# Society of Robots - Robot Forum

## Electronics => Electronics => Topic started by: icius on December 24, 2006, 12:41:23 PM

Title: Battery Ratings and Motor Ratings
Post by: icius on December 24, 2006, 12:41:23 PM
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?

Title: Re: Battery Ratings and Motor Ratings
Post by: Admin on December 24, 2006, 05:58:20 PM
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