How to Size an AGV Battery Pack: Runtime, Peak Current, and Charging Strategy

A battery can pass a capacity test and still be wrong for the AGV carrying it.

This usually happens because the initial calculation is too neat. The equipment consumes 600W on average, the required runtime is eight hours, and the battery is therefore specified at 4.8kWh. On paper, the answer appears finished.

The machine then enters a real warehouse.

It accelerates with a loaded pallet, turns on a high-friction floor, lifts at the transfer station, waits for traffic, restarts repeatedly, and occasionally reaches the charger later than expected. Current rises sharply during some of these events. Voltage falls near the end of the shift. The battery management system sees a condition that the original average-power calculation never described.

The result may be reduced speed, an undervoltage alarm, an overcurrent shutdown, incomplete routes, or an AGV that returns to charge with much less reserve than the fleet software expects.

A reliable AGV battery pack must satisfy three different requirements:

1. It must store enough usable energy for the planned operating period.

2. It must deliver the highest current demanded by the machine.

3. It must recover enough energy during the available charging windows.

Capacity, power, and charging are related, but they are not interchangeable.

The Short Answer

Start with a measured or carefully estimated duty cycle rather than a single average-current value.

Calculate the energy consumed during acceleration, cruising, turning, lifting, waiting, and auxiliary operation. Then account for the permitted state-of-charge window, electrical losses, temperature, aging, route variation, and the reserve needed to reach a charger safely.

After that, perform a separate peak-current calculation using the lowest expected battery voltage. Confirm that the cells, BMS, fuse, contactor, connector, cable assembly, and motor controller can support both the magnitude and duration of the peak.

Finally, simulate the charging schedule for the whole fleet. A battery sized for overnight charging may be different from one designed around short, frequent opportunity-charging sessions.

Why Ampere-Hours Alone Do Not Size an AGV Battery

Ampere-hours describe charge capacity. They do not provide a complete comparison between batteries operating at different voltages.

Nominal battery energy is calculated as:

Nominal energy (Wh) = Nominal voltage (V) × Rated capacity (Ah)

For example:

• 48V × 100Ah = 4,800Wh

• 51.2V × 100Ah = 5,120Wh

• 51.2V × 108Ah = 5,529.6Wh

An engineer comparing only the 100Ah and 108Ah labels would miss the effect of voltage. Watt-hours provide the more useful starting point for runtime calculations.

Even watt-hours are not the final answer. The entire nominal energy printed on a data sheet is rarely available to the machine under every operating condition. BMS limits, state-of-charge reserve, voltage cutoff, temperature, discharge rate, and aging all affect usable energy.

Victron Energy’s technical explanation of battery capacity and discharge rate also notes that rated ampere-hour capacity must be interpreted together with discharge conditions. The effect is generally more pronounced in lead-acid batteries, but lithium battery performance still depends on actual operating conditions.

Step 1: Record the Real AGV Duty Cycle

A useful duty cycle describes what the AGV does and how long it does it.

Do not merge every activity into one estimated average until the individual loads have been reviewed.

A representative operating cycle may include:

• Acceleration from rest

• Loaded travel

• Unloaded return travel

• Turning

• Slope climbing

• Lifting or conveyor transfer

• Steering

• Braking

• Waiting at intersections

• Navigation and control electronics

• Wireless communication

• Safety scanners

• Cooling fans

• Standby time

• Travel to and from the charger

The energy calculation needs the duration and average power of each phase. The peak-current review needs the maximum power and duration within those phases.

Figure 1. An AGV duty cycle contains short power peaks, steady travel, low-load waiting periods, and possible regenerative events. One average value cannot describe all of them.

If a prototype is available, measure current and voltage at a suitable sampling rate while the AGV performs a representative route. Include the heaviest payload, longest practical route, repeated starts, normal congestion, and the least favorable floor or slope condition.

A one-minute handheld-meter reading is unlikely to capture the event that determines the battery design.

Step 2: Calculate Energy Phase by Phase

Consider a hypothetical 48V-class AGV with the following one-hour operating profile.

Operating phase	Duration per hour	Average electrical power	Energy consumed
Acceleration and loaded movement	10 minutes	1,500W	250Wh
Normal transport	30 minutes	650W	325Wh
Lifting and load transfer	5 minutes	1,200W	100Wh
Waiting and control electronics	15 minutes	120W	30Wh
Total	60 minutes	—	705Wh

The hourly energy use is therefore approximately 705Wh.

If the AGV must operate for four hours between charging opportunities:

705Wh × 4 = 2,820Wh

A 2.82kWh battery might appear sufficient, but this is only the energy delivered to the equipment during the modeled cycle. It does not yet include system losses, reserve, aging, or route variation.

