Standby / Reserve
Readiness, autonomy, float or standby charging, temperature, and aging can dominate sizing.
05
Battery Selection & Sizing
Battery Selection & Sizing
Engineering reference for battery selection and sizing based on application duty, load profile, voltage architecture, discharge performance, required energy and runtime, temperature, aging, usable SOC window, charger compatibility, physical integration, and manufacturer performance data.
Battery Selection Engineering Framework
Battery selection must start with the application and load, not with chemistry, amp-hour rating, or physical size. The engineering sequence defines what the battery must do before a product family is evaluated.
Figure 1 — Battery Selection Engineering Flow
ApplicationWhat must the battery support?
→
Load ProfileCurrent, power, peaks, duration
→
Electrical ArchitectureDC bus, inverter, controller, series count
→
Runtime / AutonomyRequired support duration
→
Discharge RateConstant current, constant power, variable load
→
EnvironmentTemperature, vibration, enclosure
↓
Aging / Design CriteriaEnd-of-life duty and reserve
→
Chemistry / Product FamilyEvaluate suitable technologies
→
Charger CompatibilityVoltage, current, algorithm, recharge
→
Mechanical IntegrationSpace, mass, terminals, access
→
Protection / MonitoringFusing, BMS, alarms, communications
→
Manufacturer VerificationExact model, table, conditions, approvals
Engineering Principle
A technically suitable battery is the result of a matched system design. A product that matches only nominal voltage, Ah, or physical dimensions can still fail the required duty.
Define the Application Duty
Duty class determines which performance data matter most. This section is intentionally brief; the detailed application engineering discussion is covered in Technical Reference 03 — Battery Applications.
High-Rate Short-Duration
Power capability, end voltage, internal voltage drop, and manufacturer short-duration data become critical.
Repeated Cyclic Energy
Usable energy, SOC window, cycle duty, recharge opportunity, and efficiency are central.
Motive / Traction
Shift duty, peak current, sustained energy, battery mass, compartment fit, and charging strategy interact.
Starting
Short-duration cranking capability, temperature, reserve, fitment, and charging-system compatibility matter more than deep-cycle energy.
Mission-Critical
Required autonomy must be combined with redundancy, monitoring, maintenance, fault tolerance, and replacement strategy.
Mixed Duty
Systems combining starting, standby, cyclic, or auxiliary loads must be evaluated as a combined load profile.
Define the Load Profile
The battery must be sized against the electrical load it actually sees. The load can be approximately constant current, approximately constant power, variable or intermittent, or a base load with short peaks or surges. AC loads supplied through a UPS or inverter must be translated to the battery-side requirement using the power-conversion architecture and losses.
P = V × IInstantaneous DC power
I = P ÷ VCurrent corresponding to power at terminal voltage
E = ∫ P(t) dtEnergy over a variable load profile
For a constant-power load, battery current tends to increase as terminal voltage falls because the load continues demanding approximately the same power. This behavior is especially important for UPS and inverter-supported systems and is one reason long-duration Ah ratings alone may not represent short-duration power capability.
Determine System Voltage
Nominal system voltage is only one point in the required operating window. A battery system must remain compatible with the charger, load, inverter or controller, BMS where applicable, protective devices, and the equipment DC bus from the highest permitted charge condition to the lowest permitted discharge condition.
Figure 2 — Battery Operating-Voltage Window
Charge / Float Region
Highest normal system voltage must be accepted by connected equipment.
Higher voltage
Lower voltage
Nominal VoltageIdentification value, not a fixed operating voltage.
End / Cutoff VoltageMust remain consistent with load and battery performance-table conditions.
- Confirm nominal system or DC-bus voltage.
- Confirm charger, float, equalize or maximum charge voltage where applicable.
- Confirm minimum equipment operating voltage and protective cutoff settings.
- Determine the correct number of cells, modules, or monoblocs in series.
Amp-Hours vs Watt-Hours
Amp-hours (Ah) describe charge capacity. Watt-hours (Wh) describe energy.
