Basic Battery Concepts and Qualities

For starters, there are two major battery characteristics that directly explain its use in storage. The fundamental equation for energy is

Power (Watt) x Time (Hours) = Energy (Wh)

Therefore if you store 10MW of electric power for 1 hour %100 efficieny you get 10MWh of stored energy.

Cell, modules, and packs: A cell is the smallest, packaged form a battery can take and is generally on the order of one to six volts. A module consists of several cells generally connected in either series or parallel. A battery pack is then assembled by connecting modules together, again either in series or parallel.

Overview of Characteristics:

• Physical Requirements
• State Of Charge (Soc) & Depth Of Discharge(Dod)
• Terminal Voltage (V)
• Open-Circuit Voltage (V)
• Internal Resistance
• C-Rate
• Rate Capability
• Power To Energy Ratio
• Round Trip Efficiency
• Capacity Or Nominal Capacity (Ah For A Specific C-Rate)
• Energy Or Nominal Energy (Wh (For A Specific C-Rate))
• Cycle Life (Number For A Specific Dod) / Lifetime & Degredation
• Calendar Aging
• Discharge Curve
• Specific Energy (Wh/Kg)
• Specific Power (W/Kg)
• Energy Density (Wh/L)
• Power Density (W/L)
• Maximum Continuous Discharge Current
• Maximum 30-Sec Discharge Pulse Current
• Charge Voltage
• Float Voltage
• (Recommended) Charge Current
• (Maximum) Internal Resistance
• Cold Cranking CurrentTemperature Dependence
• Charging And Discharging Regimes

How to Compare Battery Systems:

Someone can find two commercial battery storage systems with the same rated energy of 9.8 kWh, but different capacities. Let’s call them System A and System B.

They have different capacities but the same rated energy because capacity is equal to the ratio of energy and voltage. System A has an internal battery voltage of 156 V while System B, with the higher capacity, has an internal battery voltage of 52 V.Furthermore, System A offers an output voltage of 400 V, indicating the presence of an internal DC-DC converter. System B, on the other hand, has no DC-DC converter since the output voltage is 52 V.

Key Characteristic Parameters of Batteries (Detailed):

Physical requirements

This includes the geometry of the cell, its size, weight and shape and the location of the terminals.

State of Charge (SoC) & Depth of Discharge(DoD):

• DOD = % of energy removed
• SOC = % of energy remaining

SOC% = 100% - DOD%

Terminal Voltage (V)

The voltage between the battery terminals with load applied. Terminal voltage varies with SOC and discharge/charge current.

Open-circuit voltage (V)

The voltage between the battery terminals with no load applied. The open-circuit voltage depends on the battery state of charge, increasing with state of charge.

Internal Resistance

The resistance within the battery, generally different for charging and discharging, also dependent on the battery state of charge. As internal resistance increases, the battery efficiency decreases and thermal stability is reduced as more of the charging energy is converted into heat

C-rate

Battery’s discharge rate compared to its capacity. Let’s say the base capacity of the battery is 100Ah.

• C-rate of 1C --> Discharge rate = 100A
• C-rate of 2C --> Discharge rate = 200A
• C-rate of C/2 --> Discharge rate = 50A

Rate Capability

Key points:

In summary: Critical parameter for deciding on a storage application vs. power application!

• A high power application would benefit from a high Power:Energyratio battery
• A high energy application would benefit from a low Power:Energyratio battery

Explanation:

For higher C rates, not all batteries can maintain the same stored energy because of dissipated energy such as heat during high or low discharge.

For 2 different batteries:

• Battery A = 100Ah capacity. At 1C the battery lasts 1hour. Therefore provides 100Ah. But at 2C, it lasts only 15 minutes, thus providing only 50Ah.
• Battery B = 100Ah capacity. At 1C the battery lasts 1hour. Therefore provides 100Ah. But at 2C, it lasts for 29 minutes, thus providing 97Ah.

Therefore battery B has a higher Rate Capability!

Power to Energy Ratio

Is used for directly comparing the power and energy abilities of a battery system.

