Battery C-Rate and Discharge Time Calculator

C-rate expresses charge or discharge current relative to a battery's rated capacity. A current equal to the battery's amp-hour rating is 1C, twice that current is 2C, and half that current is 0.5C, so C-rate = current (A) / capacity (Ah).

Discharge time in hours is the reciprocal of C-rate: t = 1/C. Equivalently, t = capacity (Ah) / current (A). A 1C discharge takes 1 hour, a 2C discharge takes 0.5 hours (30 minutes), and a 0.5C discharge takes 2 hours.

C-rate is itself defined as current divided by capacity, so as current (and therefore C-rate) increases, the same fixed amount of stored charge is delivered in proportionally less time. Doubling the discharge current always halves the discharge time, which is exactly the 1/C hyperbolic relationship.

Most consumer lithium-ion cells are rated for continuous discharge between 1C and 3C, with some high-power cells rated up to 10C or higher, and specialized cells even higher. Exceeding a cell's rated C-rate risks excessive heat, voltage sag, and accelerated degradation.

Not exactly, real batteries deliver somewhat less than their nameplate capacity at high discharge rates due to internal resistance and the Peukert effect, especially pronounced in lead-acid batteries. This calculator uses the ideal 1/C relationship, which is a close approximation for lithium-ion at moderate C-rates.

The same C-rate concept applies to charging, a 1C charge current would fully charge an empty battery in about 1 hour under ideal conditions. Fast-charging lithium-ion batteries often charge at rates around 0.5C to 2C, with the rate typically tapering down near full charge to protect the cells.

Battery capacity is entered in amp-hours (Ah) and discharge current in amps (A). Output C-rate is dimensionless (expressed as a multiple of C), and discharge time is given in both hours and minutes.

C-rate and energy density describe different battery properties, C-rate is about how fast energy can be delivered (power capability), while energy density is about how much energy is stored per unit mass or volume. A battery can have high energy density but a low safe C-rate, or vice versa.

Electric vehicle batteries must deliver large bursts of current for acceleration and accept fast charging, both requiring a higher sustainable C-rate than a phone battery, which typically discharges slowly over many hours and charges at a modest rate to preserve cycle life.

Yes, rearrange the same relationship: required capacity (Ah) = discharge current (A) x required runtime (h). Entering your known current and desired runtime as a C-rate (current / target capacity) lets you check whether a candidate battery's capacity meets your runtime requirement.