Battery Fundamentals

What Is a Battery?

A battery is an electrochemical source that converts chemical potential into electrical work through oxidation and reduction reactions. An individual cell is the basic electrochemical unit; a battery may consist of one cell or multiple cells connected and packaged to provide the required voltage, capacity, power, and mechanical form.

A battery does not literally store a reservoir of electricity. It stores chemical energy in reactants and electrode structures. During discharge, electrochemical reactions drive electron flow through the external circuit while ions move inside the cell. Primary systems are intended for discharge and replacement; secondary systems are designed so the electrochemical state can be restored by charging.

The useful electrical output of a battery is conditional on chemistry, temperature, state of charge, current demand, cutoff voltage, age, and the design of the cell and battery system. Nameplate values therefore describe performance under defined conditions rather than an unconditional amount of energy or runtime.

Electrochemical Cell Structure

Cell construction varies widely by chemistry and format, but the functional architecture normally includes two electrodes, an ion-conducting electrolyte, an electronically insulating separator where required, current collectors where applicable, external terminals, and an enclosure. The separator prevents direct electronic contact between electrodes while permitting ionic transport through the electrolyte.

Electrode naming requires care. During discharge of a galvanic cell, oxidation occurs at the anode and reduction at the cathode. In a rechargeable cell, the reaction direction reverses during charge, so “anode” and “cathode” can become ambiguous in general commercial discussion. For a chemistry-neutral reference, negative electrode and positive electrode are often clearer unless the operating state is explicitly stated.

How Electrochemical Cells Produce Voltage

During discharge, oxidation at one electrode releases electrons to the external circuit while reduction at the other electrode consumes electrons. To maintain charge balance, ions move through the electrolyte between the electrode regions. The combination of these coupled reactions creates an electrochemical potential difference that can perform electrical work on an external load.

The equilibrium or open-circuit voltage depends on the electrochemical potentials of the participating reactions and therefore differs among chemistries. Nominal voltage is an engineering label for a chemistry or battery system; actual voltage changes with state of charge, temperature, current, cell design, and recent operating history.

Open-Circuit Voltage vs. Terminal Voltage

Open-circuit voltage (OCV) is the voltage measured when no intentional external current is flowing and the cell has been allowed to approach a defined rest condition. Terminal voltage is the voltage measured at the external terminals during actual operation. Under discharge load it is normally lower than the corresponding rested OCV; during charge it can be higher.

A simple engineering representation is:

This expression captures the immediate ohmic contribution but does not fully describe a real battery. Charge-transfer kinetics, concentration gradients, diffusion, thermal effects, hysteresis, and other polarization processes create time-dependent voltage behavior. When a load is removed, part of the voltage recovers immediately and part relaxes more slowly toward a rested condition.

Figure 3 — Open-Circuit Voltage vs Terminal Voltage

Illustrative 12 V example — not measured chemistry-specific data.

Internal Resistance, Impedance and Polarization

Internal resistance is commonly used as an engineering shorthand for mechanisms inside the battery that oppose current flow and create terminal-voltage loss. The observed response includes purely ohmic resistance in conductors, electrolyte and interfaces, plus electrochemical contributions associated with charge transfer and mass transport. These effects are often grouped under the broader concept of polarization.

Resistance and impedance are not fixed battery constants. They vary with chemistry, state of charge, temperature, age, current level, recent history, and test method. A DC pulse-resistance measurement and an AC impedance measurement probe different time scales and electrochemical processes, so the numerical results should not be treated as interchangeable.

For a simplified ohmic component, instantaneous heat generation can be represented as:

The relationship is useful for understanding why high current causes disproportionately higher resistive heating, but total electrochemical and thermal losses in a working battery are more complex than a single fixed resistor.

Voltage, Current, Resistance, Power and Energy

Ohm’s law applies directly to resistive elements and is useful for representing the resistive component of battery voltage drop. It is not a complete model of battery terminal voltage, which also includes electrochemical polarization and time-dependent effects.

