Battery (electricity)#Battery capacity and discharging

The capacity of a battery or accumulator - summarized below only as a " battery " means - shows the amount of electric charge, which provide a battery according to the manufacturer's specification or can store. It is stated:

• As the nominal capacity in ampere-hours CN ( unit symbol: AH) - with individual cells in ampere seconds ( As) or Coulomb ( C, 1 C corresponds to 1 As)

Or

• As reserve capacity Cr, n in minutes (min ); then it is taken exactly at the reciprocal of the C- factor, see below

The capacity of a battery in the above mentioned sense may not to the electrical capacitance of a capacitor ( an electric capacity of batteries have also ) be mixed, the second per volt ampere ( As / V) and the unit Farad (F ) is indicated.

General

The available capacity of a battery depends on the Entladeverlauf from, so the discharge current, the discharge voltage of the battery ( the voltage at which the discharge is completed), and the state of charge. This results in different types of discharge:

• Discharge with constant current
• Discharge through constant resistance
• Discharge with constant power
• U.v.a.m.

Depending on Entladeverlauf the accumulator has a different capacity. In a meaningful indication of the nominal capacity must therefore be stated both the discharge current and the discharge voltage.

In general, decreases the available capacity of a battery with increasing discharge current. This effect is described by the Peukert equation. This is due among other things, increasing with increasing voltage drop across the internal resistance of the battery, which can decrease the output voltage accordingly so that the discharge voltage is reached earlier. In addition to the internal resistance of the limited speed of the electrochemical processes, and charge transport phenomena in the battery, is responsible for its decreasing with increased discharge capacity.

, After an initial high rate discharge, the current draw but reduces the level of a normal discharge, virtually the same amount of current as can be seen at a normal discharge from the beginning. With batteries, such an operation in which with low battery charge and the current consumption is reduced, but is only possible in a few cases.

In order to estimate the service life of batteries, some accumulator - specific loading method can be used during charging. The charging process itself is controlled by a charge controller.

The nature of the interconnection of several batteries has influence on the maximum amount of charge (capacity) and the available electric voltage: so the voltages of the individual batteries, while in the parallel circuit add up in the series circuit, the charge amounts.

Decrease during use

When the batteries capacity decreases with time, when properly use due to chemical reactions (aging).

For one, it comes through the charging and discharging processes at the electrodes (only partially reversible ) electrochemical processes that impede a full charge or discharge:

• Lead Technology: sulfation, crystal formation
• Nickel technology: problems such as battery inertia effect
• Lithium Chemistry: Electrode aging by irreversible parasitic chemical reactions ( calendar life ).

For another, use and lifetime usually contradictory requirements. While the load is increased at higher temperatures due to the better electron mobility, this leads to the higher reactivity of the electrode materials also decreases lifespan and capacity.

According to the wear level of the wear of the storage battery decreases in the course of the service life, the loading capacity, and thus the energy density. The durability and service life of batteries is indicated by the number of charge-discharge cycles, after which the battery only has a certain carrying capacity (generally 80%). The standards DIN 43539 part 5 and IEC 896 Part 2 to give to various procedures and guidance only.

As an indication of the remaining quality of a battery open circuit voltage can be used, in the course of life also decreases with a fully charged battery.

C- factor

General Description

The C- factor (english C factor) is a colloquial quantification of accumulators to specify the maximum allowable charge and discharge, under certain boundary conditions. It is defined as the ratio of this current and the capacity of the battery:

Since this ratio for a given accumulator and a given design over wide ranges of the absolute capacity is constant.

The dimension of the C- factor is:

The corresponding SI unit is therefore s -1. In practice, however, almost exclusively specified.

The C- factor is the reciprocal of the time for which a capacity of said battery can be discharged at the maximum discharge current.

For the entladestromabhängige capacity (see also Peukert equation), these time-dependent data have naturalized. Thus, the C20 capacity indicates the available amount of charge when the battery is discharged within 20 hours with a uniform discharge to the discharge voltage.

Multiplying the difference resulting from this nominal capacity ( in this context also referred to as K20 ) at the rated voltage (unit: V), we obtain the energy content (unit: watt hour).

Examples

The usual, but not formally correct spelling " The maximum discharge current is 15 C. " means:

That is, the considered battery can be discharged for 4 minutes at its maximum discharge current.

With a capacity

Follows:

That is, the maximum discharge current of the battery through the above-mentioned period of time is just 45 A.

Analog to this, the term " discharge at C/20 " at the same accumulator on the discharge current, which he can provide 20 hours (each with a linear factor of 300 compared to the previously determined values; Peukert equation is not taken into account):

And accordingly does " charging current 2 C " in this cell so that it should be loaded with a maximum of 6 A.