Batteries and Electrochemical Energy Storage

Introduction and context

The battery is the principal energy-storage system in most electric vehicles (EVs), hybrid electric vehicles (HEVs), and plug-in hybrid electric vehicles (PHEVs). It determines vehicle range, acceleration capability, regenerative-braking acceptance, charging time, mass, cost, safety, and service life. Battery engineering is therefore an interdisciplinary activity involving electrochemistry, thermal science, power electronics, mechanical design, control, manufacturing, diagnostics, and environmental management.

A battery does not store electricity in the same manner as a capacitor. It stores chemical free energy and converts it reversibly into electrical energy through electrochemical reactions. During discharge, oxidation occurs at the negative electrode and reduction at the positive electrode. During charging, the reactions are driven in the reverse direction by an external power source. In practical vehicle batteries, the electrochemical cells are assembled into modules and packs, with sensors, contactors, fuses, cooling systems, structural components, and a battery-management system (BMS).

This chapter develops the principles required to select, model, operate, test, and manage batteries for vehicle applications.

Learning objectives

After studying this chapter, the reader should be able to:

1. Explain cell construction, electrode chemistry, and the operation of common lithium-ion and alternative batteries.
2. Distinguish cell, module, pack, energy, power, capacity, state of charge (SOC), and state of health (SOH).
3. Use electrical equivalent-circuit models and basic aging models.
4. Explain SOC and SOH estimation methods and their limitations.
5. Analyze degradation mechanisms, fast charging, thermal management, and safety.
6. Describe performance testing, second-life assessment, recycling, and battery selection.
7. Apply these concepts to a practical vehicle battery-design problem.

1. Electrochemical foundations

1.1 Cell structure

A rechargeable cell generally contains a positive electrode (cathode during discharge), a negative electrode (anode during discharge), an electrolyte, a porous separator, current collectors, and a sealed enclosure. In a lithium-ion cell, lithium ions move through the electrolyte while electrons move through the external circuit. The separator prevents direct electronic contact between the electrodes while permitting ionic transport.

The cell voltage is related to the difference in electrochemical potential between the two electrodes. The open-circuit voltage (OCV) depends primarily on chemistry and SOC, but also on temperature, relaxation time, hysteresis, and aging. Under load, terminal voltage differs from OCV because of ohmic resistance, charge-transfer polarization, and mass-transport limitations.

For an ideal cell, electrical energy is

where E is energy in joules, V is cell voltage in volts, and q is charge in coulombs. If voltage is approximately constant, E ≈ VQ, where Q is charge capacity in ampere-hours after the appropriate conversion to watt-hours. Thus, a 3.7 V, 50 Ah cell has a nominal energy of approximately 185 Wh, although usable energy is lower because operating limits prevent complete discharge and charge.

1.2 Common rechargeable chemistries

Lithium-ion cells dominate modern EVs because of their high specific energy, high efficiency, low self-discharge, and favorable power capability. Important variants include:

No chemistry is universally best. Selection is a multi-objective decision involving energy, power, mass, volume, cost, safety, temperature range, life, charging infrastructure, supply chain, and recyclability.

1.3 Cell ratings and C-rate

Capacity is commonly stated in ampere-hours (Ah). A cell rated at Q Ah can ideally deliver Q amperes for one hour, although actual capacity depends on current, temperature, cutoff voltage, and age. The C-rate is defined as current normalized by rated capacity:

where I is current in amperes and Q_r is rated capacity in ampere-hours. A 60 Ah cell discharged at 120 A is operating at 2C; charged at 30 A, it is operating at 0.5C. C-rate is a convenient comparison, but it does not fully describe stress because heat generation also depends on resistance, temperature, SOC, and pulse duration.

2. From cells to vehicle battery packs

2.1 Series and parallel connections

Cells connected in series increase voltage. Cells connected in parallel increase capacity and current capability. For identical cells, with N_s cells in series and N_p parallel strings:

Here voltage is in volts, capacity in ampere-hours, and energy in watt-hours. The equations assume identical cells, balanced current sharing, and use of nominal rather than instantaneous voltage. The pack maximum voltage is based on the maximum allowable cell voltage, while minimum pack voltage is based on the minimum allowable cell voltage.

