A battery is a device that converts chemical energy into electrical energy and vice versa. In this summary, you will learn how hybrid, plug-in hybrid, and electric car batteries are classified, described, and compared.
This document provides a basic overview of battery operating conditions, outlines the variables used to describe the battery’s nominal and maximum characteristics, as well as the manufacturer ev battery specifications used to determine the battery’s nominal and maximum characteristics.
Cell, modules, and packs
The battery pack of hybrids and electric vehicles is composed of individual modules and cells which are arranged in parallel and serial. Cells are the smallest, packaged forms of batteries and typically range from 1 to 6 volts in size.
Modules are usually connected in series or parallel to form a battery pack. Once a battery pack is assembled, modules are connected together again, either in series or parallel.
Battery Classifications
Despite the fact that batteries may have the same chemistry, not all batteries are the same. Often manufacturers will classify batteries based on their ability to deliver high power or high energy, but not both.
A main trade-off in battery development is between power and energy. Other common classifications include High Durability, which means that the chemistry has been altered to provide longer battery life at the expense of energy and power.
C- and E- rates
Battery discharge current is often measured in terms of C-rates to normalize against battery capacity, which can vary significantly. In battery technology, a battery’s capacity is determined by the rate at which it is discharged in relation to its maximum capacity.
A 1C discharge rate means that the discharge current will discharge the entire battery within one hour. This corresponds to a discharge current of 100 Amps for a battery with 100 Amp-hrs.
Similarly, an E-rate describes the discharge power. A 1E rate describes how much power is needed to discharge the entire battery in 1 hour. If the battery’s rating is 5C, then the discharge power would be 500 Amps, and if it were C/2, then it would be 50 Amps.
Secondary and Primary Cells
Battery packs for hybrids, plug-in hybrids, and electric vehicles are all secondary batteries. A primary or non-rechargeable battery is a primary battery; a secondary battery is rechargeable.
Battery Condition
As described in the following section, a battery’s present condition can be described by a number of variables.
Recycling and repurposing used EV batteries help reduce waste, lower costs, and support sustainable energy practices.
State of Charge (SOC)(%)
As a percentage of maximum battery capacity, SOC refers to present battery capacity as a percentage of maximum capacity. The change in battery capacity over time is calculated using current integration.
Depth of Discharge (DOD) (%)
Battery capacity is expressed as a percentage of maximum capacity. A deep discharge occurs when the battery capacity is discharged to at least 80 percent of maximum capacity.
Terminal Voltage (V)
Indicated by an increase in terminal voltage when a load is applied to the battery terminals. Terminal voltage is dependent on the state of charge and the rate of discharge.
Open-circuit voltage (V)
During open-circuit operation, the battery terminals are not loaded, so the voltage there is determined by the battery charge state. The voltage there increases with a higher battery charge state.
Internal Resistance
It is also dependent on the state of charge of the battery. Resistance within the battery differs when charging and discharging. A battery’s efficiency decreases as its internal resistance increases and its thermal stability decreases as more charging energy is converted into heat.
Battery Technical Specifications
There are a number of specifications that can be found on technical specification sheets that are used to describe batteries, batteries modules, and batteries packs.
Nominal Voltage (V)
Voltage reported as the “normal” voltage of a battery, also sometimes referred to as the reported or reference voltage.
Cut-off Voltage
Batteries are generally defined as “empty” when their minimum allowable voltage is reached.
Capacity or Nominal Capacity (Ah for a specific C-rate)
A battery’s coulometric capacity is its total amount of Amp-hours available when discharged from 100 percent state of charge to the cutoff voltage at a given discharge current (specified by the C-rate). Increasing the C-rate decreases the capacity of the battery since it is calculated by multiplying the discharge current (in Amps) by the discharge time (in hours).
