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Motor Current & Battery Budget

Whether a charge lasts the day. The V5 battery holds 14 Wh and can supply 256 W, far more than the rules let you fit.

Inputs

Drivetrain

%

Share of full power averaged over the match. A drive is rarely flat out: it coasts, stops and reverses.

Everything else

%

Match

sec

Autonomous plus driver control. Check the current manual, since this changes between seasons.

Results

Matches per charge

—

Before the battery is flat.

Average power draw

—W

Energy per match

—Wh

Continuous runtime at this draw

—min

Motor power installed

—W

Peak draw against battery ceiling

—%

Peak current, roughly

—A

Peak draw, against what the battery can give—

Past the second mark the battery is the limit rather than the motors, and everything gets slower at once.

When you would use this

  1. Our battery dies before the end of the day.

    Enter your motor counts and how hard each group works. The matches-per-charge figure tells you whether you need a second battery or a lighter drive, and which one is cheaper.

  2. The robot feels sluggish in the last thirty seconds of a match.

    Compare your average draw against what the battery can supply. A drive that sits near the peak for the whole match leaves nothing for the lift when it matters.

  3. We are adding another mechanism and want to know if it fits.

    Run it once as you are and once with the new motors added. The change in energy per match is what that mechanism actually costs you, which is a better argument than a guess in a design review.

  4. Judges asked how we chose our motor layout.

    The breakdown of installed power against average draw is exactly the evidence that question wants, and the notebook export at the bottom of this page turns it into an entry.

How this is calculated

Battery life is an energy sum. The battery holds a fixed amount, and every motor spends some of it:

average draw = Σ (motor rating × how hard it works) energy/match = average draw × match length ÷ 3600 matches/charge = 14 Wh ÷ energy per match

The term worth thinking about is the effort figure, because it is the only one you are estimating. A drivetrain is not at full power for two minutes. It accelerates, coasts, stops, reverses, and spends real time doing nothing at all. A lift holding a position draws far more than one sitting at rest.

Why the battery is not your limit

The battery can supply 256 W, and VEX rates it for 10 motors at full power continuously. The rules cap you at 88 W of motors. So you cannot legally build a robot that asks more of the battery than it can give.

Which means brownouts are not usually a power-budget problem. They come from a battery that was already low, a bad connection, or a genuinely stalled mechanism pulling hard, not from having fitted too many motors.

Two things teams get wrong about this battery

  • A low battery does not make the robot slower. VEXos holds motor output steady until the battery is properly flat. If the robot felt sluggish all match, look for a mechanical cause. The battery is a convenient thing to blame and rarely the culprit.
  • Voltage is a poor fuel gauge. These cells hold a nearly flat voltage across most of their range, which is why the battery counts charge in and out rather than reading volts. Trust the percentage on the brain, not a voltmeter.

Sources & assumptions

Battery capacity (14 Wh, 1100 mAh at 12.8 V), maximum output (256 W, 20 A) and the 10-motor figure are VEX published specifications. Motor ratings of 11 W and 5.5 W are likewise published.

The "2 to 3 matches per charge" figure is community guidance rather than a VEX specification, and is included as a reality check rather than as an input.

Peak current is estimated as power ÷ nominal voltage, which understates the real figure because it ignores efficiency and because voltage sags under load. Treat it as a lower bound.

Save this run, and compare

Keeps what is on screen so you can change something and see both sides of the change. Saved in this browser only, never uploaded.

Save this as evidence

Collects what you entered, what came out, how it was worked out, and anything the tool flagged, with a timestamp and a version so someone else can reproduce it.

This is evidence, not a notebook entry. It deliberately does not write your problem statement, your reasoning, or your conclusion, because under RECF rules an Engineering Notebook has to be the students' own work and no tool may generate or organise its content. Take the numbers, decide what matters, and write it yourself.