Enominal ≈ Vnominal × Ah
Nominal comparison example
12 V × 100 Ah ≈ 1,200 Wh nominal
Nominal watt-hours are not the same as usable delivered watt-hours. Usable energy depends on the discharge-voltage profile, cutoff voltage, SOC/DoD window, discharge rate, temperature, age, battery condition, conversion losses, and system control limits.
Basic Constant-Current Sizing
For an idealized constant-current load, the theoretical charge requirement is:
Qrequired = Iload × t
Worked Screening Example — Constant-Current Load
Illustrative load: 10 A DC for 5 hours.
10 A × 5 h = 50 Ah theoretical charge requirement
The 50 Ah result is a starting calculation, not a final battery selection. The selected battery must still be verified against manufacturer discharge data at the required duration and end voltage, temperature, aging/design criteria, permitted SOC/DoD, recharge capability, and application-specific requirements.
Energy-Based Sizing
Energy-oriented systems are often screened from the required load-side energy and then corrected for the portion of nominal battery energy that can be used and for conversion losses.
Erequired = Pload × tConstant-load energy requirement
Enominal,required ≈ Eload ÷ (usable SOC fraction × conversion efficiency)Simplified screening equation
Screening Equation Only
The usable SOC fraction and conversion efficiency must be selected from the actual battery/system design. They are not universal battery constants.
Constant-Current vs Constant-Power Sizing
Constant-current and constant-power loads should not be treated as interchangeable. The appropriate manufacturer data depends on how the supported equipment behaves as battery voltage changes.
| Engineering Item | Constant Current | Constant Power |
|---|---|---|
| Load behavior | Current remains approximately fixed over the modeled interval. | Power remains approximately fixed; current increases as battery voltage falls. |
| Typical examples | Some DC reserve and telecom loads where the system behavior supports a constant-current model. | UPS inverter loads and other regulated power-conversion loads. |
| Useful manufacturer data | Constant-current discharge tables at required duration, temperature, and end voltage. | Constant-power W/cell or W/block tables at required duration, temperature, and end voltage. |
| Role of Ah | Can support first-pass estimates, but still requires table verification. | Long-duration Ah ratings can be poor predictors of short-duration power performance. |
| End-voltage importance | Defines the table condition and usable discharge point. | Critical because falling voltage changes required battery current for a fixed power load. |
Figure 3 — Constant-Current vs Constant-Power Load Behavior
Conceptual only — not measured battery data and not a discharge curve.
IEEE 1184 addresses battery selection for UPS systems together with the relationship between battery systems and UPS charging/converter components. [1]
Discharge Rate and Rate-Capacity Effect
Discharge rate affects terminal-voltage behavior, heat generation, and usable capacity or energy. In lead-acid batteries, increasing discharge rate generally reduces delivered Ah to a specified cutoff; Peukert-type behavior is one way this effect is described for many lead-acid applications. High-rate discharge also increases the significance of internal resistance and voltage sag.
Peukert's law should not be used as a universal lithium sizing method. Lithium-ion products also show rate-dependent voltage and usable energy, but the behavior depends on cell chemistry, design, temperature, current, BMS limits, and cutoff conditions. See Technical Reference 01 — Battery Fundamentals for the detailed rate-capacity discussion.
End Voltage / Cutoff Voltage
Manufacturer discharge tables are meaningful only when the stated end voltage is understood. A performance value published to one end voltage cannot be assumed to apply at a different equipment cutoff. The battery table condition and the supported equipment operating limit must both be checked.
Vstring,end = Ncells × Vend,cell
Illustrative lead-acid calculation
24 cells × 1.75 V/cell = 42.0 V string end voltage
The 1.75 V/cell value above is an illustrative table condition, not a universal limit. The correct end voltage depends on the exact battery product, discharge duration, system design, and supported equipment. IEEE 485 describes load definition and sizing methods for stationary lead-acid batteries in float service. [2]
Depth of Discharge and Usable SOC Window
Nominal capacity or energy is not necessarily available to the application. Battery manufacturers, BMS limits, system controls, life objectives, reserve requirements, and application constraints can restrict the operating SOC window.
