Peukert's law

Peukert's law, presented by the German scientist Wilhelm Peukert^{German Wikipedia} in 1897, expresses approximately the change in capacity of rechargeable lead-acid batteries at different rates of discharge. As the rate increases, the battery's available capacity decreases, approximately according to Peukert's law.

A 2006 critical study concluded that Peukert's equation could not be used to predict the state of charge of a battery accurately unless it is discharged at a constant current and constant temperature. A 50Ah lithium-ion battery tested was found to give about the same capacity at 5A and 50A; this was attributed to possible Peukert loss in capacity being countered by the increase in capacity due to the 30◦C temperature rise due to self-heating, with the conclusion that the Peukert equation is not applicable.

Manufacturers specify the capacity of a battery at a specified discharge rate. For example, a battery might be rated at 100 A·h when discharged at a rate that will fully discharge the battery in 20 hours (at 5 amperes for this example). If discharged at a faster rate the delivered capacity is less. Peukert's law describes a power relationship between the discharge current (normalized to some base rated current) and delivered capacity (normalized to the rated capacity) over some specified range of discharge currents. If Peukert's constant $k$, the exponent, were equal to unity, the delivered capacity would be independent of the current. For a real battery the exponent is greater than unity, and capacity decreases as discharge rate increases. For a lead–acid battery $k$ is typically between 1.1 and 1.3. For different lead-acid rechargeable battery technologies it generally ranges from 1.05 to 1.15 for VRSLAB AGM batteries, from 1.1 to 1.25 for gel, and from 1.2 to 1.6 for flooded batteries. The Peukert constant varies with the age of the battery, generally increasing (getting worse) with age. Application at low discharge rates must take into account the battery self-discharge current. At very high currents, practical batteries will give less capacity than predicted with a fixed exponent. The equation does not take into account the effect of temperature on battery capacity.

Formula

For a one-ampere discharge rate, Peukert's law is often stated as

$C_p = I^k t,$

where:

$C_p$ is the capacity at a one-ampere discharge rate, which must be expressed in ampere-hours,

$I$ is the actual discharge current (i.e. current drawn from a load) in amperes,

$t$ is the actual time to discharge the battery, which must be expressed in hours.

$k$ is the Peukert constant (dimensionless),

The capacity at a one-ampere discharge rate is not usually given for practical cells. As such, it can be useful to reformulate the law to a known capacity and discharge rate:

$t = H \left(\frac{C}{I H}\right)^k$

where:

$H$ is the rated discharge time (in hours),

$C$ is the rated capacity at that discharge rate (in ampere-hours),

$I$ is the actual discharge current (in amperes),

$k$ is the Peukert constant (dimensionless),

$t$ is the actual time to discharge the battery (in hours).

Using the above example, if the battery has a Peukert constant of 1.2 and is discharged at a rate of 10 amperes, it would be fully discharged in time $20{\left(\frac{100}{10 \cdot 20}\right)^{1.2}}$, which is approximately 8.7 hours. It would therefore deliver only 87 ampere-hours rather than 100.

Peukert's law can be written as

$I t = C \left(\frac{C}{I H}\right)^{k-1},$

giving $I t$, which is the effective capacity at the discharge rate $I$.

If the capacity is listed for two discharge rates, the Peukert exponent can be determined algebraically:

$\frac{Q}{Q_{0}} = \left(\frac{T}{T_{0}}\right)^{\frac{k-1}{k}}$

Another commonly used form of the Peukert's law is:

$\frac{Q}{Q_{0}} = \left(\frac{I}{I_{0}}\right)^{\alpha},$

where:

$\alpha = \frac{k-1}{2-k}$

Several representative examples of different α and corresponding k are tabulated below:

α	k	comments
0	1	ideal battery – capacity independent of current
0.1	1.09	VRSLAB AGM batteries
0.2	1.17	VRSLAB AGM batteries
0.25	1.2	Gelled
0.3	1.23	Gelled
0.33	1.25	flooded Lead-acid battery
0.5	1.33	diffusion control, Cottrell-Warburg
0.75	1.43	Example
0.8	1.44	flooded Lead-acid battery
0.9	1.47	Example
1	1.5	regardless of the current, same discharge time always, a relay switch.

Peukert's law becomes a key issue in a battery electric vehicle, where batteries rated, for example, at a 20-hour discharge time are used at a much shorter discharge time of about 1 hour. At high load currents the internal resistance of a real battery dissipates significant power, reducing the power (watts) available to the load in addition to the Peukert reduction, delivering less capacity than the simple power law equation predicts.

References

• W. Peukert, Über die Abhängigkeit der Kapazität von der Entladestromstärke bei Bleiakkumulatoren, Elektrotechnische Zeitschrift 20 (1897)

External links

• A detailed explanation of the effects of Peukert's law
• A Peukert's law calculation spreadsheet in Microsoft Excel format
• Explanation and examples
• Online Peukert's law calculator