• Option A: Lithium ion with 1MW of power capability, and 1MWh of energy storage capability. Price: 600/kW
• Option B: Lithium ion with 1MW of power capability, and 250kWh of energy storage capability. Price: $2,000/kWh

IF you want a power application (say you need 1 MW of energy storage system):

• Option A: $600/kWh x 1,000kWh= $600,000 (since you need at least that much energy stored to provide 1MW power)
• Option B: $2,000/kWh x 250kWh = $500,000

Option B is better even though the unit price is more than 3x of Option A!

However IF need at least 1 hour of energy storage (1,000kWh):

• Option A: $600/kWh x 1,000kWh = $600,000
• Option B: $2,000/kWh x 1,000kWh = $2,000,000

Option A is better by far since the application requires more storage capacity.

Round Trip Efficiency

The amount of energy that can be returned (discharged) after being stored.

Key points:

• It is never 100%
• You want higher RTE’s since it will lower your “storage fees
• Lower internal resistance means higher RTE; less wasted energy

Explanation:

It is never 100% -there is always some loss in the storage of energy.

• Storing and returning energy results in some of it being lost in heat or side reactions
• Power to Energy Ratio and Rate Capability are good indicators of the RTE capability
• Lower internal resistance means higher RTE; less wasted energy
• Operating at higher power and higher C-rates will lower RTE and vice versa; RTE can fluctuate in the same system depending on operating parameters.

RTE% impacts the operational cost of an energy storage device, because for every Wh you put in, you only get a fraction of it back out!

• This means that, on a net basis, all energy storage devices consume energy.
• This energy consumption can be considered a “storage fee”.
• Lower RTE means you pay higher storage fees!

Capacity or Nominal Capacity (Ah for a specific C-rate)

The coulometric capacity, 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.

Energy or Nominal Energy (Wh (for a specific C-rate))

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.

Cycle life (number for a specific DOD) / Lifetime & Degredation

Basically measures the lifetime of a battery in cycle lifes.

• Cycle: is when a battery is discharged and recharged from an initial SOC and back.
• Degradation: the loss of battery energy storage capacity over time which reduces the available % remaining capacity (also known as capacity “fade”)
• End of Life (EOL): The point at which a battery degrades enough to reach a % remaining capacity which the manufacture defines as the end of its useful life

!DEPENDS ON:

• Total nameplate energy capacity
• Operating temperature
• Power required per battery
• Total energy charged + discharged during use
• Operating SOC range
• % Remaining capacity at End of Life

Explanation:

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

Calendar Aging

In addition to loss of energy capacity due to cycling, it is important to note that batteries also undergo calendar aging. This comprises all aging processes that lead to a degradation of a battery cell independent of chargedischarge cycling and is an important factor in situations where the operational periods are shorter than the idle periods. It has been found that calendar aging can be more predominant in cycle aging studies when cycle depths and current rates are low.

Discharge Curve

The discharge curve is a plot of voltage against percentage of capacity discharged. A flat discharge curve is desirable as this means that the voltage remains constant as the battery is used up.

Specific Energy (Wh/kg)

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. Along with the energy consumption of the vehicle, it determines the battery weight required to achieve a given electric range.

Specific Power (W/kg)

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.

Energy Density (Wh/L)

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

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. Along with the maximum continuous power of the motor, this defines the top sustainable speed and acceleration of the vehicle.

Maximum 30-sec Discharge Pulse Current

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. Along with the peak power of the electric motor, this defines the acceleration performance (0-60 mph time) of the vehicle.

Charge Voltage

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 reaching the charge voltage, then constant voltage charging, allowing the charge current to taper until it is very small

Float Voltage

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.

(Maximum) Internal Resistance

The resistance within the battery, generally different for charging and discharging.

Cold Cranking Current

The maximum amount of current a battery can provide for a short period of time is called the cranking current. This parameter is often specified for transport applications, in which the battery must provide enough current to start a large engine. However, it is typically not an important parameter in PV systems.

Temperature dependence

A graph of Voltage vs time of charge for different heats.

Charging and Discharging Regimes

Each battery type has a particular set of restraints and conditions related to its charging and discharging regime, and many types of batteries require specific charging regimes or charge controllers. For example, nickel cadmium batteries should be nearly completely discharged before charging, while lead acid batteries should never be fully discharged. Furthermore, the voltage and current during the charge cycle will be different for each type of battery. Typically, a battery charger or charge controller designed for one type of battery cannot be used with another type.