Tune your battery monitoring with Peukert's exponent

Battery capacity is expressed in Ampere hours (Ah) and indicates how much current a battery can deliver over time. For example, if a 100Ah battery is discharged with a constant current of 5A, the battery is completely discharged in 20 hours.

The rate at which a battery discharges is expressed as C -rating. The C rating indicates how many hours a battery with a given capacity will last. 1C is the 1h rate and means that the discharge current will discharge the entire battery at 1 hour. For a battery with a capacity of 100Ah corresponds to a discharge current of 100A. A 5C rate for this battery would be 500A for 12 minutes (1/5 hour) and a C5 rate would be 20A for 5 hours.

Note that there are two; way of expressing the C capacity of a battery. Either with a number before C or with a number after C.

For example:
  - 5C is the same as C0.5
  - 1C is the same as C1
  - 0.2C is the same as C2

The capacity of the battery depends on discharge rate. The faster the discharge rate, the less capacity will be available. The relationship between slow or fast discharge can be calculated according to Peukert's law and is expressed by the Peukert exponent. Some battery chemicals suffer more from this phenomenon than others. Lead acid is more affected by this than lithium batteries. The battery monitor takes this phenomenon into account with Peukert's exponent.

Example på discharge rate
A lead acid battery is rated at 100Ah at C20, this means that this battery can deliver a total current of 100A for 20 hours at a rate of 5A per hour. C20 = 100Ah (5 x 20 = 100).

When the same 100Ah battery is discharged completely at två hours greatly reduces its capacity. På due to the higher discharge rate, it can only give C2 = 56Ah.

Peukert's formula
The value that can be adjusted in Peukert's formula is the exponent n: see the formula below.

In the battery monitor, the Peukert exponent can be adjusted from 1.00 to 1.50. The higher the Peukert exponent, the faster the effective capacity "shrinks" with decreasing discharge rate. An ideal (theoretical) battery has a Peukert exponent på 1.00 and has a fixed capacity regardless of the size of the på the discharge current. The default setting is the battery monitor for the Peukert exponent is 1.25. This is an acceptable average value for most lead acid batteries.

In order to facilitate the development of the Peukert exponent, Victron Energy has developed
a Peukert tube more available at to download from: http://www.victronenergy. com/support-and-downloads/software

Note that the Peukert exponent is no more than a rough approximation of reality, and that at very high currents, batteries will give even less capacity than what is obtained from a fixed exponent. We recommend that you do not change the default value of the battery monitor without first getting into the subject, except for lithium batteries.