Worked example. A pack uses 96 series groups, each group containing 4 parallel 50 Ah cells. If nominal cell voltage is 3.7 V:

• Nominal voltage = 96 × 3.7 = 355.2 V.
• Pack capacity = 4 × 50 = 200 Ah.
• Nominal energy = 355.2 × 200 = 71,040 Wh ≈ 71.0 kWh.

The pack does not normally expose the full 0–100% chemical capacity. If the control system permits 90% of nominal capacity for durability, usable nominal energy is approximately 63.9 kWh before considering voltage variation and temperature.

2.2 Pack construction

A traction battery pack typically includes cells, modules or cell-to-pack structures, busbars, interconnects, contactors, pre-charge circuitry, fuses, current sensors, voltage taps, temperature sensors, cooling plates, insulation, crash protection, venting or pressure-relief provisions, and a BMS. The enclosure must provide mechanical stiffness, sealing against water and dust, electrical isolation, electromagnetic compatibility, and controlled thermal behavior.

The BMS performs four central functions:

• measurement of cell voltage, pack current, temperature, and insulation resistance;
• estimation of SOC, SOH, available power, and remaining energy;
• protection against overvoltage, undervoltage, overcurrent, overheating, and isolation faults;
• balancing and communication with the vehicle control unit and charger.

Cell balancing may be passive, in which excess energy is dissipated through resistors, or active, in which energy is transferred between cells or modules. Passive balancing is simple and inexpensive but wastes energy and is slow for large imbalances.

2.3 Electrical and thermal design

A battery produces heat from internal resistance and electrochemical polarization. A first approximation for irreversible heat is:

where P_loss is heat-generation power in watts, I is current in amperes, and R is effective internal resistance in ohms. The approximation is most useful for short-term electrical analysis; detailed thermal models also include entropic heat, reaction heat, tab and busbar losses, and temperature-dependent parameters.

Cooling may use air, liquid plates, refrigerant, or immersion fluids. Liquid cooling generally provides better heat-transfer uniformity and compactness, but increases cost, mass, sealing requirements, and service complexity. Thermal design must limit both maximum temperature and cell-to-cell temperature variation because nonuniformity accelerates imbalance and uneven aging.

3. Battery models

3.1 Energy and power models

A simple energy model treats the battery as an energy reservoir. The stored energy changes according to current and voltage:

or, during charging, dE/dt = η_c V I, where t is time, V is terminal voltage, I is current magnitude, and η_d and η_c are discharge and charge efficiencies. Sign conventions must be stated because some models define discharge current as positive and others define charging current as positive.

A battery’s maximum power is constrained by voltage limits, current limits, temperature, SOC, and the ability of the cells and interconnects to reject heat. Consequently, a battery can have high energy but modest power, or high power but limited energy.

3.2 Thevenin equivalent-circuit model

A practical first-order model consists of an SOC-dependent ideal voltage source in series with an ohmic resistance and one or more resistor-capacitor polarization branches:

where V_t is terminal voltage, V_oc is open-circuit voltage, T is temperature, R_0 is instantaneous series resistance, V_p is polarization voltage, R_p is polarization resistance, and C_p is polarization capacitance. The sign convention here takes positive I as discharge. The time constant is τ = R_p C_p. Additional RC branches improve representation of multiple dynamic time scales but increase parameter-identification effort.

The SOC state can be approximated by coulomb counting:

where Q_n is usable nominal capacity and η_I is a coulombic-efficiency factor. This method is simple but accumulates current-sensor bias and depends on an accurate initial SOC and capacity estimate.

3.3 Model identification and validation

Model parameters can be obtained from pulse tests, constant-current charge/discharge tests, electrochemical impedance spectroscopy, or manufacturer data. A suitable test separates ohmic voltage drop, relaxation dynamics, and OCV-SOC behavior. Validation must use drive cycles or operating conditions different from those used for fitting; otherwise, a model may fit the calibration data but fail in real service.

A model should be no more complex than required by its purpose. A control-oriented BMS model may use one or two RC branches, whereas cell-development research may use electrochemical porous-electrode models. Model uncertainty is particularly important near low temperature, high current, high SOC, and end of life.

4. State estimation

4.1 State of charge

SOC is the available charge relative to a defined reference capacity:

The reference must be specified. It may be rated capacity, current available capacity, or a certified usable window. SOC is not directly measurable by a single sensor. It is estimated using combinations of current integration, OCV correlation, voltage response, temperature, and model-based observers.