The example also depends on the quality of its inputs. If the five-minute lifting period actually averages 1,700W, or congestion causes more stop-start operation, real energy consumption will be higher.

Step 3: Convert Operating Energy Into Required Nominal Capacity

A practical preliminary formula is:

Required nominal energy = Load energy ÷ Combined usable-energy factors

The factors may include:

• Electrical-system efficiency

• Permitted state-of-charge window

• End-of-life capacity allowance

• Temperature allowance

• Operational reserve

For the example above, assume:

• 90% system efficiency

• 80% planned state-of-charge window

• 85% remaining capacity at the selected end-of-life criterion

The preliminary nominal-energy requirement becomes:

2.82kWh ÷ 0.90 ÷ 0.80 ÷ 0.85 = approximately 4.61kWh

This is already much higher than the original 2.82kWh load-energy figure.

A separate temperature or route-variation allowance may still be required. Avoid multiplying arbitrary “safety factors” without defining what each factor covers; otherwise, the calculation can hide double-counting or leave an important condition unaddressed.

What the State-of-Charge Reserve Is For

Reserve energy is not unused capacity without a purpose. It can allow the AGV to:

• Finish its current movement safely

• Reach a charger after a route delay

• Avoid routine operation close to BMS cutoff

• Tolerate normal capacity variation

• Continue operating when a charging station is unavailable

• Handle a temporary increase in route length

• Support battery aging without immediate replacement

The correct reserve depends on fleet architecture. An AGV operating beside multiple available chargers may need a different reserve strategy from a vehicle that must cross a large warehouse to reach a single charging station.

Step 4: Check Peak Current Separately

A battery large enough in watt-hours may still be unable to deliver the required current.

Current can be estimated from:

Current (A) = Electrical power (W) ÷ Battery voltage (V)

For peak-load calculations, use the lowest expected battery voltage under the relevant operating condition—not nominal voltage alone. Controller efficiency and DC-DC losses must also be considered.

Suppose a hypothetical AGV requires 6.5kW for several seconds while accelerating with its maximum payload. If the battery can fall to 44V under that condition and drivetrain efficiency is estimated at 92%:

Peak battery current = 6,500W ÷ 44V ÷ 0.92

Peak battery current ≈ 161A

The designer must now verify:

• Cell peak-discharge capability

• Duration allowed at 161A

• BMS overcurrent threshold

• BMS overcurrent delay

• Fuse time-current behavior

• Contactor making and carrying capacity

• Connector current and temperature rise

• Cable voltage drop

• Busbar and PCB current path

• Battery internal resistance

• Controller undervoltage threshold

Do not substitute a continuous-current rating for a short-term peak rating—or the reverse.

A BMS may permit a high current for only a few seconds. Another design may allow the same current for longer but at a higher cell temperature. The specification needs a current-versus-time profile, not just one maximum number.

Voltage Sag Can End the Shift Before Capacity Does

The AGV may show remaining state of charge and still shut down during acceleration.

Every cell, interconnect, busbar, cable, contactor, fuse, and connector contributes resistance. When current rises, the voltage drop across that resistance also rises.

The simplified relationship is:

Voltage drop = Current × Resistance

At low state of charge or low temperature, the battery’s effective resistance may be higher. A current peak then pulls the pack voltage below either the BMS undervoltage threshold or the motor controller’s minimum input voltage.

When the load disappears, the voltage recovers. Technicians may then find that the battery appears normal at rest.

This is why end-of-shift testing must include the heaviest credible load. A no-load voltage reading cannot prove that the battery can finish the route.

Step 5: Decide Whether the Voltage Platform Is Still Appropriate

For equal power, a higher-voltage platform carries less current in an ideal comparison.

A 4.8kW load requires approximately:

• 200A at 24V

• 100A at 48V

Lower current may reduce cable heating, voltage drop, connector burden, and the current rating required from some power-path components. However, changing from 24V to 48V is a system redesign rather than a simple battery upgrade.

The following equipment must match the battery’s complete voltage range:

• Motor controller

• Motors

• Charger

• DC-DC converter

• Contactors

• Fuses

• Connectors

• Auxiliary electronics

• Pre-charge circuit

• Service and diagnostic equipment

The published voltage label is not enough. A “48V” and a “51.2V” battery can have different chemistries, charge voltages, and cutoff limits.

Step 6: Make Charging Part of the Capacity Calculation

Battery sizing and charging strategy should be developed together.

There are three common approaches.

Scheduled Full Charging

The AGV operates for a defined period and then remains at a charger for a longer session.