Eusable ≈ Enominal × usable SOC fraction
This is a simplified relationship. The permitted SOC/DoD window must come from the exact battery/system design and should not be replaced with a generic percentage.
Temperature Correction
Temperature changes electrochemical kinetics, impedance, available capacity, power capability, charge acceptance, aging rate, and in some chemistries the permitted charging window. Low temperature commonly increases impedance and can reduce available power and capacity. Higher temperature may temporarily improve kinetics but generally accelerates degradation and can increase safety risk.
There is no universal temperature derating factor. Use manufacturer-specific data or the applicable application standard or guide.
Figure 4 — Manufacturer-Specific Temperature Capacity Factors
EnerSys PowerSafe OPzV Lead-Acid Example
Aging / End-of-Life Design
A system sized only for a new battery can lose required autonomy or power capability as the battery ages. Aging can reduce available capacity, increase resistance or impedance, increase voltage drop under load, and reduce high-rate performance.
The correct aging or end-of-life criterion depends on the application, battery technology, project specification, maintenance practice, and applicable standard. Do not apply a universal replacement percentage across every chemistry and application. IEEE 1188 provides maintenance, testing, and replacement guidance for stationary VRLA batteries, while other technologies and applications use their own criteria. [4]
New-Battery DutyBase load and runtime requirement
+
Temperature CriterionProduct-specific performance correction
+
Aging / End-of-Life CriterionProject or standard-specific reserve
+
Reliability / RedundancySeparate design objective where required
Conceptual sizing inputs only; the factors are not intended to be added arithmetically without the governing sizing method.
System Efficiency and Auxiliary Losses
Battery sizing should distinguish battery-side energy from load-side delivered energy. Depending on the architecture, the battery may also support inverter losses, DC/DC conversion losses, cable losses, BMS/control consumption, thermal-management auxiliaries, communications, contactor coils, and other parasitic loads.
Battery-Side EnergyEnergy withdrawn from battery terminals
→
Conversion + Auxiliary LossesInverter, DC/DC, cables, BMS, controls, thermal auxiliaries
→
Load-Side Delivered EnergyEnergy actually delivered to the supported load
Charger efficiency is relevant when assessing recharge energy and infrastructure, while discharge-path efficiency is relevant when determining how much battery-side energy is required to satisfy the supported load.
Series and Parallel String Sizing
For identical batteries in an idealized bank, series connection increases voltage while Ah remains approximately the same. Parallel connection increases available Ah/current capability while nominal voltage remains approximately the same.
| Configuration | Nominal Voltage | Ah | Nominal Energy |
|---|---|---|---|
| 1 battery | 12 V | 100 Ah | 1.2 kWh |
| 2S | 24 V | 100 Ah | 2.4 kWh |
| 2P | 12 V | 200 Ah | 2.4 kWh |
| 2S2P | 24 V | 200 Ah | 4.8 kWh |
Figure 5 — Series, Parallel and Redundant String Architecture
12 V
100 Ah+12 V
100 Ah
24 V · 100 Ah
100 Ah+12 V
100 Ah
12 V · 100 Ah12 V · 100 Ah
12 V · 200 Ah
String AString B
Capacity or redundancy depends on design objective
Real battery banks require review of current sharing, cable and interconnect resistance, balancing, protection, string mismatch, monitoring, and manufacturer limits. Equal cable resistance and matched battery condition become increasingly important with parallel strings.
Parallel Strings and Redundancy
A parallel string can be added because the load requires more current or capacity, or because the system requires redundancy. These are different design objectives and should be documented separately.
Parallel strings can introduce unequal current sharing, cable-resistance sensitivity, fault-isolation challenges, monitoring requirements, and maintenance complexity. There is no single universal maximum number of parallel strings; follow the battery manufacturer and system design requirements. IEEE 946 addresses battery, charger, distribution, instrumentation, and protection considerations in stationary DC power systems. [3]
Charger and Recharge Capability
Battery capacity cannot be evaluated independently of the charger. Increasing installed Ah can increase the charge that must be replaced after discharge and can therefore increase recharge time if available charging current does not increase.
trecharge ≈ charge to replace ÷ available net charging current
Simplified conceptual relationship only. Actual recharge depends on battery chemistry, SOC, charge acceptance, voltage control, current limits, charge algorithm, temperature, losses, and charge-stage behavior.
