How to Size Battery Pack Wires for an OEM Device: AWG/mm², Peak Current and Voltage Drop

Battery pack wires connect the stored energy in the pack to the host device. Their size affects voltage drop, heat generation, peak-load performance, mechanical flexibility and overall system reliability.

For OEM products, wire selection should be completed from the device’s real current profile rather than from battery capacity alone. A 3,000mAh battery may be used in a low-current sensor, a handheld instrument or a motor-driven device. These applications can require very different wire sizes.

The same wire must also work with:

• the battery voltage;

• continuous current;

• peak or inrush current;

• cable length;

• allowable voltage drop;

• operating temperature;

• insulation;

• connector and crimp;

• enclosure routing;

• vibration and repeated movement.

This article explains how to select battery pack wire size using AWG and mm², how to estimate voltage drop, and how to validate the final cable assembly for an OEM device.

Quick Answer: How Do You Choose Battery Pack Wire Size?

Choose battery pack wire size by evaluating the highest relevant current, total positive-and-negative cable length, allowable voltage drop, temperature rise, installation conditions and mechanical requirements.

The basic process is:

1. Define the battery voltage and device load.

2. Record continuous, peak and inrush current.

3. Measure the complete one-way cable route.

4. Convert it into total loop length for positive and negative conductors.

5. Set the maximum acceptable voltage drop.

6. Calculate the approximate conductor cross-sectional area.

7. Select an AWG or mm² size that meets both electrical and mechanical requirements.

8. Check the connector, crimp, fuse, BMS and cable temperature.

9. Validate the assembly inside the finished device.

The wire should not be selected only by:

• battery capacity in mAh;

• battery voltage;

• a generic online ampacity chart;

• maximum discharge current;

• connector brand;

• cable appearance.

For an OEM battery pack, the wire, connector, protection system and enclosure should be treated as one electrical and mechanical assembly.

Main Body

1. Define the Battery Pack and Device Current Profile

The first step is to understand what the cable must carry.

Document:

Continuous current and peak current affect wire selection in different ways.

Continuous current mainly affects:

• long-term heating;

• conductor temperature;

• insulation temperature;

• bundled-cable performance;

• sustained voltage drop.

Peak current mainly affects:

• instantaneous voltage drop;

• device reset or shutdown;

• motor and actuator performance;

• BMS overcurrent protection;

• connector and crimp stress.

A short current pulse may not create the same thermal condition as continuous current, but it can still cause a system failure if the voltage at the device falls too far.

2. Do Not Select Wire Size by Battery Capacity Alone

Battery capacity indicates how much charge the battery can store. It does not directly determine the required wire size.

For example, two battery packs may both have a capacity of 3,000mAh:

• one may supply a small IoT sensor at 0.2A;

• another may supply a motor at 8A continuous and 15A peak.

The second application requires much greater attention to conductor size, voltage drop, connector resistance and thermal behavior.

Other factors may also change the cable requirement:

• a low-voltage battery system;

• a long cable route;

• a high-power converter;

• a motor startup event;

• a narrow enclosure;

• a high ambient temperature;

• repeated cable movement;

• charging and discharging through the same leads.

Capacity can help describe the battery, but it is not a substitute for the load profile.

3. Understand AWG and mm²

Battery wires are commonly specified using either:

• AWG, or American Wire Gauge;

• conductor cross-sectional area in mm².

The two systems describe conductor size in different ways. In the AWG system, a smaller AWG number generally indicates a larger conductor. In the metric system, a larger mm² value indicates a larger cross-sectional area.

The following values are approximate copper-conductor references at around 20°C:

These figures are planning references, not final approval values. Actual performance also depends on:

• conductor material;

• stranded or solid construction;

• insulation;

• conductor temperature;

• cable length;

• manufacturing tolerance;

• crimp and terminal resistance;

• bundling and installation conditions.

The cable manufacturer’s specification should be used for final selection.

4. Calculate the Total Loop Length

Voltage drop occurs through the complete current path.