This approach may be simple to manage, but it normally requires enough stored energy to cover the interval between charges. Fleet availability can also become sensitive to a missed or incomplete charging session.

Opportunity Charging

The AGV charges briefly while waiting at transfer stations, during breaks, or between tasks.

Opportunity charging may reduce the onboard energy required for a given working day. However, the benefit depends on whether the vehicle actually reaches the charger at the planned time and remains connected long enough.

Short charging events also increase the importance of:

• Reliable docking alignment

• Charger-to-BMS communication

• Charge acceptance at the current temperature

• Contact condition

• Fleet scheduling

• Charge-current limits

• SOC estimation

• Charger availability

Battery Swapping

A discharged battery is removed and replaced with a charged unit.

Swapping can reduce vehicle downtime, but it introduces battery inventory, mechanical handling, connectors, locking systems, identification, charging-room management, and operator procedures.

Figure 2. A scheduled charging model concentrates charging into longer parked periods, while opportunity charging adds smaller amounts of energy between operating tasks.

Do Not Calculate Charging Time by Division Alone

A 108Ah battery charged at 20A has a theoretical capacity-to-current ratio of 5.4 hours. That does not mean a complete charge will always finish in exactly 5.4 hours.

Real charging time also depends on:

• Initial state of charge

• Constant-current and constant-voltage phases

• BMS current limits

• Cell balancing

• Temperature

• Charger efficiency

• Cable and connector limits

• Charge taper near the upper voltage

• Charger communication

• Auxiliary loads that remain active

The public 51.2V 108Ah Dailymag battery page lists a 20A standard charge and RS485/CAN communication. These published values are useful starting points, but the final charging time and charger protocol still need project-specific confirmation.

Step 7: Treat Regenerative Braking Carefully

Regenerative braking can return some energy to the battery, but it should not be counted twice or assumed to be available in every situation.

Regeneration depends on:

• Motor-controller design

• Vehicle mass

• Speed

• Route gradient

• Braking frequency

• Battery state of charge

• Cell temperature

• BMS charge-current limit

• Maximum battery voltage

• Charge acceptance

• Friction-brake blending

A nearly full or cold battery may not be able to accept the expected regenerative current. The controller needs an alternative way to control bus voltage and stop the machine safely.

Use measured net energy from a representative route when possible. If regeneration is estimated during early design, document the assumption and verify it during complete-machine testing.

Step 8: Define What the BMS Must Tell the AGV

An AGV lithium battery is part of the machine’s control system, even when the BMS does not directly control vehicle movement.

Useful BMS information may include:

• Pack voltage

• Current

• State of charge

• Temperature

• Charge permission

• Discharge permission

• Recommended charge-current limit

• Recommended discharge-current limit

• Alarm and protection status

• Contactor status

• Remaining energy estimate

• Cell-voltage extremes

• Hardware and firmware versions

The physical presence of CAN or RS485 does not confirm protocol compatibility.

The OEM must still define message identifiers or registers, scaling, byte order, update rate, timeouts, alarms, startup behavior, and the safe response to lost communication.

An AGV should not continue using a stale current limit indefinitely because one CAN message stopped arriving.

Step 9: Include Temperature, Floor Conditions, and Payload Variation

Battery performance measured in a comfortable laboratory does not automatically represent a working warehouse.

Temperature affects charge acceptance, resistance, available energy, power capability, and aging. The US Department of Energy has noted that battery temperature influences performance, reliability, safety, and life-cycle cost.

The AGV’s environment may add other variations:

• Cold rooms or loading docks

• Hot charging areas

• Uneven floors

• High rolling resistance

• Ramps

• Dust or moisture

• Repeated mechanical shock

• Vibration

• Increased payload

• Wheel wear

• Obstructed cooling paths

A battery enclosure also needs suitable mounting, sealing, service access, connector protection, and thermal design.

AGVs and AMRs fall within broader machine and vehicle safety frameworks. ISO 3691-4:2023, for example, addresses safety requirements and verification for driverless industrial trucks and their systems. The standard’s published scope does not provide the project-specific battery sizing calculation, so the power system still requires its own engineering assessment and validation.

How Published Battery Data Can Be Used

Dailymag Energy publicly positions its battery solutions for applications including industrial robots and other professional equipment.

Its 51.2V 108Ah product page publishes:

• 51.2V nominal voltage

• 108Ah rated capacity

• 20A standard charge

• 50A standard discharge

• 150A maximum continuous discharge

• RS485/CAN communication

Multiplying the published voltage and capacity gives approximately 5.53kWh of nominal energy.

This does not mean the battery will suit every 5kWh AGV application. The engineering team must still confirm chemistry, operating-voltage range, peak-current capability, temperature limits, IP requirement, cycle-life target, connector design, protocol, charge time, and mechanical compatibility.