Energy or Nominal Energy (Wh (for a specific C-rate))
The “energy capacity” of the battery, a measure of the total Watt-hours available when discharged from 100 percent state of charge to the cutoff voltage at a certain discharge current (specified as a C-rate). In order to calculate energy, the discharge power multiplies the discharge time by the discharge power (in Watts). Energy decreases with a higher C-rate, just like capacity does.
Cycle Life (number for a specific DOD)
Cycle life is estimated for specific charge and discharge conditions. Before the battery fails to meet specific performance criteria, it can undergo a specific number of discharge-charge cycles. In addition to cycling rate and depth, the cycle life of a battery is affected by temperature and humidity, as well. The higher the DOD, the shorter the cycle life.
Specific Energy (Wh/kg)
It is sometimes called gravimetric energy density because it indicates the nominal energy of a battery per unit mass. Specific energy is determined by the battery chemistry and packaging. It also determines how much battery weight is required to achieve a given electric range, along with the vehicle’s energy consumption.
Specific Power (W/kg)
A measure of how much power can be delivered per unit mass. Specific power is determined by the battery chemistry and packaging. However, a given performance target requires a battery weighing a particular amount to be effective.
Energy Density (Wh/L)
Often called volumetric energy density, the nominal battery energy per unit volume. Specific energy, along with the vehicle’s energy consumption, determines the size of the battery required in order to achieve a given electric range. Specific energy is determined by the battery chemistry and packaging.
Power Density (W/L)
The maximum power that can be delivered by a battery per unit volume. It depends on the packaging and chemistry of the battery and determines the size that is necessary to meet a given performance request.
Maximum Continuous Discharge Current
An amount of current that can be continuously discharged from a battery. It is usually the manufacturer of the battery that sets this limit in order to prevent excessive discharge rates from damaging it or reducing its capacity. This also determines the maximum sustained speed and acceleration of the vehicle along with the maximum continuous power of the motor.
Maximum 30-sec Discharge Pulse Current
It is defined by the battery manufacturer as the maximum current that the battery can be discharged at for up to 30 seconds at a time. In order to prevent excessive discharge rates that could damage the battery or reduce its capacity, the manufacturer sets this limit.
It determines the vehicle’s acceleration performance (0-60 mph time) along with the peak power of the electric motor.
Charge Voltage
When charged to full capacity, the battery is charged to this voltage. There are mainly two types of charging schemes: constant current charging and constant voltage charging, allowing charge current to taper down to a very small amount before the battery voltage reaches the charge voltage.
Float Voltage
A voltage that is maintained after a battery is fully charged to maintain the battery’s 100 percent capacity by compensating for self-discharge.
(Recommended) Charge Current
In a constant current charging scheme, the ideal current is reached when approximately 70 percent of the battery’s SOC is reached before proceeding to constant voltage charging.
Frequently Asked Questions
1. What type of battery is used in electric vehicles?
It is common for electric vehicles to use lithium-ion batteries, which are known for their high energy density. All lithium-ion batteries are small and lightweight, making them capable of storing a large amount of energy.
2. What are the requirements of EV batteries?
The battery for electric vehicles must meet a number of key requirements. Their energy storage capacity should be sufficient to meet the range requirements of the vehicle, they must deliver power efficiently to the electric motor, they must be durable enough to withstand frequent charging and discharge cycles, and they must be safe and reliable.
3. How many amps is an EV car battery?
Based on the vehicle and battery configuration, an electric car battery can have a different amperage. Electric motors require a high amount of power to accelerate and operate, so EV batteries typically have a high amperage capacity. The battery pack size and design can determine the amp capacity, which can range from a few hundred to a few thousand amps.
4. How many kW are EV batteries?
Batteries for electric vehicles are almost always rated in terms of kilowatt-hours (kWh) rather than kilowatts (kW). Kilowatt-hours represent how much energy the battery can store and deliver over time. Kw refers to how much power a battery can supply.
The range of battery capacities for electric vehicles varies depending on their model and manufacturer, with smaller EVs typically having 20 kWh batteries and larger, longer-range cars often having over 100 kWh batteries.
