How can I prevent undervoltage in a DC motor powered electric vehicle?

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Discussion Overview

The discussion revolves around preventing undervoltage conditions in a DC motor powered electric vehicle, specifically focusing on the challenges posed by lithium polymer batteries and potential solutions involving capacitors and other circuit elements. Participants explore both theoretical and practical aspects of the issue.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested
  • Mathematical reasoning

Main Points Raised

  • Jake describes an issue with the voltage dropping from ~26V to ~17V when the accelerator is fully engaged, leading to an undervoltage condition that trips the breaker.
  • Some participants suggest that a series diode and parallel capacitor might help smooth out voltage spikes, while others express skepticism about their effectiveness.
  • One participant questions the duration of the undervoltage condition, suggesting that if it lasts only milliseconds, a capacitor might be beneficial, but emphasizes that capacitors have much less capacity than batteries.
  • There is a discussion about the possibility of adding more lithium polymer batteries, but constraints related to space and budget are noted.
  • Participants inquire about the utility of large automotive "audio" capacitors, which are known for their high capacitance, and whether they could be a viable solution.
  • A mathematical approach is proposed for calculating the required capacitor size based on the duration of the undervoltage condition and the current draw of the motor.

Areas of Agreement / Disagreement

Participants express differing views on the effectiveness of capacitors and diodes in addressing the undervoltage issue. There is no consensus on a definitive solution, and multiple competing ideas are presented regarding potential approaches.

Contextual Notes

Participants acknowledge limitations in their experience with power electronics and the challenges of finding effective solutions. The discussion highlights the dependence on specific conditions such as the duration of the undervoltage and the current draw of the motor.

Who May Find This Useful

This discussion may be of interest to individuals working on electric vehicle design, particularly those dealing with power management in DC motor systems and lithium polymer battery applications.

solacedagony
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I am currently running a DC motor in an electric vehicle. A lithium polymer battery powers a motor controller which is connected to said DC motor to drive the car.

The lithium polymer battery as you would imagine has a battery monitoring system on it. One of the conditions it protects against is an undervoltage condition. When the car accelerator is set to 100%, the voltage will instantaneously drop from ~26V to ~17V for a short time tripping the undervoltage condition and opening the breaker. This shuts off the car.

What solutions exist? Could a series diode and parallel capacitor be added to smooth the instantaneous spikes? If so, what method could be used to calculate the required capacitance of the smoothing capacitor? If not, what other circuit elements could be used to prevent this instantaneous voltage drop?

Thank you in advance for any help or suggestions you can provide.
Jake
 
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From Wikipedia:
The voltage of a Li-poly cell varies from about 2.7 V (discharged) to about 4.23 V (fully charged), and Li-poly cells have to be protected from overcharge by limiting the applied voltage to no more than 4.235 V per cell used in a series combination. Overcharging a Li-poly battery can cause an explosion or fire. During discharge on load, the load has to be removed as soon as the voltage drops below approximately 3.0 V[citation needed]per cell (used in a series combination), or else the battery will subsequently no longer accept a full charge and may experience problems holding voltage under load. This can be achieved, as with other lithium-ion batteries, also harmed by under- and over-voltage, by circuitry that prevents overcharge and deep discharge.

The easiest solution would be to set the accelerator to a percentage that does not cause the undervoltage breaker to trip.

I doubt if a diode and capacitor will help.
 
We realize the problems with these batteries. That's why we want to minimize the occurrence of the undervoltage condition using capacitors or some other method. We have little to no experience with power electronics and researching hasn't given me any useful information.
 
How long does the undervoltage condition last? If it's real short maybe a capacitor would help, however the capacity of even a large capacitor is miniscule compared to a battery. I don't think a diode will help, though.

Is there any possibility of adding more of the same lithium polymer batteries that you have now?

Maybe one or more of the existing batteries needs to be replaced. Has this just started recently?
 
skeptic2 said:
How long does the undervoltage condition last? If it's real short maybe a capacitor would help, however the capacity of even a large capacitor is miniscule compared to a battery. I don't think a diode will help, though.

Is there any possibility of adding more of the same lithium polymer batteries that you have now?

Maybe one or more of the existing batteries needs to be replaced. Has this just started recently?
The condition happens in the time of probably milliseconds. Half the time, the instrumentation doesn't even pick it up. Sometimes it does and that's how I figured out that was the problem.

We can't fit anymore battery in the space we have. We also don't have the budget for another lithium battery even if we could fit it. It happens generally when the battery becomes slightly discharged. When it's fresh, it normally doesn't happen.

Would those large automotive "audio" capacitors be useful? They normally have very large capacitance.
 
If you know how long the undervoltage condition lasts and how much current the motors are drawing, you can calculate what size capacitor you'll need.

Amps x seconds = Coulombs
Capacitance (Farads) = Coulombs / Volts.
 
solacedagony said:
Would those large automotive "audio" capacitors be useful? They normally have very large capacitance.


But how physically big are they? Bigger than another battery once you do the above calculation?
 

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