Common methods include:

1. Coulomb counting: fast and inexpensive, but sensitive to current offset, capacity uncertainty, and initial-condition error.
2. OCV lookup: useful after sufficient rest, but unsuitable for dynamic operation and chemistries with flat OCV-SOC curves.
3. Extended Kalman filter (EKF): combines a nonlinear battery model with voltage and current measurements. It can correct integration drift but requires a credible model and noise covariance tuning.
4. Unscented Kalman filter or particle filter: useful for stronger nonlinearities, at higher computational cost.
5. Data-driven estimation: neural networks and other machine-learning methods can identify complex relationships, but require representative data, validation, explainability, and protection against distribution shift.

A robust estimator usually combines methods rather than relying on voltage alone. Sensor calibration, synchronization, observability, and fault detection are as important as the mathematical observer.

4.2 State of health

SOH describes degradation relative to a reference condition. It has no universal single definition. Two common indicators are capacity retention and resistance growth:

where Q_BOL and R_BOL are beginning-of-life capacity and resistance, and Q_EOL and R_EOL are the corresponding end-of-life or current values. The second expression is an index in which increasing resistance produces a lower SOH value; alternative conventions instead report resistance growth directly. End-of-life thresholds depend on application, such as 70–80% capacity retention or a specified power loss, and must not be treated as universal laws.

SOH estimation uses periodic capacity tests, resistance or power tests, incremental-capacity analysis, differential-voltage analysis, impedance features, temperature history, and accumulated charge throughput. Since complete capacity tests are inconvenient in vehicles, the BMS often estimates SOH opportunistically from partial operating data. Confidence intervals should accompany estimates when safety or warranty decisions depend on them.

4.3 Available power and energy

The BMS must estimate whether the battery can accept regenerative-braking power or deliver traction power without violating voltage, current, temperature, and SOC limits. Available power is therefore a constrained prediction, not merely nominal voltage multiplied by a maximum current. At high SOC, charging power may be restricted; at low temperature, both charge acceptance and discharge power may be reduced.

5. Degradation, charging, and safety

5.1 Principal degradation mechanisms

Battery aging is commonly divided into calendar aging and cycle aging. Calendar aging occurs with time, even when the vehicle is not driven, and is accelerated by high temperature and high SOC. Cycle aging results from charge-discharge operation and depends on depth of discharge, current, temperature, rest periods, and mechanical stress.

Important mechanisms include:

• growth of the solid-electrolyte interphase on the negative electrode;
• lithium plating during high-rate or low-temperature charging;
• loss of active lithium and active electrode material;
• particle cracking and loss of electrical contact;
• electrolyte oxidation and gas generation;
• transition-metal dissolution and cross-talk between electrodes;
• separator damage, dendrite formation, and internal short circuits.

A useful empirical capacity-retention expression is:

where Q_loss is capacity loss, t is calendar time, N_eq is equivalent full cycles, and k_c and k_cyc are fitted coefficients. This is an empirical approximation, not a universal degradation law. It is valid only over the tested range and must be recalibrated for chemistry, temperature, SOC window, and current profile.

5.2 Fast charging

Fast charging reduces vehicle downtime but increases demands on the cell, charger, cooling system, grid connection, and user protection. A common charging strategy is constant-current/constant-voltage (CC-CV). The charger first supplies approximately constant current until the upper voltage limit is reached, then holds voltage while current gradually decreases. Charging terminates when current falls below a specified threshold or another safety condition is met.

Fast charging is limited by lithium plating, heat generation, voltage rise, electrode diffusion, and pack-level imbalance. Effective mitigation includes preconditioning the battery, controlling charge current as a function of temperature and SOC, restricting high-SOC charging, improving electrode design, and using accurate cell-level sensing. A high-power charger cannot safely force a battery to accept power beyond its electrochemical limits.

5.3 Safety and thermal runaway

Thermal runaway is a self-accelerating exothermic process that can occur when heat generation exceeds heat removal and internal reactions become unstable. Initiating events include overcharge, internal short circuit, mechanical penetration, manufacturing defects, external heating, and severe collision. Safety engineering uses prevention, detection, containment, and mitigation:

• prevent abuse with electrical limits and robust mechanical design;
• detect abnormal voltage, temperature, pressure, gas, and isolation conditions;
• contain propagation using barriers, spacing, vent paths, and module design;
• mitigate hazards through crash disconnects, warning systems, fire-response procedures, and emergency-service information.