Charge VoltageFloat, absorption, boost, equalize, maximum voltage, or lithium charge limit as applicable.
Maximum Charge CurrentBattery acceptance, charger output, and connected-load sharing.
Charge AlgorithmMust match the battery product and approved operating method.
Temperature CompensationUse where required by the specific battery/charger design.
BMS CommunicationSome lithium systems require charge-enable, CAN, RS485, or other control integration.
Recharge RequirementConfirm required time to restore the system after a design discharge.
Engineering Significance
A larger Ah battery is not automatically a better replacement if the installed charger cannot correctly recharge it within the required operating cycle.
Battery Chemistry Selection
Chemistry should be selected after the duty, electrical architecture, environmental conditions, charging infrastructure, mechanical constraints, safety requirements, and lifecycle objectives are defined. Detailed chemistry comparison belongs in Technical Reference 02 — Battery Types & Technologies.
- Application duty and discharge-rate requirement
- Cycling frequency and permitted SOC window
- Maintenance model and inspection access
- Temperature and environmental exposure
- Existing charger and power-conversion equipment
- Space, mass, installation, and lifting constraints
- Safety, compliance, monitoring, and communications
- Lifecycle cost and replacement strategy
- OEM or system-approved compatibility
No battery chemistry is universally superior for every application.
Mechanical and Physical Integration
Electrical compatibility does not guarantee physical integration. A replacement must fit the battery compartment or rack while preserving safe terminal clearance, ventilation or thermal-management requirements, service access, lifting access, and cable routing.
DimensionsLength, width, height, handles, protrusions, terminal height.
Terminal ClearanceProtective covers, live-part clearance, tooling access.
Polarity & OrientationCable reach and approved mounting orientation.
Rack / Tray CompatibilityFootprint, restraints, seismic or application-specific requirements.
MassRack/floor loading, lifting, service handling, equipment balance.
Ventilation / ThermalGas ventilation or thermal management as required by technology and installation.
Service ClearanceInspection, testing, replacement, and connector access.
Cable RoutingBend radius, support, protection, and equal path resistance where required.
In motive applications, battery mass can be an equipment-design variable or contribute to vehicle balance. It should not automatically be reduced during a chemistry conversion without reviewing the equipment manufacturer's requirements.
Terminals, Cables and Connections
Connection details affect voltage drop, heat generation, mechanical reliability, serviceability, and fault protection. Battery selection should confirm terminal style, polarity, cable size, connector type, terminal hardware, torque requirements, protective devices, and cable routing without turning the selection process into a substitute for the installation manual.
Engineering Significance
Loose, undersized, corroded, mismatched, or high-resistance connections can create local heating and voltage drop even when the battery itself is correctly sized. Use the exact battery and equipment installation instructions for terminal hardware and torque requirements.
Battery Datasheet Interpretation
A specification value without its test conditions can be misleading. The exact product revision, test temperature, discharge duration, end voltage, SOC, charge condition, pulse duration, and other limits determine how a datasheet value should be used.
