I am currently using two 12V CIM Motor (Spec Sheet: http://content.vexrobotics.com/docs/217-2000-CIM-motor-specs.pdf) with a gearbox at a 49:1 ratio.

I have a LiFePO4 battery that is rated at a continuous current draw of 25A or a peak of 70A for 30 seconds. When the motor spun it was drawing 40A. After a few minutes a sulfur odor was noticed coming from the battery.

How much do I change the gear ratio so that the motor draws less current?

Also how long would I have to turn the motor off between the 30 second spurts if I just ran at 40A on and off?

EDIT: There are two motors that are being fed into one gearbox.
Battery Information: http://www.filedropper.com/ctcbatteryproductspec128v12ahv2

The two motors are for a robot drive system. The 40A is for the total draw from the battery (2xMotors, Sabertooth 2x60 controller, arduino). When the motor controller was commanded to go full forward on both motors 40A is drawn.

I can send a command to the motor controller to drive the motor slower. Would it be more effective to send a slower command to the motor controller or change the gearbox so that the motor controller could use full throttle?

  • \$\begingroup\$ To a first approximation, increase the gearing by at least 40/25. This ignores no-load current, increased gearbox friction, increased speed becaus load is reduced, etc so I'd suggest 2:1, i.e. approx 100:1 overall gearing. \$\endgroup\$
    – user16324
    Commented May 11, 2015 at 8:45
  • \$\begingroup\$ When you say "how do I change the gear ratio" do you means "HOW MUCH do I need to change the ratio" or "literally how can I change it mechanically?" \$\endgroup\$
    – Russell McMahon
    Commented May 11, 2015 at 14:50
  • \$\begingroup\$ I am looking to find out HOW MUCH \$\endgroup\$
    – brad123664
    Commented May 11, 2015 at 15:52

2 Answers 2


First we need to look at the uncertainties before we address possible solutions:

  • Are there two motors in use or one - it could be either from your text.

  • And, is the 40A for two motors in use or one?

  • You say " ... motor was spun ..." - what does this mean? An unloaded motor is rated at 2.7A, at normal load it's 27A and max power = 68A.
    Where on that continuum is "was spun" ?

  • You need to describe the nature of the load and your application. Whether it is possible to decrease input power to the motor to within battery specs and still meet your requirement is unknowable to us as you have not provided enough information.

  • What is the mechanical arrangement?
    What attaches to the shaft(s)?
    Why do you need to run it at that power?
    Why can you not run it at lower power?
    Do you use a motor controller or just connect the battery to the motor directly?

  • Please supply battery model and brand and very ideally a internet link.

  • What else can you tell us that may help us to help you?


Changing physical gear ratio inside the gearbox is a mechanical task outside the scope of this form. It is possibly possible, but more likely not to be. Doing this depends on the gearbox design. The manufacturer and/or user community will be able to comment on this.

If you are using or can use a belt or chain drive externally you may be able to change the ratio of pulleys or sprockets used to reduce speed and increase torque.

You MAY be able to move the motor back from where it is mounted so there is miore room and add an inline speed reducer - maybe 2:1.


You can PWM the motor(s) to limit battery current to 25A continuous this will reduce power to probably about 25/40 = 65% of that at 40A but maybe not. Motor efficiency rises usefully with decreasing current but it is not certain how it will run at 25A mean under your load.

Depending on facytors such as PWM frequency and duty cycle, motor imp[edance and more PWM may reduce peak moptor current to mean current or may still cause peaks greater than motor continuous maximum. So, if you use PWM yiou will ideally add capacitance to the PWM input = battery output that will supply the PWNM pulse currents while limiting mean and peak battery currents to no more than the continuous maximum rating.

Battery rated maximim continuous output is 25A.
Based on what you say the unseen by us data sheet says you can run it at 40A for 30 seconds followed by the rest period for 70A in the data sheet - ie at the 70A ratings. You MAY be able to run it for longer safely or with a lower rest period. Or not. If you provide a link to the battery data sheet and tell us the brand and model we may be able to tell you more. Maybe not, but without that it's a guess at best.


You might find these two blogs to be helpful: FIRST Robotics (FRC) Motor Modeling and Controlling Motors With The Talon SRX

They show you how to model/simulate a feedback control system with a CIM motor and a Talon SRX controller. You can see the impact of gearing and multiple motors. You can see how the voltage, current, and mechanical loads all interact. Motor Control System: CIM & Talon SRX


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