Battery Storage & Capacity
The amount of available and maximum energy stored in a battery is related to the battery capacity, the discharged capacity, described as depth of discharge (DOD), and the state of charge (SOC). The relationship between these characteristics is shown in
and described in detail in the following sections.
As an equation, it can be described as the following:
Rated capacity = Discharged capacity + Available capacity
„Lorem modi tempora incidunt ut labore et dolore magnam.”
Energy Storage in Batteries
Energy produced and used by various electrical devices is typically measured in Watt-hours (Wh) or kilowatt-hours (kWh). However, the recorded measurement that reflects the energy stored in batteries is Ampere-hours (Ah): this term is known as the battery capacity.
Note: Ampere hours is not a unit of energy, it is a unit of charge within the battery. The energy stored in a battery can be determined from the battery capacity by using the voltage of the battery as shown in the following equations.
To convert between Wh and Ah, where V is the nominal battery voltage
Wh = Ah x V
Battery Capacity
Every battery cell, when fully charged, has the potential to deliver a certain amount of charge over a period of time, measured in hours. Therefore, the unit for battery capacity is Amp-hour (Ah).
The capacity of the battery is specified by its manufacturer in Ah for given discharge rate and cell temperature. This information is important: the capacity of a battery will change depending on the discharge rate (i.e., discharge current) and cell temperature. #C ratings are used lithium batteries where this effect is less pronounced and the battery is discharged over a much shorter period of time compared to lead acid batteries.
The number quoted in the '#C' rating (for a lithium battery) indicates a fraction of an hour during which the battery is being discharged. The 1C rating indicates the current discharge over a period of 1 hour (equivalent to the C, rate); the 0.2C rate is the current discharge over a period of five hours (1 ± 0.2 = 5).
It is possible to distinguish between these two uses of the 'C' rating by the commonly used notations described below:
Lithium ion batteries: the number is quoted before the 'C' and is used as the denominator of 1 hour (60 minutes). E.g. 0.2C Rating = 60 minutes divided by 0.2 = 300 minutes = 5 hours; 0.5C Rating = 60 minutes divided by 0.5 = 120 minutes = 2 hours.
Depth of Discharge (DOD)
Depth of discharge describes how much of a battery's total capacity has been discharged and is usually given as a percentage.
Batteries typically have a maximum recommended depth of discharge (DOD) to prevent damage to the battery. For example, the maximum recommended DOD for lead-acid batteries is usually around 70%. The battery manufacturer's specifications should be used for this information.
There is also a relationship between the regular depth of discharge, also known as 'daily depth of discharge' (daily DOD), and the lifetime of the battery. For example, regularly discharging a lead-acid battery to a DOD of 20%, which is 50% less than the recommended maximum DOD, should result in a longer lifetime compared to regularly discharging a lead-acid battery to a DOD of 60%, which is only 10% less than the recommended maximum DOD.
State of Charge (SOC)
State of charge (SOC) describes how much of a battery's initial capacity is available. It is expressed in terms of a percentage of the battery's rated capacity. The sum of SOC and DOD will always equal 100%. For example, a battery that is at 25% depth of discharge (DOD) would have a state of charge (SOC) of 75%.
To summaries
DOD (%) =discharged capacity/rated capacity x I00%
SOC (%) = available capacity/rated capacity x 100%
DOD (%) + SOC (%) = 100% of rated capacity