Power: the battery, the switch and the fuse

Every motor, servo, sensor and radio on your robot runs off one battery, through one switch, protected by one fuse. When any of those three is weak, the whole robot is weak — and it usually shows up as a disconnect in the middle of a match, not as an obvious fault on the bench.

The battery

FTC robots run on a 12V nickel-metal hydride (NiMH) pack. The Competition Manual lists exactly which packs are legal (rule R601 in the 2026–27 manual), and only those part numbers are allowed, even if another pack looks identical. The list changes between seasons, so check the current Competition Manual before you buy.

The most common pack is the REV 12V Slim Battery (REV-31-1302). Its published specs make a good worked example:

Spec	REV 12V Slim Battery
Cells	10 NiMH cells in series
Nominal voltage	12V (10 cells × 1.2V)
Capacity	3000 mAh
Fuse	20A ATM blade fuse, in-line on the red lead
Connector	XT30
Wire	16 AWG, 150 mm long
Mass	567 g

Nominal voltage

The label voltage. A freshly charged NiMH pack reads noticeably higher than 12V with no load on it, and the reading falls as the pack drains and whenever the robot pulls a lot of current.

Capacity (mAh)

How much charge the pack holds. 3000 mAh means, in theory, 3A for one hour, or 30A for six minutes. In practice the voltage sags too low to run the robot well long before the pack is truly "empty".

Worked example — how much energy is that? Energy is voltage × capacity: 12V × 3.0Ah = 36 watt-hours. A robot can pull a large share of that out in a few minutes of hard driving — which is why the battery gets warm and why the fuse matters.

The fuse

The fuse is the robot's only over-current protection. On the REV Slim Battery it is a standard 20A ATM mini blade fuse in a black holder on the red wire. If the robot tries to pull far more than 20A for long enough, the fuse blows and the robot goes dead.

Worked example — can four drive motors blow the fuse? goBILDA lists its 312 RPM Yellow Jacket gearmotor at 9.2A stall current. Four of them stalled at once — say, pushing against another robot — would ask for 4 × 9.2 = 36.8A, nearly double the fuse rating. Game Manual 0's advice follows directly: avoid designs where more than two motors stall at the same time.

To check a fuse, REV's guidance is to carefully open the black fuse housing and look at the metal bridge inside the fuse. If it is broken, replace the fuse. Keep a few spare 20A ATM fuses in your pit kit; they are sold at auto-parts stores.

The main power switch

Every robot needs exactly one approved main power switch, and it must control all power from the battery (rule R603 in the 2026–27 manual). Approved switches include the REV Switch Cable and Bracket (REV-31-1387) and the goBILDA Floodgate Power Switch; the manual has the full list. The switch is not a breaker — it does no current protection. Its job is to let you, and field staff, kill the robot instantly.

Where you put it matters as much as which one you buy:

• Reachable. Field staff may need to turn your robot off, so it should be easy to find and operate from outside the robot.
• Away from moving parts and pinch points. The manual requires this.
• Protected from other robots. REV suggests mounting it slightly recessed, and FIRST's wiring guide recommends not mounting it flush with or outside the frame perimeter, angling it upwards, or putting it behind a shield with a finger hole.
• Clear of game pieces, so nothing can land on it and flip it.

Voltage sag, brownouts and disconnects

A battery is not a perfect 12V source. When the robot pulls a lot of current — accelerating a drivetrain, lifting a heavy arm, stalling a motor — the voltage at the hub drops for as long as the load lasts. That is voltage sag, and it is worse with a low or worn-out battery and with loose connections.

If the voltage sags far enough, the electronics stop working properly. That is a brownout. REV lists the symptoms:

• The Driver Station shows about 9V or lower while code is running.
• The Driver Station makes its disconnect sound, or shows power warnings.
• Motors run slower than they are commanded to.
• The robot connects fine until a particular mechanism runs.

The Control Hub's status LED also helps: blinking orange means the hub is seeing less than 7V from the battery. REV has also linked unexpected IMU resets to poor battery connections and low voltage.

REV also gives a floor for the battery itself: do not run a pack below about 9.0V with no load on it. Running the robot until it stops responding can permanently damage the cells. Brief dips below 9V under heavy load are expected and fine.

Charging

Use a charger made for NiMH robot packs, such as the REV 12V Slim Battery Charger. REV's published guidance:

Battery safety

These rules come from FIRST's Robot Wiring Guide and REV's battery guidance. They are not optional.

• Never leave batteries charging unattended. FIRST notes that a faulty battery can overheat on the charger and become a fire hazard.
• Protect the terminals. Do not store or carry batteries loose in a bag with tools, screws or other metal that could short across the connector.
• Never have bare wire on both battery leads. If two exposed ends touch, the battery shorts out — that can damage the pack and start a fire.
• Retire damaged packs. REV says not to use a battery if the wire sheathing is fraying or the pack is visibly damaged. Take it out of service, tell your coach or mentor, and recycle it through a proper battery-recycling drop-off rather than the bin.
• Unplug by the connector body, not the wires. Pulling on the leads can tear the XT30 solder joints loose.
• Mount it securely, with no sharp edges. FIRST's wiring guide recommends a snug holder plus a Velcro strap, low in the robot, with nothing that can cut into the pack or its leads.

Managing batteries as a team

Rookie teams usually lose battery problems in the pits, not on the field. A few habits fix most of them:

• Number every battery and keep a simple log of when each was charged and how it performed.
• Bring enough packs that each one can fully charge and cool between matches.
• Retire slow packs. REV notes that a battery that no longer holds charge like it used to may simply be at the end of its life. If one pack always browns out first, stop using it in matches.

Check yourself

Your battery's fuse blew during practice. A teammate has a spare 30A blade fuse that fits the same holder. What should you do?

• AUse the 30A fuse — it is less likely to blow again
• BReplace it with a 20A ATM fuse, then find out what made it blow
• CBypass the fuse holder until you can buy a spare
• DSwap in a self-resetting fuse so it never needs replacing

Check yourself

Your robot drives fine, but every time the lift runs at full power the Driver Station shows about 8.5V and sometimes plays the disconnect sound. What is the most likely cause?

• AA bug in the lift code
• BA brownout from a low or worn battery, or a loose power connection, under heavy load
• CWi-Fi interference from other teams
• DThe main power switch is the wrong brand

Check yourself

A battery just came off the charger and feels warm. The next match is in 40 minutes. What is the right call?

• APut it in the robot immediately so it is ready
• BPut it straight back on the charger to top it up
• CLet it cool to room temperature, then install it before the match
• DCool it in a freezer to speed things up

Next

With power understood, the next lesson wires the rest of the robot to it — in the correct order.

Sources and further reading

• REV 12V Slim Battery (REV-31-1302) — specifications and best practices
• REV: 12V Battery Best Practices and Troubleshooting
• REV: General Troubleshooting — never plug a charger into a hub
• REV: Control Hub Troubleshooting — brownout symptoms
• REV: Status LED blink codes
• REV Switch Cable and Bracket (REV-31-1387)
• FIRST Robot Wiring Guide — battery security, battery safety, switch placement
• FTC Competition Manual — section 12.6, Power Distribution
• Game Manual 0: Motor Power — current draw and the 20A limit
• goBILDA 5203 Yellow Jacket, 312 RPM — stall current used in the example