Published products are reference configurations. Final selection should be based on the complete machine.

A Practical AGV Battery Validation Plan

1. Bench-Level Electrical Test

Confirm pack voltage, usable capacity, BMS thresholds, charger behavior, communication, contactor operation, and protection recovery.

2. Static Machine Test

Install the battery in the AGV and test startup, auxiliary loads, controller pre-charge, emergency stop, charging connection, sleep, and wake behavior.

3. Peak-Load Test

Run maximum-payload acceleration, turning, lifting, slope, and motor-stall scenarios where applicable. Record pack voltage, cell-voltage extremes, current, and temperature.

4. Representative Route Test

Operate the AGV through a complete real or replicated route with normal traffic, load handling, waiting, and charger travel.

5. Low-SOC Test

Repeat critical load events near the minimum planned state of charge. This is often where voltage-sag problems become visible.

6. Temperature Test

Verify discharge, charging, shutdown, and recovery behavior at the project’s required temperature limits.

7. Fleet Charging Test

Test several vehicles, charging-station availability, interrupted charging, communication loss, docking errors, and schedule variation.

8. Aging Review

Confirm that the AGV still meets runtime and peak-power requirements at the defined end-of-life capacity and resistance.

Information to Include in an AGV Battery RFQ

Provide the battery supplier with:

1. Nominal system voltage and permitted voltage window.

2. Measured or estimated time-based load profile.

3. Continuous current.

4. Peak current, duration, and repetition rate.

5. Required operating time between charges.

6. Planned SOC reserve.

7. Charging method and available charging time.

8. Charger voltage, current, and communication.

9. Regenerative current and duration.

10. Maximum payload and slope.

11. Operating and charging temperatures.

12. Installation dimensions and mass limit.

13. Connector, cable, and pinout.

14. CAN or RS485 protocol requirements.

15. Shock, vibration, sealing, and environmental conditions.

16. Target service life and end-of-life criterion.

17. Applicable market and certification requirements.

18. Prototype and complete-machine validation plan.

Statements such as “48V, eight hours” or “100Ah required” are not enough for a reliable proposal.

Frequently Asked Questions

How many ampere-hours does an AGV need?

Calculate the required usable watt-hours first, then convert the result to ampere-hours at the selected nominal voltage. Add defined allowances for SOC reserve, efficiency, temperature, aging, and operational variation.

Is a larger battery always safer?

No. More energy may improve runtime but also increases mass, size, charging time, cost, fault energy, and mechanical load. Peak-current capability and system compatibility still require separate verification.

Can average current be used to choose the BMS?

Not by itself. The BMS must support the continuous load and every credible peak-current event for the required duration without nuisance protection or unsafe heating.

Can opportunity charging reduce battery size?

Yes, if charging opportunities are frequent, long enough, and operationally reliable. Model missed charging sessions and charger congestion before reducing onboard reserve.

Is CAN required for an AMR battery pack?

Not universally. CAN is common in mobile machinery, while some systems use RS485 or other interfaces. Protocol compatibility and fault behavior matter more than the connector label.

How much reserve capacity should an AGV have?

There is no universal percentage. Reserve should reflect charger distance, route variability, battery aging, temperature, fleet redundancy, and the consequences of stopping before reaching the charger.

Can regenerative braking be included in the runtime calculation?

Yes, but preferably from measured route data. The battery may restrict regenerative charging when it is cold, nearly full, or close to a charge-current limit.

When should the battery be replaced?

Replacement criteria may include remaining capacity, increased resistance, inability to support peak load, abnormal cell imbalance, repeated faults, physical damage, or failure to complete the required duty cycle.

Should the final test use a new battery?

Use a new battery for initial integration, but also verify the design against modeled or tested end-of-life conditions. A machine that works only with a new pack has insufficient lifecycle margin.

Final Recommendation

The best AGV battery capacity calculation is not a single equation. It is a controlled comparison between the machine’s time-based load profile, the battery’s usable energy, the peak-current path, and the actual charging schedule.

Begin with the route. Measure what the AGV does during acceleration, transport, lifting, waiting, braking, and charging. Convert those events into energy and current requirements. Define every margin rather than adding one unexplained safety factor.

Then validate the battery in the complete machine at low state of charge, maximum payload, unfavorable temperature, and realistic fleet conditions.

Dailymag Energy provides customized lithium battery matching for industrial equipment. To evaluate an AMR battery pack or AGV project, prepare the load profile, voltage window, charging plan, communication requirements, installation drawing, and operating environment.

Contact the Dailymag Energy engineering and sales team for a project-specific battery review.