Voltage, Current, and Resistance

The three quantities that run your robot#

Everything electrical on an FRC robot comes back to three quantities. The water-in-a-hose picture is the fastest way to keep them straight:

• Voltage (volts, V) is the pressure pushing current through the circuit. A fully charged FRC battery supplies about 12-13 volts of pressure.
• Current (amps, A) is the flow rate — how much charge moves past a point each second. A stalled drivetrain motor pulls a lot of current; a blinking LED pulls almost none.
• Resistance (ohms, Ω) is the friction that opposes flow. Thin wire, a loose crimp, or a motor's windings all add resistance.

The relationship to lock in: voltage causes current, and resistance limits it. No voltage difference, no current. Add resistance for the same voltage, and current drops.

One practical detail: voltage is always measured between two points — that's why your multimeter has two probes. Current flows through a component; voltage exists across it.

Your whole robot hangs off one battery#

An FRC robot runs on a single 12V, 18Ah sealed lead-acid (SLA) battery. "12V" is the nominal pressure; a healthy, fully charged pack actually reads 12.7 to 13.5 volts open-circuit (sitting with nothing connected). "18Ah" (amp-hours) is roughly how much current it can deliver over time before it's drained.

Here's the insight that trips up every rookie: voltage sags under load. When six motors slam to full power at the start of a match, they demand huge current all at once. Because the battery and wiring have small but real resistance, that current drags the measured voltage down. Sag far enough and the roboRIO triggers a brownout to protect itself — outputs cut out and your robot goes limp for a moment. Knowing how voltage, current, and resistance interact is the difference between guessing why the robot "died" and actually diagnosing it.

Units and the tools that read them#

A multimeter reads all three. Measure voltage by touching both probes across two points while the circuit is live. Measure resistance on a component with power off — checking resistance on a powered circuit gives garbage readings and can damage the meter. You don't even need a meter for current on the bot: the PDH (Power Distribution Hub) or PDP (Power Distribution Panel) reports per-channel current to your code over CAN, so you can watch amps live during a match.

Get comfortable with cause and effect here — voltage drives, resistance limits, current is the result. The next lesson ties all three into one equation you'll use constantly.