In a DC battery system, current is the rate of charge flow demanded or supplied by the circuit, voltage is the electrical potential difference at the terminals, and power is the instantaneous rate of energy transfer. Energy accumulates power over time.

Current demand also changes the battery's terminal voltage and internal heat generation, which is why a battery that has adequate nominal energy may still be unsuitable for a high-power load.

Capacity: Amp-Hours

Amp-hour capacity expresses electrical charge delivered over a specified discharge test:

For constant current:

Amp-hours measure charge, not energy. Rated capacity is conditional on the manufacturer's stated discharge rate or duration, end/cutoff voltage, temperature, initial state, chemistry, battery condition, and test procedure. A value such as “100 Ah” therefore cannot be interpreted correctly without its rating conditions.

Lead-acid batteries show a pronounced rate-capacity effect often described with Peukert-type behavior: as discharge current rises, delivered amp-hour capacity to a specified cutoff generally falls. Peukert relationships are useful for defined lead-acid applications but should not be applied universally to all battery chemistries.

Energy: Watt-Hours

Electrical energy delivered by a battery is obtained by integrating voltage and current over time:

A simple nominal-energy estimate is:

Example: 12 V × 100 Ah ≈ 1,200 Wh nominal energy. This is useful for first-pass comparison but is not the same as measured delivered or usable energy.

Actual energy depends on the discharge voltage profile, cutoff voltage, rate, temperature, efficiency, allowed SOC window, battery age, and control-system limits.

C-Rate and Discharge Rate

C-rate expresses charge or discharge current relative to a stated capacity reference. For a 100 Ah battery, an idealized 1C current is 100 A, 0.5C is 50 A, and 0.1C is 10 A. The notation is convenient for comparing current severity across battery sizes.

C-rate does not guarantee a specific runtime or delivered capacity. Manufacturer rating conventions, allowable cutoff voltage, chemistry, temperature, and cell design all influence actual performance.

Capacity vs. Discharge Rate

Battery capacity is specified under defined discharge conditions. In lead-acid batteries, the available amp-hour capacity generally decreases as discharge current increases. This behavior is commonly described by the rate-capacity effect and, for many lead-acid applications, by Peukert-type behavior.

The Trojan T-105 provides a real manufacturer example:

These are ratings for the same 6 V flooded lead-acid battery under different discharge durations.

Figure 4 — Rated Capacity vs. Discharge Duration

Source: Trojan Battery Company — T-105 6V Flooded Lead Acid Battery / official manufacturer datasheet.

Application note: This is a flooded lead-acid example and should not be generalized quantitatively to lithium, LiFePO₄, or other battery chemistries.

Temperature Effects

Temperature changes reaction kinetics, ionic conductivity, charge-transfer behavior, diffusion, and therefore internal impedance, available power, capacity, charge acceptance, aging rate, and safety margins.

Low temperature: typically increases impedance and reduces available power and capacity. Charging can also become more restrictive because some chemistries have temperature-dependent charge limits.

High temperature: can temporarily improve reaction kinetics and reduce some resistive losses, but generally accelerates degradation processes and may increase safety risk. The magnitude of these effects is chemistry- and design-specific; there is no single universal temperature correction percentage for all batteries.

State of Charge (SOC)

State of charge is an estimate of the remaining charge relative to a defined usable or reference capacity:

SOC normally cannot be measured directly. Practical systems estimate it using combinations of coulomb counting, rested OCV correlation, measured voltage/current/temperature, electrochemical or equivalent-circuit models, and BMS estimation algorithms.

Each method has limitations. Coulomb counting accumulates sensor and integration error; OCV methods require chemistry-specific relationships and sufficient rest; voltage under load is influenced by resistance and polarization; model-based estimators depend on parameter accuracy and calibration.

State of Health (SOH)

State of health is not one universal directly measured variable. It is a condition indicator defined relative to a chosen performance criterion. A system may estimate SOH from remaining capacity, resistance or impedance growth, power capability, self-discharge, or an application-specific combination of metrics.

