Tool 08 · Power System
Most "mystery" robot failures are power failures. Add up what your motors, servos and electronics actually draw, and this tool sizes the three things that keep the lights on: the BEC, the fuse, and the wire.
≈ 30–50% of stall current while driving
SG90 ≈ 0.5 A · standard ≈ 1 A · high-torque ≈ 2.5 A
MCU + sensors ≈ 0.3–0.5 A · Raspberry Pi ≈ 1 A+
Please enter non-negative values in every field.
Power system spec
5 V load = servos (if on BEC) + electronics
Battery current = motor current + 5 V load × 5 ÷ V_bat ÷ 0.85
BEC rating = 5 V load × 1.5 (rounded up to a real product)
Fuse ≈ 1.4 × continuous current · wire sized from ampacity table
The subtle line is the second one: a switching BEC converts power, not current, so 4 A on the 5 V rail costs only about 2.1 A from a 3S battery (at ~85% conversion efficiency). Linear regulators don't get this discount — they draw the full rail current from the battery and burn the voltage difference as heat, which is why anything above ~1 A on 5 V should come from a switching UBEC.
The fuse protects the wire, not the electronics — it's sized above your continuous draw so it never nuisance-blows, but below what the wiring can survive in a dead short. The gauge recommendation applies to the battery leads (which carry everything); branch wires to a single motor or servo can be one or two sizes thinner.
If your motor working current is a guess, get a real number from the Motor Sizing Calculator, then confirm the pack can actually deliver the total with the Battery C-Rating Checker.
Wire and fuse should survive brief stall events, but sizing everything for simultaneous all-motor stall produces an absurdly heavy power system. The convention: continuous parts (BEC, battery) sized for working current with headroom; protective parts (wire, connectors) checked against realistic stall peaks.
One main fuse on the battery positive lead is the minimum. Bigger robots add a smaller fuse per branch — a motor short then kills one branch instead of the whole robot.
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