| Datasheet Parameter | What It Means | What Must Be Checked | Common Misinterpretation |
|---|---|---|---|
| Nominal voltage | Conventional identification voltage. | Cell count and full operating-voltage window. | Treating nominal voltage as a fixed operating voltage. |
| Rated Ah | Charge delivered under stated test conditions. | Rate/duration, end voltage, temperature, condition. | Assuming the same Ah is available at every discharge rate. |
| Energy / Wh | Nominal or measured energy depending on datasheet definition. | Voltage basis, test method, cutoff, SOC window. | Equating nominal Wh with usable delivered Wh. |
| C-rate | Current normalized to a stated capacity reference. | Reference capacity and allowed operating range. | Assuming C-rate predicts runtime without voltage/cutoff data. |
| Constant-current table | Published current for defined time/end-voltage/temperature. | Exact row, column, end voltage and temperature. | Using data from a different cutoff or temperature. |
| Constant-power table | Published W/cell or W/block under defined conditions. | Duration, end voltage, temperature, cell/block basis. | Converting a long-hour Ah rating into short UPS power without manufacturer data. |
| End voltage | Voltage at which the published discharge test terminates. | Compatibility with equipment minimum voltage. | Ignoring the table's end-voltage assumption. |
| Maximum continuous current | Product current limit under specified conditions. | Temperature, duration, terminals, BMS/protection limits. | Treating it as guaranteed usable capacity or energy. |
| Peak / pulse current | Short-duration current capability. | Pulse length, SOC, temperature, recovery interval. | Treating pulse current as continuous current. |
| Charge voltage | Specified voltage for a charge stage or operating mode. | Temperature basis, charger algorithm, tolerance. | Using a generic charger because nominal voltage matches. |
| Float voltage | Standby maintenance voltage where applicable. | Temperature compensation and product chemistry. | Applying float charging to products not designed for it. |
| Cycle charge | Charge method for cyclic service. | Voltage/current limits and charge termination. | Assuming cycle and float settings are interchangeable. |
| Operating temperature | Specified operating range. | Charge vs discharge limits and performance derating. | Assuming full rated performance across the entire range. |
| Storage temperature | Permitted storage environment. | Storage SOC and refresh-charge requirements. | Ignoring time-dependent storage limitations. |
| Cycle life | Cycles under a defined laboratory/profile condition. | DoD, current, temperature, charge method, EOL criterion. | Comparing cycle numbers without matching test conditions. |
| Calendar / design life | Time-based service/design expectation under specified conditions. | Temperature, float condition, maintenance, EOL definition. | Treating design life as a guaranteed replacement interval. |
| Internal resistance | Method-dependent electrical parameter. | Test method, SOC, temperature, frequency/pulse method. | Comparing values measured by different methods as equivalent. |
| Self-discharge | Open-circuit charge loss over time. | Temperature, storage SOC and measurement period. | Assuming the same storage interval at all temperatures. |
| Dimensions | Physical envelope. | Terminal height, handles, cable and service clearance. | Checking footprint only. |
| Weight | Product mass. | Rack/floor loading, lifting, motive counterbalance. | Assuming lighter is automatically better. |
| Terminals | Connection interface. | Type, polarity, hardware, torque and cable compatibility. | Assuming adapters are always acceptable. |
| Certification / compliance | Evidence for a defined standard, scope or regulation. | Exact model, certificate scope, edition and jurisdiction. | Assuming a company-level certificate covers every product. |
Figure 6 — Datasheet Verification Flow
Exact ModelConfirm product code and revision
→
Published ValueAh, W/cell, current, voltage, cycle data
→
Test ConditionsTime, end voltage, temperature, SOC, method
→
Application MatchLoad, runtime, environment, charger, mechanics
→
Engineering DecisionAccept, resize, or select another model
Manufacturer-Data Worked Example
Worked Example — UPS Constant-Power Sizing
This screening example uses an official EnerSys DataSafe HX constant-power table. It demonstrates the sizing method only; final UPS design requires all applicable system margins and manufacturer requirements.
UPS Constant-Power Screening — DataSafe 12HX400
Published manufacturer condition
- Battery: EnerSys DataSafe 12HX400
- Runtime: 10 minutes
- End voltage: 1.75 V/cell
- Temperature: 25°C / 77°F
- Published constant-power rating: 493.2 W/cell
Illustrative battery-side UPS load: 10.0 kW for 10 minutes.
Nominal string: four 12 V monoblocs = 24 cells.
493.2 W/cell × 24 cells = 11,836.8 W ≈ 11.84 kW per string
Required screening string quantity = 10,000 W ÷ 11,836.8 W/string = 0.845 → 1 string
At the exact published table condition, one four-monobloc string has a published 10-minute rating above the illustrative 10.0 kW battery-side load.
Design items not yet included
Aging/end-of-life criterion · temperature correction · redundancy · UPS conversion losses if the starting load is specified on the AC side · recharge capability · manufacturer/system rules · cable and protection design.