Because the definition and algorithm differ among manufacturers and applications, two systems can report different SOH values for the same physical battery. SOH should therefore be interpreted together with the method, reference state, operating history, and service requirement.

Depth of Discharge and Usable Capacity

Depth of discharge expresses the portion of a reference capacity removed from the battery:

When SOC and DoD use the same reference capacity, DoD is approximately 100% − SOC. Reported values may differ in practical BMS implementations because usable buffers, reference capacities, calibration, and estimation methods can differ.

Manufacturers may restrict the permitted SOC/DoD window for life, safety, power, or warranty reasons. A 100 Ah nameplate rating therefore does not automatically mean an application should routinely extract the full 100 Ah.

Coulombic, Energy and Round-Trip Efficiency

Coulombic efficiency compares charge removed during discharge with charge supplied during charge over a defined cycle.

Energy efficiency compares electrical energy out with electrical energy in. Because charge and discharge occur at different terminal voltages and include internal losses, high coulombic efficiency does not imply identical energy efficiency.

Round-trip efficiency is a system-level energy ratio measured over a complete charge/discharge process with clearly defined boundaries. It may include battery-only losses or broader conversion and auxiliary losses depending on the stated test boundary. DOE FEMP evaluates deployed BESS efficiency from measured charge and discharge energy over a defined analysis period, reinforcing the importance of the measurement boundary and period. [3]

Cells, Modules, Packs and Battery Banks

Battery-system terminology varies among industries and manufacturers, so the following hierarchy should be treated as a general engineering model rather than an absolute naming standard.

Depending on chemistry and application, higher-level assemblies can include BMS electronics, contactors, fuses, interconnects, thermal-management hardware, enclosure systems, sensors, communication interfaces, and supervisory monitoring.

Series and Parallel Configurations

In an idealized battery bank, series connection adds voltage while amp-hour capacity remains the same; parallel connection adds amp-hour capacity while nominal voltage remains the same. Total nominal energy increases with the number of equivalent batteries in either architecture.

Real banks require attention to current sharing, cell/battery mismatch, state-of-charge balance, cable and interconnect resistance, fuse/protection coordination, charger compatibility, and manufacturer limits on parallel strings.

Batteries combined in a bank should be compatible in chemistry, voltage, design, capacity, age, condition, and approved application.

Primary vs. Secondary Batteries

Battery Rating Conditions and Datasheet Interpretation

Battery ratings must be read together with their test conditions. A specification value without the associated discharge rate or duration, end voltage, temperature, charge state, and measurement method can be misleading.

For stationary batteries, constant-current and constant-power data may both be important. For high-power batteries, continuous and pulse limits must be interpreted with time, temperature, SOC, and protection limits. DOE/INL battery test methodology likewise ties performance characterization to defined test profiles, procedures, and operating conditions. [2]

Essential Battery Terminology

Terminology is aligned with standard battery engineering usage and IEC 60050-482 / Electropedia concepts where applicable. [1]

Technical Summary

• Battery voltage originates from electrochemical potential and changes with chemistry and operating condition.
• Terminal voltage differs from rested open-circuit voltage whenever current and polarization effects are present.
• Amp-hours measure charge; watt-hours measure energy.
• Rated capacity is conditional on discharge rate or duration, cutoff voltage, temperature, initial state, and test method.
• Internal resistance and polarization affect voltage drop, heat generation, available power, and usable energy.
• SOC and SOH are estimated, method-dependent quantities rather than simple direct voltage readings.
• Temperature affects impedance, capacity, power capability, charge acceptance, degradation rate, and safety.
• Series and parallel architecture changes system voltage, capacity, current distribution, and protection requirements.
• Nominal energy is not the same as delivered or usable energy.
• Datasheet values should always be interpreted together with their test conditions and operating limits.

Technical Interpretation Notes