# will limiting the current draw of a dc motor increase the the run time form a battery.

#### everettdale

Joined Jan 26, 2022
8
I have a 150mah Lipo battery that is running a d/c motor that is drawing 1.4 amps (the motor and battery are directly hooked up)
As the motor runs the amperage draw slowly decreases over time until its below .85 amps (takes about 5 min) then the battery circuitry shuts everything down.

My question is: can I build a circuit that limits the amount of current provided to the motor to 1 amp. Without decreasing the amount of time the motor will run. or hopefully increase run time.

Or should I lower the voltage going to the motor with, I assume, a resister in series with the motor. but will the current draw of the motor and resister equal the draw of the motor alone. or will I get a longer runtime on the motor?

#### Ian0

Joined Aug 7, 2020
8,939
If you assume a constant efficiency for the motor, then work done is power x time. The reduction in power and the increase in run time will cancel out and the energy taken from the battery will remain the same.
Copper loss in the motor will vary with the square of current, so reducing the current will increase the efficiency and take less energy from the battery.
However, putting a resistor in series will increase the work done, by heating up the resistor, so the energy taken from the battery will be greater.
The losses in your current limit circuit would be the key to it!

#### everettdale

Joined Jan 26, 2022
8
so reducing the current will increase the efficiency and take less energy from the battery.
Ok this is what I was thinking. so what type of circuit am I looking for? It needs as to be simple as possible, and fewest components as possible. I looked at current limiting circuits and just cant get my head around them.

thanks

#### Ya’akov

Joined Jan 27, 2019
8,497
A high efficiency solution to this problem will not be simple. One approach is PWM, which can be done relatively easily thanks fo many readily available modules.

In any case, an explanation of the actual application will make any help we can give much more relevant and will reduce the frustrations of your helpers. Drawings, schematic, and the like are preferable but even a concise narrative will make things a lot more clear than this minimalist request.

#### crutschow

Joined Mar 14, 2008
33,331
Will the motor still drive its load if you reduce the current to 1A?

#### Carl_MIT

Joined Jan 26, 2022
5

#### k1ng 1337

Joined Sep 11, 2020
818
I have a 150mah Lipo battery that is running a d/c motor that is drawing 1.4 amps (the motor and battery are directly hooked up)
As the motor runs the amperage draw slowly decreases over time until its below .85 amps (takes about 5 min) then the battery circuitry shuts everything down.

My question is: can I build a circuit that limits the amount of current provided to the motor to 1 amp. Without decreasing the amount of time the motor will run. or hopefully increase run time.

Or should I lower the voltage going to the motor with, I assume, a resister in series with the motor. but will the current draw of the motor and resister equal the draw of the motor alone. or will I get a longer runtime on the motor?
It might be helpful to conduct empirical measurements using a stopwatch and multimeter with different combinations of parameters to learn the nature of the motor. You could try different battery voltages, PWM, a bench power supply on voltage or current limiting setting, a buck/boost converter, a voltage/current regulator such as LM317 and even some power resistors. Each of these will behave differently with different losses to be calculated which would be a good mathematical exercise. You could also increase/decrease the load on the motor to see how it responds electronically to a given power source. Doing this has helped me bridge the gap between theory and understanding regarding inductive loads especially when the loading on a motor is dynamic such as a RC car going up a hill. The above are not practical circuit suggestions rather investigative learning techniques as some will yield considerable power losses. On the flip side providing more information of your goal, components on hand and what level of electrical knowledge you posses will significantly help the community to provide you with a solution.

#### everettdale

Joined Jan 26, 2022
8
I did basically what you said. I ran some testes and wasn't able to get better results with limited parts. I was just thinking I was missing something but really wasn't. thanks for the help and suggestions.