Battery safety must be designed across the cell, module, pack, vehicle, charger, manufacturing process, transport system, and service organization. Compliance with applicable vehicle, electrical, transport, and battery-recycling regulations is essential; exact requirements depend on jurisdiction and vehicle category.

6. Performance testing

Battery testing should reproduce intended use while controlling variables that influence results. A test plan normally specifies cell format, SOC window, temperature, current profile, rest periods, measurement accuracy, cutoff voltage, safety limits, and data-sampling rate.

Important tests include:

Energy efficiency over a complete cycle can be defined as:

where E_in is electrical energy supplied during charging and E_out is energy recovered during discharge. The boundary must be identified: cell terminals, pack terminals, or wall-to-wheel. Charger and thermal-system losses produce different values at different boundaries.

Repeatability requires controlled preconditioning and adequate replicates. Test results should report uncertainty, not just a single number. For example, a capacity difference may reflect temperature variation or measurement error rather than real degradation.

7. Battery selection for vehicle applications

Selection begins with vehicle requirements rather than chemistry preference. Required traction energy can be estimated from vehicle energy consumption:

where e_d is energy consumption at the wheels in Wh/km, d is required distance in km, and η_system is the overall battery-to-wheel efficiency. An engineering design adds reserve energy for temperature, aging, auxiliary loads, traffic, and user expectations.

Power must also be checked independently. If the vehicle requires peak wheel power P_wheel and drivetrain efficiency is η_d, battery peak power is approximately P_wheel/η_d, with additional margin for accessory loads and aging. The selected pack must meet both energy and power requirements at the relevant temperature and SOC.

Decision criteria include:

• usable energy per unit mass and volume;
• continuous and peak power;
• charge time and charging infrastructure;
• cycle and calendar life;
• thermal stability and abuse tolerance;
• cost and supply-chain exposure;
• maintenance and diagnostics;
• manufacturability and serviceability;
• recyclability and expected second-life value.

A high-energy cell may reduce pack mass but may have lower power or more demanding thermal management. A lower-energy LFP pack may be attractive where cost, safety, and long life outweigh volume and mass penalties.

8. Second life, recycling, and sustainability

A battery removed from a vehicle may retain useful capacity and power capability. In second-life applications it can be used for stationary storage, renewable-energy buffering, backup power, or charging infrastructure. Before reuse, the battery requires identification, electrical and insulation testing, capacity and resistance characterization, thermal inspection, and safe repackaging. The economic benefit is not automatic: transport, testing, disassembly, warranty, integration, and safety certification may exceed the value of recovered energy.

Recycling routes include:

• Pyrometallurgy: high-temperature processing that recovers selected metals but consumes substantial energy and may lose some materials.
• Hydrometallurgy: chemical leaching and separation with potentially high recovery rates, but requiring reagent management and wastewater control.
• Direct recycling: attempts to preserve cathode structure and restore performance, potentially reducing energy use but requiring careful sorting and chemistry control.

Responsible battery life-cycle management includes traceability, design for disassembly, standardized labeling, worker protection, transport controls, producer responsibility, and transparent reporting of material recovery. Ethical concerns include mining impacts, labor conditions, community exposure, data privacy from connected BMS systems, and equitable access to charging and electrified transport.

9. Applied case: illustrative design of a compact EV battery

Background

Consider an illustrative compact EV requiring 360 km of practical range. Measured road-load energy consumption is 155 Wh/km at the wheels. The design team assumes battery-to-wheel efficiency of 88%, a 10% reserve for auxiliary loads and environmental variation, and a usable SOC window of 90% of nominal pack energy.

Problem

Estimate nominal battery energy and examine whether a 400 V-class, 150 kW battery is adequate for a 120 kW peak traction demand. The vehicle must also accept 100 kW regenerative or fast-charge power under suitable conditions.

Analysis and approach

Wheel energy is:

Battery energy delivered to the drivetrain is:

Including the 10% design allowance:

If only 90% of nominal energy is made usable:

A nominal pack of approximately 78 kWh is therefore a reasonable starting point, subject to validation with climate, grade, speed, tire, and degradation studies.

