Battery C-Rate Calculator

C-rate to current and current to C-rate, and then the part the arithmetic hides: the capacity a battery actually gives at that rate, which is below its rating whenever the rate is above the one the rating was measured at. With a check against the datasheet’s continuous and pulse current limits.

Last updated 24 Sep 2026

C-rate and current

C-rate ↔ current, and the capacity you really get

Which way?

Rated capacityThe capacity printed on the label, in the units beside it.

Capacity unit

C-rate (multiples of the rated capacity)0.5C of a 200 Ah battery is 100 A. 1C empties the rating in an hour — on paper.

Discharge current (A)

Chemistry (sets the exponent and the rating rate)

Peukert exponent (1.0 = ideal)From your own datasheet if it lists capacity at two rates: k = ln(t2 ÷ t1) ÷ ln(I1 ÷ I2). Use the figures from your part’s datasheet; typical values vary widely between manufacturers.

Hour-rate the capacity is quoted at (h)20 for most lead-acid (C20), 1 for many LiFePO4 packs, 5 for Li-ion cells. Getting this wrong moves every answer below.

Nominal voltage (V)Only used for the power and energy rows.

Datasheet maximum CONTINUOUS current (A)From the cell or battery datasheet. Use the figures from your part’s datasheet; typical values vary widely between manufacturers.

Datasheet maximum PULSE current (A)The short-burst rating, usually quoted with a duration such as 5 or 30 seconds.

The battery, an ammeter and the load. The C-rate is nothing more than this current divided by the rated capacity, so 0.5C of a 200 Ah battery is 100 A. The capacity written beside the battery is the capacity actually available at THIS rate, which is less than the rating whenever the current is above the rate the rating was measured at. The load turns amber past the datasheet's continuous current and red past its pulse limit. The dots move at the discharge current.

20AExample

a 100 Ah AGM battery rated at the 20-hour rate, discharged at 0.2C, Peukert exponent 1.15

C-rate, and the capacity it costs you

I = C-rate × C_rated · C-rate = I ÷ C_rated
available capacity: C_eff = H × I × (C_rated ÷ (I × H))^k when I is above C_rated ÷ H; otherwise C_eff = C_rated

C rated: the capacity on the label, in amp-hours
H: the hour-rate that capacity was measured at — 20 h for most lead-acid, 1 h for many LiFePO4 packs, 5 h for Li-ion cells
k: Peukert’s exponent. 1.0 is an ideal battery; lead-acid runs 1.1 to 1.3; lithium is close to 1
C eff ÷ I: the runtime to empty at this rate, in hours

Worked example

a 100 Ah AGM battery rated at the 20-hour rate, discharged at 0.2C, Peukert exponent 1.15
I = 0.2 × 100 Ah = 20 A
The rating was measured at 100 Ah ÷ 20 h = 5 A, so 20 A is 4.0 times the rating rate and Peukert applies
C_eff = 20 × 20 × (100 ÷ (20 × 20))^1.15 = 81.23 Ah, 18.8% below the rating
Runtime = 81.23 Ah ÷ 20 A = 4.06 h, where capacity ÷ current alone would have promised 5.00 h
At 12 V that is 240 W for 974.7 Wh of energy — and 40% of this battery's 50 A continuous rating

Peukert against a real datasheet: Trojan T-105, 225 Ah at the 20-hour rate

k was fitted to two rows only — the 20-hour and 5-hour capacities — and the model then asked about the other four. It is within 2% at moderate rates, 6% out at 25 A and nearly 15% optimistic at 75 A, and 10% pessimistic at the 100-hour rate because this page credits no capacity above the rating at low rates. Peukert is a fit, not a law of nature; use it near the rates it was fitted at.
Discharge	Current	Datasheet	This model, k = 1.1644	Error
100-hour rate	2.50 A	250.0 Ah	225.0 Ah	-10.0%
20-hour rate	11.25 A	225.0 Ah	225.0 Ah	0.0%
10-hour rate	20.70 A	207.0 Ah	203.5 Ah	-1.7%
25 A constant current	25.00 A	186.3 Ah	197.3 Ah	5.9%
5-hour rate	37.00 A	185.0 Ah	185.0 Ah	0.0%
75 A constant current	75.00 A	143.8 Ah	164.7 Ah	14.6%

The defaults each chemistry loads

Starting points only, and the reason the page asks which chemistry you have before it asks anything else: the same 100 Ah label means a different current depending on the rate it was measured at. Choose “Enter my own” and take both numbers from your datasheet.
Chemistry	Peukert exponent	Rating hour-rate	Rating C-rate
Flooded lead-acid	1.25	20 h (C20)	0.05C
AGM or gel lead-acid	1.15	20 h (C20)	0.05C
LiFePO4	1.02	1 h (1C)	1C
Li-ion (NMC cylindrical cell)	1.05	5 h (0.2C)	0.2C

What a C-rate is, and what it quietly costs

The C-rate is a current expressed as a multiple of the battery’s rated capacity. 0.5C of a 200 Ah battery is 100 A; 2C of a 3,000 mAh cell is 6 A. It is a convenient way to compare batteries of different sizes, and it is the unit almost every datasheet uses for its charge and discharge limits. The arithmetic is one multiplication in each direction and this page does both.

