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Voltage Drop & Power Spikes

How far the battery voltage dips when every motor pulls at once, how much power is lost as heat on the way, and how to measure your own battery instead of guessing.

Inputs

The battery

V

Nothing moving. A full V5 battery reads about 14.6 V; it counts as flat at 10 V.

V

Example value. Drive into a wall at full power and read it after a second, before the motors heat up.

A

Example value. In VEXcode, print Brain.Battery.current() next to Brain.Battery.voltage().

The spike

A drive launching from rest: every drive motor. A pushing match: the drive plus anything else working.

A

VEXos limits an 11W motor to 2.5 A when eight or fewer are plugged in, and lower when there are more.

A

Brain, radio, sensors and lightly loaded motors. An estimate.

s

A drive reaches speed in well under a second; a pushing match can last several.

V

10 V is where VEX treats the battery as flat.

Results

Voltage at the robot during the spike

—V

Voltage lost to resistance

—V

Current during the spike

—A

Of the battery's rated maximum

—%

Rated for 20 A.

Power reaching the robot

—W

Power turned into heat on the way

—W

Charge the spike uses

—mAh

Most current before you reach your floor

—A

Most power this battery could ever give

—W

Resistance used

—Ω

Show the dip for each number of motors

The voltage and power for every number of motors spiking at once.

Hit the throttle with more motors

Hit the throttle with more motorsvolts0amps
6

A battery behaves like a perfect battery with a small resistor inside it. Every amp you draw loses a little voltage across that resistor, so the voltage at the robot falls in a straight line as the current climbs. Each motor you add pulls the dot further down the line. The dashed line is the lowest voltage you said you are happy with; where the two cross is the most current you can draw.

When you would use this

  1. The robot stutters when it launches off the line.

    Measure the battery, then check the voltage during a full-drive spike here. If it dips near your floor, try a fresher battery before blaming the code.

  2. One battery always seems weaker than the others.

    Take the two readings on each battery with the same load. The one with the biggest drop has the highest resistance, and it will sag most in a pushing match.

  3. We want to know whether an extra motor is worth it electrically.

    Open the table. Each extra motor at its limit adds the same current and the same dip, so you can see where you cross your floor.

How this is calculated

A battery acts like a perfect voltage source with a small resistance in series. The more current flows, the more voltage is lost across that resistance before it reaches the robot.

resistance = (voltage at rest − voltage under load) ÷ load current current = motors × current each + everything else dip = current × resistance at robot = voltage at rest − dip heat = current² × resistance charge = current × seconds ÷ 3.6 (mAh)

Why there is a ceiling on power

Drawing more current delivers more power, until the dip eats half the voltage. Past that point, extra current only makes more heat. The most any load can get is

maximum power = voltage at rest² ÷ (4 × resistance)

V5 motors are nowhere near it: VEXos limits their current long before. That limit is also why the dip is predictable: at their limit, the motors draw a fixed current whatever the voltage does.

Taking the readings

  • Charge the battery fully, and let it rest a few minutes.
  • Read the voltage with nothing moving.
  • Drive into a wall at full power and read voltage and current together after about a second.
  • Stop before the motors warm up: hot motors cut their current and spoil the reading.

Make a notebook page

Turns this calculation into a dated engineering notebook page: the setup, data, results, figures, working and sources, numbered and cited. You add the goal, the decision and the reflection in your own words, plus photos and sketches. Print it, paste it into Google Docs, or download a Word file or slides.

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          Lengths in holes are also shown in inches between the end holes. Weights are only added up where you type them.

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                              Things the calculator flagged, such as a motor near stall. They’re quoted and credited to the calculator, so they never read as your conclusion.

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                              What “Fit on one sheet” is for

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                                Sources & assumptions

                                Published. Battery full and flat voltages, its 20 A rated maximum, and the 2.5 A motor current limit with eight or fewer 11W motors.

                                Not published. The battery's internal resistance. That is why this page measures it by default. The example readings in the fields are illustrations, not V5 figures; replace them with your own.

                                Simplification. Real resistance changes with charge, temperature and age, and a battery recovers a little voltage after a spike. Treat the result as the dip at the moment of the spike, with the battery as it was when you measured it.

                                Checked. scripts/verify-electrical.mjs simulates 500 batteries and checks every figure against the circuit itself: currents by bisection, power by conservation, and resistance recovered from simulated meter readings.

                                Every source the site uses, and when it was checked

                                Where this goes next

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                                Comes after

                                Motor Current & Battery Budget

                                You are here

                                Voltage Drop & Power Spikes

                                Where your answer goes

                                Motor Current & Battery Budgetsee whether one charge lasts a full day of matchesMotor Diagnostics Dashboardfind which motor is pulling the most currentBattery Over a Dayplan how many charged batteries a competition day takes

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