For peak traction, a 120 kW wheel demand at 94% drivetrain efficiency requires approximately 127.7 kW from the battery before auxiliary loads. A 150 kW battery rating provides nominal margin, but the BMS must verify that 150 kW remains available at the intended SOC, temperature, and end-of-life condition. For a 100 kW charging request, cell temperature, voltage, current, and pack cooling must be checked; the charger rating alone does not guarantee a 100 kW battery acceptance rate.

Outcome and learning points

The analysis shows that energy and power are separate design constraints. A pack may meet range requirements but fail peak-power or fast-charge requirements. The final design would require cell selection, series-parallel configuration, thermal simulation, crash analysis, aging tests, charger compatibility, and a life-cycle-cost assessment. The example is illustrative; it is not a certification calculation.

10. Contemporary developments and professional practice

Current battery engineering is moving toward cell-to-pack and structural-pack architectures, silicon-rich negative electrodes, high-manganese and cobalt-reduced cathodes, solid-state concepts, sodium-ion systems, improved fast-charge protocols, and digital battery passports. These developments promise lower cost, higher energy density, improved sustainability, or better safety, but each introduces trade-offs in manufacturing maturity, swelling, interfaces, cycle life, quality control, or recycling.

Artificial intelligence can assist with fault diagnosis, remaining-useful-life prediction, adaptive charging, and manufacturing inspection. It should augment—not replace—physical safety limits. Data-driven models require representative data across temperature, aging, manufacturing variation, and operating conditions. Their decisions should be monitored, validated, and made explainable where they affect safety or warranty.

Best practice is to combine electrochemical understanding, conservative protection limits, validated models, redundant sensing, controlled testing, traceable manufacturing, and life-cycle planning. Battery engineering is successful when performance, safety, durability, cost, and environmental responsibility are optimized together.

Chapter Summary

Electrochemical cells convert chemical energy into electrical energy through reversible electrode reactions. Lithium-ion chemistries dominate vehicle applications, but NMC, NCA, LFP, LTO, NiMH, lead-acid, and emerging sodium-ion batteries offer different compromises. Cells are connected in series to increase voltage and in parallel to increase capacity and current capability. A vehicle pack also requires mechanical protection, thermal management, contactors, sensors, fuses, balancing, and a BMS.

Equivalent-circuit models represent OCV, resistance, and dynamic polarization and are useful for simulation and control. SOC is estimated using coulomb counting, OCV information, model-based observers, and sometimes data-driven methods. SOH may be defined by capacity retention, resistance growth, or available power, and its definition must be stated explicitly. Aging arises from calendar and cycle effects, including interphase growth, lithium plating, cracking, electrolyte degradation, and loss of active material.

Fast charging requires control of current, voltage, temperature, and SOC to avoid plating and excessive degradation. Testing must control temperature, cutoff conditions, current profiles, and measurement uncertainty. Battery selection is a systems decision involving energy, power, mass, cost, safety, life, charging, supply chain, second life, and recycling. Sustainable practice extends from responsible material sourcing and design for service to second-life evaluation and recovery of valuable materials.

Key Terms

Electrochemical cell: A device that converts chemical energy to electrical energy and can be rechargeable when reactions are sufficiently reversible.

Battery pack: An integrated assembly of cells, electrical protection, thermal management, structure, sensors, and controls.

Cathode: The positive electrode during discharge in the conventional battery description.

Anode: The negative electrode during discharge.

Electrolyte: Ion-conducting medium between the electrodes.

State of charge (SOC): Available charge expressed relative to a defined reference capacity.

State of health (SOH): An indicator of degradation relative to a beginning-of-life or specified reference condition.

C-rate: Current normalized by rated capacity.

Open-circuit voltage (OCV): Voltage measured with negligible current after an appropriate relaxation period.

Equivalent-circuit model: An electrical model using ideal sources, resistors, capacitors, and dependent states to approximate battery behavior.

Coulomb counting: SOC estimation by integrating measured current over time.

Calendar aging: Degradation caused primarily by time, temperature, and storage conditions.

Cycle aging: Degradation associated with charge-discharge operation.

Lithium plating: Deposition of metallic lithium on the negative electrode, often promoted by cold or rapid charging.

Thermal runaway: Self-accelerating exothermic failure involving rapid heat generation.

Cell balancing: Equalization of SOC or voltage among series-connected cells.