Source: EnerSys, DataSafe HX & HX Plus Performance Data, constant power to 1.75 V/cell at 25°C / 77°F.
[5]
Manufacturer-Data Worked Example
Worked Example — Telecom Autonomy
Telecom reserve systems can be screened using manufacturer constant-current data when the load model supports that approach. The table condition, end voltage, and temperature must match the engineering check.
Telecom Autonomy Screening — PowerSafe 12V190F
Published manufacturer condition
- Battery: EnerSys PowerSafe 12V190F
- Autonomy: 8 hours
- End voltage: 1.80 V/cell
- Temperature: 25°C / 77°F
- Published discharge current: 23.77 A
Nominal −48 V architecture: four 12 V monoblocs in series = 24 cells.
String end voltage = 24 cells × 1.80 V/cell = 43.2 V
Illustrative DC load: 20 A for 8 hours.
Published screening: 20 A < 23.77 A → PASS at the cited table condition
Design items not yet included
Outdoor/site temperature · aging/design criterion · rectifier capacity · low-voltage disconnect settings · monitoring · parallel-string behavior · recharge requirement · operator standard.
Source: EnerSys, PowerSafe V-FT Performance Data, constant-current table to 1.80 V/cell at 25°C / 77°F.
[6]
Illustrative Energy-Sizing Example
Worked Example — Stationary Storage / Solar
This example uses explicitly labeled illustrative assumptions to show the relationship between load-side energy, usable SOC window, and inverter efficiency. The assumptions are not universal battery ratings.
Stationary Storage Nominal Energy from Usable AC Energy
Illustrative project assumptions
- Required AC energy delivered to load: 10.0 kWh
- Allowed usable SOC window: 80%
- Inverter discharge efficiency used for screening: 94%
Enominal = EAC,required ÷ (usable SOC fraction × inverter efficiency)
Enominal = 10.0 kWh ÷ (0.80 × 0.94)
Enominal = 13.30 kWh
Under these stated assumptions, at least 13.3 kWh nominal battery energy is required before applying any project-specific temperature, aging, reserve, auxiliary-load, reliability, or other design allowances.
Fire Alarm / Life-Safety Sizing
Fire-alarm and other life-safety battery sizing must follow the system or panel manufacturer documentation, the applicable fire-alarm code or standard, the actual connected standby load, the actual alarm/emergency load, required durations, charger/recharge capability, enclosure capacity, approved battery type, and jurisdictional requirements.
Do not use a single universal fire-alarm sizing formula without the governing equipment and code context. NFPA 72 provides requirements for fire alarm and signaling systems; the applicable edition and local adoption should be confirmed for the project. [11]
Panel / Power SupplyExact manufacturer and model.
Standby LoadActual connected quiescent current.
Alarm / Emergency LoadActual connected alarm or emergency current.
Required DurationsEquipment, code and jurisdiction specific.
Charger / RechargePanel charger rating and allowed battery capacity.
Enclosure / ListingBattery fit, approved configuration, documentation and inspection requirements.
Forklift / Motive Selection
Motive selection combines electrical duty with truck integration. Confirm system voltage, required Ah/kWh, peak power, battery compartment, battery mass where required by the equipment design, connectors, cable position, shift duty, charging strategy, charger compatibility, thermal limits, and communications for lithium systems.
OSHA 29 CFR 1910.178 includes requirements relevant to powered industrial trucks and battery charging/changing practices. [12] Detailed application behavior is covered in Technical Reference 03.
