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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.

Drive harder

Drive harder
45%

The top bar is the robot's average power over a match: the drive in the darker part, everything else in the lighter part. Underneath is one full battery, cut into the slices a match takes. Drive harder and the drive's share grows, every slice gets wider, and fewer matches fit on a charge.

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 truly 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.

Every source the site uses, and when it was checked

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.

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.

My notebook

Notebook page

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Open My notebook

    This page

    Our own layouts

    A layout remembers how a page is set up: its type, its system, which parts print and which of your tables it uses. It never remembers a word, a number or a photo.

    Your team kept on this device
    Add someone to the team list
    Your words never written for you

    These are the parts a judge or teacher reads to see your thinking, so the page leaves them to you. Leave a box empty to get lined space for writing by hand. Start lines with "- " for a list. The notes under each box only count and point things out; they never change what you wrote.

    Rough notes

    Rough notes

    Rough notes

    Rough notes

    Rough notes

    Rough notes

    Rough notes

    Rough notes

    Rough notes

    The word counts can be measured against a target your team sets on My notebook. The reading level is Flesch-Kincaid grade level = 0.39 × (words ÷ sentences) + 11.8 × (syllables ÷ words) − 15.59. Syllables are counted by vowel groups, so the grade is an estimate of the school year the writing reads at. It says nothing about whether the writing is any good.

    One part at a time, with a clock. At the end of practice, while you remember it.
    How you tested your details

    Fill in what applies. These print as a small table under the method.

    Before and after

    Compare this calculation with a run you saved earlier on this tool (under “Saved runs”). The page shows what changed and by how much.

    A line through the data from your recorded table

    Pick two measured columns from the same table. The page plots them and draws the least-squares line, only across the range you measured.

    Robot log a CSV from the robot's SD card

    Load a CSV your robot saved. Pick the time column and up to three columns to plot, and type their units. Only those columns go on the page; the file stays on this device.

    Two versions' trials your data, compared

    Pick a column of recorded trials for each version, from this page or another saved page. The page shows both averages and a 95% range for the difference. The numbers are copied in when you pick them, so later changes to the other page don't change this one.

    Options compared your scores

    Score each option 1 to 5 on each criterion, and weight each criterion 1 to 5 by how much it matters. The page only multiplies and adds.

    Photos and sketches stay on this device

    Up to 4. Paste a screenshot with Ctrl+V, drop pictures here, or pick them. Photos are shrunk before they’re kept, and an unsaved page keeps them in its draft. Use Mark up to draw arrows, labels, dimensions and numbered callouts on a photo. Mark one photo Before and one After to print them side by side.

      Test plan written before testing

      Write this before you test: the question, what you expect with a number, and what result would make you drop the idea.

        Build steps our build

        One row per step, in order, so someone could rebuild from this page.

          Parts used our parts list

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

            Plan the cuts: packs every piece of one part (sizes in holes, or inches on the hole grid) onto whole sticks of the stock length you type.

            Ways it could fail our scores

            List the ways this could fail. Score how likely (1 to 5) and how bad (1 to 5); the page multiplies them and puts the biggest risks first.

              Designed against built our measurements

              What the CAD or the plan said, and what you measured on the robot. The tolerance is yours.

                Changes from the last version our changes

                What changed since the last version, why, and which result it was meant to move.

                  That page’s settings, results and first photo are copied in and set beside this page’s. The result each change should move is looked up on both pages.

                  Tasks our plan

                  What needs doing, who, and by when. Open tasks carry to the next meeting page until someone ticks them done.

                    The pages behind this idea our evidence

                    The pages in this notebook that show the idea being tried, measured or changed. A judge reading the claim can turn straight to them.

                      Time spent our hours

                      Who worked on this and for how long.

                        Session log what we did, as we did it

                        Notes taken during a session. The time comes from the clock when the note was taken; every word is yours.

                          Size limit does it fit?

                          Type the starting size limit from the current game manual, and the robot's size as designed or measured, in inches. The site doesn't store the limit, because it changes with the game.

                          What broke from your breakage log

                          The breakage log is on My notebook. Copy it onto this page as a table, as it stands now; later entries don't change this page.

