore, it is safe to replace AA with D (or C).

dmitryvm
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