Electronics > Electronics

Battery Ratings and Motor Ratings

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icius:
Hello all,

I am trying to match motors to batteries for my robot.  I am hung up on looking at the current draw on the motor spec sheet and the current rating of the battery.  My first ( probably flawed) thought is that the current rating of the battery should be able to handle the current draw of the motor for at least the "maximum efficiency" of the motor.  So just as an example I look at this motor:

http://www.johnsonelectric.com/product/product_sheet/fect_sheet.php?ProdID=129--A

And let's say this battery:

http://www2.towerhobbies.com/cgi-bin/wti0001p?&I=LXJHT3&P=7

Seems like kind of a pretty standard setup for an RC car right?  (I think).  The battery is rated at 1100 mAh (milliamp hours right?).  The spec sheet  for the motor says it draws 6.794 Amps at maximum efficiency.  So that's 6,794 mA right?

So from what I understand (or don't understand) this is saying that this battery pack could power this motor for 9.7 minutes at maximum efficiency.  Does that seem right?

So at maximum efficiency, it requires 6,794mA. Since the battery can supply 1100mAh, that means your battery will last 1100/6794*60 = 9.7 minutes as you said. Your calculations are correct.

1100mAh is actually a very small amount . . . i typically use on average 1800mAh+

here is a good list for 7.2V NiMH batteries:
http://www.onlybatterypacks.com/items.asp?db=8
the discharge rates on those batteries wont be able to handle more than one motor, so if you have two motors, you will need to use a NiCad, or put two NiMH in parallel

http://www.onlybatterypacks.com/items.asp?db=3

icius:
Thanks for the quick reply.  I mulled over that message a little bit and recalled my days messing with RC cars eons ago.  Come to think about it 10-15 minutes on a charged pack is about what I got out of those RC cars.  Thanks for the confirmation!

Militoy:
As Admin alluded to, when you make your calculation as to operating time, you also need to be careful to watch the batteries' discharge curves. The mAH/Time function isn't always linear. Some batteries are rated at lower discharge rates than others - so the mAH rating may be misleading if the battery is run at a higher current than its capacity is spec'ed at (this can work in your favor as well, if you drain the batteries at a lower rate than typical). Many battery manufacturers have published curves, to help you make an accurate estimate of battery holdup at various discharge rates.