Tool 07 · Power System
A C-rating is a promise about current, and robots are very good at testing promises. Enter your pack and your robot's draw, and this tool shows the real current ceiling, your safety headroom, and whether brownouts are in your future.
The number printed on the pack (e.g. 25C)
All motors driving + electronics
Pushing, climbing, or stall events
Please enter positive values in every field.
Discharge spec
Max continuous current = capacity (Ah) × C-rating
Realistic max = rated max × trust factor
Headroom = 1 − (your draw ÷ realistic max)
Runtime ≈ capacity (Ah) ÷ draw × 60 × 0.8 usable
The C-rating multiplies capacity into current: a 2.2 Ah pack rated 25C claims 55 A continuous. The catch is the word claims. Independent testing consistently shows budget packs sagging hard and heating up well below their printed rating, which is why this tool applies a trust factor — the printed number is a marketing ceiling, not an engineering guarantee.
Peak draw matters separately: most LiPo packs tolerate short bursts of roughly twice their continuous rating, but every deep sag event stresses the cells. If your stall current blows past the burst estimate, the fix is a higher-C pack, a bigger pack (more Ah at the same C = more amps), or current-limiting in your motor driver.
The runtime figure here uses the same honest 80%-usable-capacity assumption as the Battery Runtime Calculator — use that tool for the full runtime picture, and the Motor Sizing Calculator if you don't yet know your motors' current draw.
Higher-C packs cost more and are often heavier for the same capacity. Buy the C-rating your draw requires with 2× headroom — paying for 100C on a slow rover that draws 3C is money that should have gone into capacity.
Effectively yes: internal resistance rises with cycles and abuse, so an old pack sags more at the same current. A pack that browned out "suddenly" usually degraded gradually and crossed the threshold quietly.
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