                          CAD model which version this page is about

                          Paste a link to the model (Onshape, Fusion or anything else) and the name you use for this version. For Onshape, the page reads from the link whether it points at a saved version or a workspace that keeps changing. Nothing is fetched.

                          Your code
                          Comments notes for the team, not the page

                          A teammate or coach can leave a note here. Comments stay beside the page, travel with it in exports and links, and don't print unless you tick them under Include.

                            Sources you used besides this tool

                            Videos, other teams, manuals, people. The tool’s own sources are cited for you.

                              Sign-off
                              Include

                              Press a ? to see what an option is for, or read them all on How the notebook tools work.

                              What “The calculator’s figures” is for

                              The calculator’s own diagrams and charts, numbered as figures and drawn from your numbers. Judges look for pictures that explain a design.

                              What “How it was calculated” is for

                              The formulas and steps the calculator used, so a reader can check the arithmetic for themselves.

                              What “The calculator’s warnings” is for

                              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.

                              What “The calculator’s notes” is for

                              The calculator’s plain-language notes on the result. Off at first, because they read like conclusions, and the conclusions on a page should be yours.

                              What “Lined space” is for

                              Ruled lines under any part you haven’t written yet, so a printed page can be finished by hand.

                              What “Writing space as” is for

                              The blank space for handwriting as ruled lines, a dot grid or quarter-inch squares. Dots and squares suit sketches and hand-drawn graphs.

                              What “A summary under recorded data” is for

                              The number of readings, the average, the spread, the lowest and the highest under each table you recorded, worked out from your numbers. It shows every run counted, not just the best one.

                              What “Words used” is for

                              A small box with the site’s one-line definitions of glossary terms the calculator used, for readers who don’t know them. It never looks at your own writing.

                              What “Robot at the time” is for

                              The Robot Profile as it was when the page was first saved, so weeks later you can tell which version of the robot a test was run on.

                              What “Long tables in full” is for

                              Long calculated tables printed complete on an extra page, instead of every few rows on the page itself.

                              What “Fit on one sheet” is for

                              Smaller type and shorter tables, aiming for one printed sheet. Use it when every entry in your notebook should be a single page.

                              What “Wider left margin” is for

                              A wider left edge, so the page can be hole-punched or bound without losing any words.

                              What “Larger type, for reading aloud” is for

                              Bigger print with more room between lines, for the team member who reads the page to a judge. It overrides Fit on one sheet.

                              What “Our rubric scores” is for

                              Your own scores against the rubric, on a sheet of their own, for checking a page before a judge does.

                              What “When this page was first saved” is for

                              The day the page was first saved, printed beside its date. It shows the notebook was kept while the work happened.

                              What “Teammates’ comments” is for

                              Prints the comments teammates left on this page at the end, marked as not part of the page. Off at first, because comments are notes for the team, not for judges.

                              What “Show every table and block” is for

                              Shows every table and block in the editor, not only the ones this type of page usually uses. It changes the editor, not the printed page.

                              Rubric check

                                This checks what is on the page, not how good it is. Clear, specific writing in your own words is what earns the top marks.

                                Preview, letter size

                                Write it now

                                Where this goes next

                                Explore further on the full map

                                Where the answers on this page can be used, and which tools could have given you the numbers it asks for.

                                Comes after

                                Motor Power Budget Checkerfills in your motor count hereWhole Robot Design Checkfills in your motor count hereTorque & Stall Checkgives you your motor countAcceleration & Time-to-Speedgives you your motor countFlywheel Design Studiogives you your motor count

                                You are here

                                Motor Current & Battery Budget

                                Where your answer goes

                                Motor Power Budget Checkercheck that motor mix is legal before you plan around itbrings your motor count with itMotor Diagnostics Dashboardcompare that predicted current with what your motors really pulleduses your current draw againPneumatic or Motor?if the battery runs short, see if a piston could take a motor's jobbrings your motor wattage with itVoltage Drop & Power Spikessee how far the voltage dips when those motors all pull at once

                                a number carries across the next question it raises

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