# Battery says 7.2V 2600 mAh, Motor says 40 mA free-run so...

Discussion in 'General Electronics Chat' started by RyanD, Jan 20, 2010.

Jan 14, 2010
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2. ### SgtWookie Expert

Jul 17, 2007
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The motor is rated for 6v, not 7.2v. 7.2v is 120% of the motors' rated voltage. It would probably draw 50mA or more at 7.2v at no load.

If there is any load on the motor (such as starting from a stop, or if it's loaded down) it will use much more current. Stall current is 360mA, or 9 times the no-load current.

3. ### RyanD Thread Starter Member

Jan 14, 2010
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okay, so the math is right I just have to figure out the variable values. I'm looking for motors to control a small robot and I want it to run for as long as possible without a recharge. When there are wheels and weight on the motor the load will jump up but even if it's running at it's full stall current(which would kill it, no?) then using this battery it would run for about 7 hours?

More info. My plan is to have two of these on the robot in parallel to control the motors and some other power source to control the IC and other sensors. Even if I can get these to hold up for 5 hours that would be great.

4. ### SgtWookie Expert

Jul 17, 2007
22,201
1,809
15 oz-in isn't a whole lot of torque, and it doesn't say if that is stall torque - but you could assume that.

Batteries are rated for mAh based on a 20-hour discharge rate. 2600mAh / 20 = 130mA.

If you draw current at a rate greater than the 20-hour discharge rate, you will get much less life out of the battery. As current draw increases, more power is dissipated inside the battery. That is why they get hot if you put a heavy load on them; if the terminals are shorted, they might explode due to the high temperatures generated.

However, if your current draw is less than the 20-hour discharge rate, you will likely find that you get more life out of the batteries than you would expect.

If you stall the motor by causing it to try to exert at/more than 15 in-oz of torque, you will get the stall current of 360mA. This is more than twice the 20-hour discharge rate, and you will have a very short run-time.

Anything you can do to reduce the torque required by the motor will extend the run-time.

5. ### Von Active Member

Oct 29, 2008
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Sgt. Wookie said:
"Batteries are rated for mAh based on a 20-hour discharge rate."

Sgt. can you provide a link to this?

I have been involved in testing batteries in the past and have not used this method.

Capacity was determined statistically by testing a large group (30+) of batteries at a "typical" discharge rate of about 1/5 of the "C", or designed capacity, until a "cut off" voltage is reached.

Then manufacturing changes were made as required to obtain the desired or specified capacity.

Thus the "time" ( 20 hrs.) is the variable not a constant.

6. ### Audioguru Expert

Dec 20, 2007
11,251
1,350
Your 1/5th of C is a 5 hour rate.
A lead-acid battery is rated at a 10 hour or 20 hour rate.
A Ni-cad or Ni-MH battery performs almost the same at a 20 hour, 10 hour, 5 hour or 1 hour rate.

Here is the capacity of an Energizer Ni-MH AA cell that is rated at 2500mAh:

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7. ### Von Active Member

Oct 29, 2008
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Look at the graph again.

Time is the variable. The discharge rate and cut off voltage are the constants.

While different chemistries need different cut-off voltages and may use (require) different discharge rates to determine capacity, the graph illustrates the need for standardized discharge rates to "keep the playing filed level".

Jan 14, 2010
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