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DC Motor Voltage Smoothing

  1. Aug 31, 2011 #1
    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.
  2. jcsd
  3. Sep 1, 2011 #2
    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.
  4. Sep 1, 2011 #3
    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.
  5. Sep 1, 2011 #4
    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?
  6. Sep 1, 2011 #5
    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.
  7. Sep 1, 2011 #6
    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.
  8. Sep 2, 2011 #7


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