The part that is not arithmetic. A rated capacity is a measurement, and the measurement has a discharge rate attached to it. Most lead-acid batteries are rated at the 20-hour rate: a 100 Ah battery means one that delivered 5 A for 20 hours. Draw 20 A instead and you do not get five hours — you get about four, because the faster the discharge the less charge the plates can deliver. W. Peukert put a number on that in 1897: I^k × t is constant, with k above 1 for lead-acid and close to 1 for lithium. At 0.2C the example battery gives 81.2 Ah of its 100 Ah rating, and the missing 18.8% is the whole reason this page exists.

Where the rating rate comes from. Lead-acid is quoted at C20 almost universally, sometimes also at C10 and C5, and the C5 number is always smaller. Lithium iron phosphate packs are usually quoted at 1C or 0.5C, and cylindrical Li-ion cells at 0.2C — Samsung’s INR21700-50E, for instance, rates 4,900 mAh at a 980 mA discharge. Comparing a lead-acid C20 figure with a lithium 1C figure is comparing two different measurements, and the lithium cell is being flattered less than it looks.

The rate limits are separate questions. Peukert says what you get; the datasheet’s continuous and pulse current ratings say what you are allowed to take. They are set by heating and by internal resistance, not by capacity, and exceeding them shortens life or trips protection long before capacity becomes the issue. This page reports the current as a percentage of both. For runtime with a depth of discharge, a duty cycle or a load quoted in watts, the battery life calculator carries all of that; for the conversion between mAh, Ah and watt-hours, the battery capacity converter.

What the model leaves out. Temperature, which matters more than Peukert for lead-acid below about 10 °C. Ageing, which reduces the rating itself. The voltage sag that makes a fast discharge end at a higher state of charge than a slow one. And the fact that Peukert’s exponent is not a constant: it drifts with temperature and with the battery’s age, and the table above shows what that costs when a k fitted at moderate rates is asked about a high one.

Frequently asked questions

What does 0.5C mean on a battery?

A current of half the rated capacity per hour. For a 200 Ah battery, 0.5C is 100 A. For a 3,000 mAh cell it is 1.5 A. The C-rate says nothing by itself about how long the battery will last at that current.

How do I convert a C-rate to amps?

Multiply by the rated capacity in amp-hours: current = C-rate × capacity. Going the other way, C-rate = current ÷ capacity. A 40 A draw from a 100 Ah battery is 0.4C.

Why does a 100 Ah battery not give 100 A for one hour?

Because the 100 Ah was measured at a slow rate, usually over 20 hours. At 100 A a typical lead-acid battery delivers around half its rating, so it lasts well under half an hour. Peukert’s law is the usual way to estimate how much you lose, and this page applies it.

Does the C-rate affect lithium batteries the same way?

Much less. LiFePO4 and Li-ion have Peukert exponents close to 1.0 to 1.05, so capacity barely falls with rate until the cell’s own current limit is reached. The binding constraint for lithium is usually the datasheet’s continuous current rating and the BMS, not lost capacity.

What C-rate should I charge at?

Follow the datasheet. Flooded lead-acid is usually charged at 0.1C to 0.2C, AGM up to about 0.3C, and LiFePO4 commonly at 0.2C to 0.5C with 1C allowed by some manufacturers. Charging faster than the sheet allows is the quickest way to shorten a battery’s life.

Where do I find the Peukert exponent?

From two capacities at two different rates on the same datasheet: k = ln(t2 ÷ t1) ÷ ln(I1 ÷ I2). A battery giving 225 Ah over 20 hours and 185 Ah over 5 hours has k = 1.164. If the sheet gives only one rate, 1.25 for lead-acid and 1.05 for lithium are the usual defaults.

Related calculators

References

Peukert W. Über die Abhängigkeit der Kapazität von der Entladestromstärke bei Bleiakkumulatoren. Elektrotechnische Zeitschrift 1897;18:287–288.
Trojan Battery Company. T-105 data sheet: 6 V flooded deep-cycle battery, capacity 185 Ah at the 5-hour rate, 207 Ah at the 10-hour rate, 225 Ah at the 20-hour rate and 250 Ah at the 100-hour rate; 447 minutes at 25 A and 115 minutes at 75 A. The validation table above is built from those six rows.
Samsung SDI. INR21700-50E cell specification, V1.0: 4,900 mAh nominal capacity at a 980 mA (0.2C) standard discharge, 3.6 V nominal, 9,800 mA maximum continuous discharge, internal impedance 35 mΩ or less at AC 1 kHz.
Victron Energy. SmartShunt / BMV battery monitor manual: Peukert exponent 1.25 for lead-acid and 1.05 for lithium when the datasheet gives nothing better; 1.00 is an ideal battery.
IEC 61434:1996. Secondary cells and batteries containing alkaline or other non-acid electrolytes — Guide to the designation of current in alkaline secondary cell and battery standards, which defines the I(t) notation the C-rate is the everyday name for.