Battery C-Rate Calculator

Battery discharge rate as a normalized multiple of capacity first appeared in lead-acid battery standards in the early 1900s. The convention was simple: pick a reference discharge time (commonly 20 hours, giving the "C/20" rating), and then express any other discharge in terms of that reference. The letter "C" stands for capacity, and a 1C rate means the battery is fully discharged in 1 hour — the reciprocal of the reference rate.

The notation became universal when the IEEE and IEC standardized it in the 1960s for nickel-cadmium and later lithium chemistries. It lets an engineer compare discharge stress across radically different battery sizes: a 1000 mAh LiPo at 10C (10 A) and a 100 Ah EV pack at 10C (1000 A) experience the same relative stress on their chemistry, despite the absolute currents differing by 100×. Every modern battery datasheet specifies both continuous and peak C-ratings — a direct descendant of that early 20th-century lead-acid convention.

The physics underlying chemistry-specific C-rate limits comes from internal resistance. At high currents, I²R losses inside the cell generate heat; if the heat can't escape fast enough, the electrolyte degrades, separator films melt, and thermal runaway can occur. Lithium polymer packs for R/C aircraft were the first commercial cells to push continuous C-rates into the 25–50× range in the 2000s, using thin pouch construction and high-surface-area electrodes specifically to move heat out quickly.

Pick what to solve for (C-rate, current, or capacity), enter the other two with engineering suffixes, and this tool returns the third via C = I / Q. A chemistry safety guide on the right compares your calculated rate against typical continuous and burst limits for Li-ion 18650, high-drain 21700, LiPo drone cells, LiFePO4, NiMH, lead-acid, and alkaline — so you can spot when your design is asking a cell for more than it can safely give.

Design rule of thumb: size cells for 20–50% more than peak current. Running below the datasheet rating extends cycle life significantly, because heat (not total energy moved) is what kills cells. Everything runs client-side; no values leave your browser.