All modules
Beginner15 min read

Power Budget 101

How to calculate runtime, avoid brownouts, and size your battery and regulators. The math that saves your robot from dying mid-demo.

Why power budgets fail

Most robotics builders estimate power consumption from spec-sheet maximums, add them up, and buy a battery that covers the total. Then the robot browns out during a motor stall or dies 20 minutes into a demo.

The problem: spec sheets list peak draw, not average draw. Real systems spend most of their time in steady-state, not at peak. And peak occurs simultaneously (startup surge) or intermittently (stall). Budget for both.

The three numbers you need

1
Average current (A)
What the system draws in normal operation. This determines runtime.
2
Peak current (A)
Startup or stall current. Determines wire gauge and fuse rating.
3
Capacity (Ah)
Battery capacity. Divide by average current to get runtime in hours.

Runtime formula

# Runtime calculation
capacity_Ah = 5.2 # 5200mAh
usable_Ah = capacity_Ah * 0.80 # 80% DoD — safe for LiPo
avg_draw_A = 1.26 # 14W average at 11.1V
runtime_hr = usable_Ah / avg_draw_A
# → 3.3 hours (we measured 2.1hr at peak load)

Always apply an 80% depth-of-discharge (DoD) factor for LiPo cells — discharging below 20% damages them. For harsh environments or high current draws, use 70%.

Room-Nav power audit (real numbers)

SUBSYSTEMIDLE (W)ACTIVE (W)NOTE
Jetson Orin Nano5W10WMAXN mode, varies with AI workload
OAK-D Lite2W2.5WMyriadX inference adds ~0.5W
RPLIDAR A12W2.5WMotor on during active scan
ReSpeaker 6-mic0.5W0.8WUSB bus power
Motors ×20.5W3.6WN20 at cruise vs. stall
USB hub + misc0.5W0.8WHub, display, wiring overhead
Total10.5W20.2W

At 14W average draw (between idle and active), a 3S 5200mAh LiPo (57.7Wh × 80%) gives 3.3hr calculated. We measure 2.1hr because our average load is closer to 18W — nav2 + DepthAI + voice is heavy.

Regulator selection

LiPo voltage varies: a 3S pack is 12.6V full, 11.1V nominal, 9.9V cutoff. Your compute board needs a stable 5V (or 12V for some motor drivers). Choose a regulator with:

  • Input range covering full LiPo discharge: 9V–14V for 3S
  • Output current ≥ 120% of your peak load (headroom matters)
  • Efficiency ≥ 90% — wasted power becomes heat, heat kills runtime
  • Soft-start to suppress startup surge — prevents voltage dip on power-on
TIP — Build the spreadsheet first

Before ordering any power components, fill in a simple spreadsheet: list every subsystem, look up idle and peak current from the datasheet, multiply by supply voltage for watts. Sum for average draw. Pick a battery that gives your target runtime × 1.2 safety margin.

Related kit: Room-Nav BOM has a full verified power audit in the engineering checklist.

All modulesNext: DC Motor Control
SEE IT IN PRACTICE

The RP2350 kit is the best way to practice power budgeting with a real build — simple enough to see every watt clearly.

View RP2350 BOM →