Understanding Battery Technical Specifications.

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Commonly in a specification sheet for a typical battery, you have all kinds of technical terms that need to be understood so as to be able to use the battery in the right way to get maximum benefit from  the battery in a particular application. Summarized below are some of the key technical terms used in battery specifications:

Nominal Voltage (V)
This is the reference voltage of the battery, also sometimes thought of as the “normal” voltage of the battery.

Cut-off Voltage (V)
This is the minimum allowable voltage of a battery. It is this voltage that generally defines the “empty” state of the battery.

Capacity or Nominal Capacity (AH for a specific C rate)
This is the total Amp-hours available when the battery is discharged at a certain discharge current (specified as a C-rate) from 100 percent state-of-charge to the cut-off voltage. Capacity is calculated by multiplying the discharge current (in Amps) by the discharge time (in hours) and decreases with increasing C-rate.

State of Charge (% SOC)
SOC is defined as the remaining capacity of a battery and it is affected by its operating conditions such as load current and temperature. It is calculated as:

$$  SOC  = {\frac {Remaining \ Capacity}{Rated \ Capacity}} $$

Depth of Discharge
DOD is used to indicate the percentage of the total battery capacity that has been discharged.

$$ DOD = 1 - SOC $$

Energy or Nominal Energy (Wh for a specific C rate)
This is the “energy capacity” of the battery, the total Watt-hours available when the battery is discharged at a certain discharge current (specified as a C-rate) from 100 percent state-of-charge to the cut-off voltage. Energy is calculated by multiplying the discharge power (in Watts) by the discharge time (in hours). Like capacity, energy decreases with increasing C-rate.
The rated Wh capacity of a battery can be calculated as:

$Rated \ Wh = Rated \ Ah \ Capacity \ * \ Rated \ Battery \ Voltage$


Cycle Life (Number for a specific DOD)
This is the number of discharge-charge cycles the battery can experience before it fails to meet specific performance criteria. Cycle life is estimated for specific charge and discharge conditions. The actual operating life of the battery is affected by the rate and depth of cycles and by other conditions such as temperature and humidity. The higher the DOD, the lower the cycle life.

Specific Energy (Wh/Kg)
This is the nominal battery energy per unit mass, sometimes referred to as the gravimetric energy density. Specific energy is a characteristic of the battery chemistry and packaging. It is expressed in Watt-hours per kilogram (Wh/kg) as:

$$Specific \ Energy = {\frac {Rated \ Wh \ Capacity}{Battery \ Mass \ in \ Kg}}$$

Specific Power (W/Kg)
This is the maximum available power per unit mass. Specific power is a characteristic of the battery chemistry and packaging. It determines the battery weight required to achieve a given performance target. It is expressed in W/kg as:



$$Specific \ Power = {\frac {Rated \ Peak \ Power}{Battery \ Mass \ in \ Kg}}$$

Peak Power
The peak power of a battery is defined as:
$$P = {\frac{2V_{oc}^2}{9r}}$$
Where:
$V_{oc}$ is the open circuit voltage of battery
$r$ is the internal resistance of the battery

Energy Density (Wh/L)
This is the nominal battery energy per unit volume, sometimes referred to as the volumetric energy density. Specific energy is a characteristic of the battery chemistry and packaging. Along with the energy consumption of the vehicle, it determines the battery size required to achieve a given electric range.

Power Density (W/L)
The maximum available power per unit volume. Specific power is a characteristic of the battery chemistry and packaging. It determines the battery size required to achieve a given performance target.

Maximum Continuous Discharge Current
This is the maximum current at which the battery can be discharged continuously. This limit is usually defined by the battery manufacturer in order to prevent excessive discharge rates that would damage the battery or reduce its capacity. 

Maximum 30-sec Discharge Pulse Current
This is the maximum current at which the battery can be discharged for pulses of up to 30 seconds. This limit is usually defined by the battery manufacturer in order to prevent excessive discharge rates that would damage the battery or reduce its capacity.

Charge Voltage (V)
This is the voltage that the battery is charged to when charged to full capacity. Charging schemes generally consist of a constant current charging until the battery voltage reaches the charge voltage, then constant voltage charging, allowing the charge current to taper until it is very small.

Float Voltage (V)
This is the voltage at which the battery is maintained after being charge to 100 percent SOC to maintain that capacity by compensating for self-discharge of the battery.

(Recommended) Charge Current
The ideal current at which the battery is initially charged (to roughly 70 percent SOC) under constant charging scheme before transitioning into constant voltage charging.

Internal Resistance (Maximum)
This is the resistance within the battery, generally different for charging and discharging.