Fast charging: Charging at a high power level subject to cell and pack safety constraints.

Second life: Use of a retired vehicle battery in another application before recycling.

Direct recycling: Recovery and restoration of battery materials while attempting to preserve electrode structure.

Battery-management system: Electronic system that monitors, estimates, protects, balances, and communicates battery operating states.

Review Questions

Short Answer Questions

1. Why are cells connected in series and parallel in a traction battery?
2. Define SOC and explain why it cannot be measured directly by a single sensor.
3. State two advantages and two limitations of LFP chemistry.
4. What is the purpose of a battery-management system?
5. Distinguish calendar aging from cycle aging.

Long Answer Questions

1. Explain the construction and operation of a lithium-ion cell, including the roles of the separator, electrolyte, electrodes, and current collectors.
2. Develop and discuss a first-order Thevenin equivalent-circuit model, including the meaning and limitations of its parameters.
3. Compare coulomb counting, OCV methods, Kalman filtering, and data-driven SOC estimation.
4. Explain the causes of thermal runaway and describe prevention, detection, containment, and mitigation strategies.
5. Discuss the technical and environmental trade-offs involved in battery recycling and second-life deployment.

Analytical / Application Questions

1. A battery uses 100 series groups and 3 parallel 45 Ah cells per group. Each cell has a nominal voltage of 3.65 V. Calculate nominal pack voltage, capacity, and energy.
2. A vehicle consumes 165 Wh/km at the wheels and must travel 300 km. If battery-to-wheel efficiency is 90%, a 12% design allowance is required, and only 88% of nominal energy is usable, estimate the required nominal battery energy.
3. A BMS reports increasing resistance but nearly unchanged capacity. Explain how this condition can reduce vehicle performance and identify two tests that could confirm the diagnosis.

Multiple Choice Questions with Answers

Question 1

A battery pack contains 96 series groups, each with four 50 Ah cells in parallel. What is the pack capacity?

A. 50 Ah

B. 96 Ah

C. 200 Ah

D. 19,200 Ah

Correct Answer: C. 200 Ah

Question 2

What is the primary purpose of connecting cells in series?

A. To increase pack voltage

B. To increase electrolyte conductivity

C. To eliminate thermal management

D. To reduce all internal resistance to zero

Correct Answer: A. To increase pack voltage

Question 3

In the stated Thevenin model, which term represents the instantaneous ohmic voltage drop?

A. V_oc

B. I R_0

C. V_p

D. R_p C_p

Correct Answer: B. I R_0

Question 4

A 60 Ah cell is discharged at 120 A. What is its approximate C-rate?

A. 0.5C

B. 1C

C. 2C

D. 7,200C

Correct Answer: C. 2C

Question 5

Which condition most strongly increases the risk of lithium plating during charging?

A. Moderate-temperature, low-rate charging

B. Low-temperature, high-rate charging

C. Long rest after charging

D. Low-SOC storage without charging

Correct Answer: B. Low-temperature, high-rate charging

Question 6

Which method estimates SOC by integrating current over time?

A. Coulomb counting

B. Pyrometallurgy

C. Passive balancing

D. Thermal imaging alone

Correct Answer: A. Coulomb counting

Question 7

What does SOH_Q commonly represent?

A. Pack voltage divided by current

B. End-of-life capacity divided by beginning-of-life capacity

C. Charge energy divided by discharge energy

D. Cell temperature divided by ambient temperature

Correct Answer: B. End-of-life capacity divided by beginning-of-life capacity

Question 8

In a constant-current/constant-voltage charging strategy, what happens during the constant-voltage phase?

A. Voltage falls to zero and current remains fixed

B. Voltage is held near its upper limit while current decreases

C. Both voltage and current increase without limits

D. The battery is disconnected from all measurement systems

Correct Answer: B. Voltage is held near its upper limit while current decreases

Question 9

Which recycling route uses chemical leaching and separation?

A. Hydrometallurgy

B. Pyrometallurgy

C. Mechanical balancing

D. Coulomb counting

Correct Answer: A. Hydrometallurgy

Question 10

A 3.7 V, 50 Ah cell has an approximate nominal energy of:

A. 18.5 Wh

B. 53.7 Wh

C. 185 Wh

D. 1,850 Wh

Correct Answer: C. 185 Wh