Replacement Battery Equivalence
Equal voltage and Ah do not establish replacement equivalence. The replacement must be reviewed electrically, mechanically, thermally, operationally, and with respect to charging, protection, communications, safety, and equipment approval.
| Parameter | Must Match? | Can Vary? | Engineering Review Required? |
|---|---|---|---|
| Nominal voltage | Normally yes for direct replacement | Only with system redesign/approval | Yes if changed |
| Operating voltage range | Must be compatible | Can differ by technology | Yes |
| Chemistry | Not necessarily | Possible only where system supports it | Yes |
| Capacity | Must meet required duty | Can vary within system limits | Yes if changed materially |
| Power capability | Must meet load | Can exceed requirement if system-compatible | Yes |
| Dimensions | Must fit safely | Can vary within compartment/rack limits | As needed |
| Terminal type | Normally must be compatible | Can vary only with approved connection design | Yes if changed |
| Polarity | Must be compatible | No for direct cable connection | Yes if system modified |
| Weight | Must meet installation/equipment constraints | Can vary within approved limits | Yes for motive/rack/loading changes |
| Charger | Must be compatible | May require replacement/reprogramming | Yes |
| BMS / communications | Must be compatible where used | Can vary with validated integration | Yes |
| Protection | Must remain correctly coordinated | May require redesign | Yes |
| Certifications / compliance | Must meet application requirement | Evidence may differ by product | Yes |
| Temperature limits | Must cover application | Can differ | Yes |
| Application approval | Must satisfy OEM/system requirement | Only with documented approval | Yes |
Battery Bank Replacement
Series and parallel banks perform best when units are compatible in model, capacity, design, age, condition, state of charge, and electrical connection. Mixing old and new batteries can create unequal voltage behavior, current sharing, and aging within the bank, especially in critical series strings.
Replacement Principle
Full-set replacement is often technically preferred in matched series strings, but the correct decision depends on system design, manufacturer guidance, measured condition, maintenance practice, redundancy, and application criticality. Avoid universal statements that every battery bank must always be replaced as a complete set.
Common Selection and Sizing Errors
| Error | Technical Consequence |
|---|---|
| Selecting by Ah alone | Can miss short-duration power, end-voltage, rate, energy, charger, or environmental requirements. |
| Ignoring end voltage | Uses manufacturer data under conditions that may not match the equipment cutoff. |
| Ignoring discharge rate | Can overestimate usable capacity or power and underpredict voltage sag. |
| Using nominal Wh as usable Wh | Overstates energy available to the load after SOC limits and system losses. |
| Ignoring charger capacity | Can produce excessive recharge time or incompatible charging. |
| Ignoring temperature | Can reduce available power/capacity or violate charge limits. |
| No aging criterion | System may meet duty when new but fail before the planned replacement point. |
| Mixing incompatible batteries | Can create imbalance, unequal current sharing, shortened life, or unsafe operation. |
| Ignoring terminals / polarity | Can prevent safe connection or create incorrect cable routing and resistance. |
| Ignoring weight | Can exceed rack/floor/equipment limits or alter motive-equipment balance. |
| Unsupported chemistry conversion | Can create charger, voltage-window, BMS, protection, or approval conflicts. |
| Ignoring BMS / communications | Can prevent correct charge/discharge control, alarms, or system enable functions. |
| Using generic curves instead of product data | Can produce sizing results unrelated to the exact battery model. |
Professional Battery Selection Checklist
This checklist is intended for technical quoting, procurement, replacement review, and application engineering. It does not replace the manufacturer or application-specific design procedure.
| Selection Item | Information to Record / Verify | Status |
|---|---|---|
| Application | Equipment/system function and supported duty. | □ |
| Equipment model | Manufacturer, exact model, revision where relevant. | □ |
| Load | DC current, DC power, AC load, peaks/surges, base load, measured profile. | □ |
| Voltage | Nominal bus, charge voltage, operating range, minimum/end voltage. | □ |
| Runtime | Required autonomy at defined load. | □ |
| Duty cycle | Standby, high-rate, cyclic, motive, starting, mixed. | □ |
| Chemistry | Approved or technically suitable chemistry/product family. | □ |
| Capacity / energy | Required Ah/kWh under applicable method. | □ |
| Discharge rate | Constant current, constant power, C-rate, pulse profile. | □ |
| End voltage | Manufacturer table condition and equipment cutoff. | □ |
| Temperature | Battery temperature range and performance correction. | □ |
| Aging criterion | Project/standard/manufacturer end-of-life basis. | □ |
| SOC / DoD window | Permitted operating window and reserve requirement. | □ |
| Charger | Model, voltage, current, algorithm, communication. | □ |
| Recharge time | Required recovery time after defined discharge. | □ |
| Dimensions | Length, width, height, terminal clearance. | □ |
| Weight | Battery mass and rack/floor/equipment limits. | □ |
| Terminals | Type, orientation, polarity, hardware. | □ |
| Cables / connectors | Size, connector, routing, equal path resistance where required. | □ |
| BMS / communications | CAN, RS485, dry contacts, enable lines, monitoring compatibility. | □ |
| Safety / compliance | Application, jurisdiction, transport and product requirements. | □ |
| Manufacturer model | Exact model number and current product revision. | □ |
| Performance table / reference | Exact table, duration, temperature, end voltage and source document. | □ |
| Warranty | Coverage, operating conditions, exclusions, claim requirements. | □ |
| Quantity / string configuration | Series count, parallel strings, redundancy, protection. | □ |
Technical Summary
- Battery sizing begins with the application, load profile, and required duty.
- Amp-hours alone do not determine runtime, power capability, or replacement equivalence.
- Constant-power and constant-current loads require different interpretation and often different manufacturer tables.
- Manufacturer performance data must be matched to the required duration, end voltage, and temperature.
- Nominal energy is not equal to usable delivered energy.
- Temperature and aging can materially change required installed capacity or power capability.
- Charger capability and required recharge time are part of battery sizing.
- Parallel capacity and redundancy are separate design objectives.
- Replacement equivalence requires electrical, mechanical, charging, control, protection, temperature, and approval compatibility.
- Critical systems should be verified against current manufacturer data, equipment documentation, and application-specific standards.
Technical References
- IEEE Standards Association, IEEE 1184-2022 — IEEE Guide for Batteries for Uninterruptible Power Supply Systems. IEEE standard
- IEEE Standards Association, IEEE 485-2020 — IEEE Recommended Practice for Sizing Lead-Acid Batteries for Stationary Applications. IEEE standard
- IEEE Standards Association, IEEE 946-2020 — IEEE Recommended Practice for the Design of DC Power Systems for Stationary Applications. IEEE standard
- IEEE Standards Association, IEEE 1188-2025 — Recommended Practice for Maintenance, Testing, and Replacement of Valve-Regulated Lead-Acid Batteries for Stationary Applications. IEEE standard
- EnerSys, DataSafe HX & HX Plus Performance Data, constant-current and constant-power discharge performance tables. Manufacturer performance data
- EnerSys, PowerSafe V-FT Performance Data, October 2022, constant-current discharge tables including 1.80 V/cell at 25°C / 77°F. Manufacturer performance data
- EnerSys, PowerSafe OPzV Battery Installation, Operation and Maintenance Instructions, AMER-EN-M-PS-OPZV-0126, 2026; end-voltage guidance, recharge information, and temperature correction factors. Manufacturer instructions
- IEC, IEC 62485-2:2010 — Safety requirements for secondary batteries and battery installations — Part 2: Stationary batteries. IEC standard
- IEC, IEC 62485-5:2020 — Safety requirements for secondary batteries and battery installations — Part 5: Safe operation of stationary lithium ion batteries. IEC standard
- IEC, IEC 62619:2022 — Safety requirements for secondary lithium cells and batteries for use in industrial applications. IEC standard
- National Fire Protection Association, NFPA 72 — National Fire Alarm and Signaling Code, current code-development resource / 2025 publication. NFPA reference
- U.S. Occupational Safety and Health Administration, 29 CFR 1910.178 — Powered Industrial Trucks, including battery-changing and charging provisions. OSHA regulation
- U.S. Department of Energy Federal Energy Management Program, Battery Energy Storage System Evaluation Method. DOE reference
- U.S. Department of Energy, Solar Integration: Solar Energy and Storage Basics and power-electronics guidance. DOE reference
Standards Note
The applicable sizing, safety, installation, testing, fire/life-safety, transportation, and equipment requirements vary by battery technology, application, system architecture, jurisdiction, and project specification. The exact edition and scope of each standard or code should be confirmed for the project.
Last technical review: August 2026