For a typical two-wire battery connection, current travels:

1. from the positive battery terminal to the device;

2. through the device load;

3. back through the negative wire to the battery.

Therefore, the electrical length is usually the total positive and negative route, not only the physical distance between the battery and device.

Total loop length = Positive-wire length + Negative-wire length

If both wires follow the same route:

Total loop length ≈ 2 × One-way cable length

For example, if the one-way route is 0.8m:

Total loop length ≈ 2 × 0.8m = 1.6m

The measurement should include the actual installed route, including:

• bends;

• cable exits;

• movement around enclosure ribs;

• service loops;

• connector leads;

• internal routing.

Using only the straight-line distance can underestimate the actual resistance.

5. Use Voltage Drop as a Primary Design Constraint

The basic voltage-drop relationship is:

Voltage drop = Current × Resistance

The resistance of a conductor can be estimated using:

R = ρ × L ÷ A

Where:

• R is conductor resistance in ohms;

• ρ is the conductor resistivity;

• L is the total loop length in meters;

• A is the conductor cross-sectional area in mm².

For copper at approximately 20°C, a planning value for resistivity is:

ρ ≈ 0.0175 Ω·mm²/m

Combining the formulas gives:

Vdrop ≈ ρ × L × I ÷ A

The minimum theoretical conductor area can therefore be estimated as:

A ≥ ρ × L × I ÷ Vdrop,max

This formula provides a starting point. It does not replace thermal testing or manufacturer data.

6. Estimate Power Loss and Heat

The cable also produces heat when current passes through its resistance.

Power loss = Current² × Resistance

This relationship is important because heat increases with the square of current.

For example, if the total cable resistance is 0.02Ω:

• at 3A, power loss is approximately 0.18W;

• at 6A, power loss is approximately 0.72W;

• at 10A, power loss is approximately 2.0W.

The same cable can therefore perform acceptably at a low current but become a significant heat source at a higher current.

Power loss can occur in:

• the wire;

• crimped terminals;

• connector contacts;

• fuse holders;

• battery tabs;

• BMS terminals;

• PCB traces.

The wire should not be checked in isolation. A large conductor connected through a high-resistance crimp can still create localized heating and voltage loss.

7. Worked Example: Selecting a Preliminary Wire Size

Consider an illustrative OEM device with the following requirements:

• battery pack: 7.4V nominal;

• continuous current: 6A;

• peak current: 12A for 1 second;

• one-way cable route: 0.8m;

• total loop length: 1.6m;

• maximum continuous voltage drop target: 0.15V.

Using the continuous current:

A ≥ 0.0175 × 1.6 × 6 ÷ 0.15
A ≥ 1.12 mm²

A conductor around 1.31mm², commonly associated with 16 AWG, may be a preliminary candidate for the continuous-current requirement.

Its approximate continuous voltage drop would be:

Vdrop ≈ 0.0175 × 1.6 × 6 ÷ 1.31
Vdrop ≈ 0.13V

The peak event must then be checked separately:

Vdrop,peak ≈ 0.0175 × 1.6 × 12 ÷ 1.31
Vdrop,peak ≈ 0.26V

The design team should determine whether a 0.26V transient loss is acceptable for the host device. If it is not, a larger conductor may be required.

This is an illustrative calculation. Final selection must also consider:

• connector and crimp resistance;

• BMS resistance;

• fuse resistance;

• temperature;

• cable flexibility;

• installation space;

• peak-current duration;

• device undervoltage limit.

8. Consider Low-Voltage Systems Carefully

Low-voltage battery systems are more sensitive to voltage drop because the same device power requires more current.

A simplified relationship is:

Current ≈ Power ÷ Voltage

For the same 24W load:

• at 12V, the current is approximately 2A before conversion losses;

• at 6V, the current is approximately 4A;

• at 3.7V, the current is approximately 6.5A.

Higher current increases:

• cable voltage drop;

• connector loss;

• BMS stress;

• fuse requirements;

• thermal load;

• battery-side current demand.

This is why a cable that seems adequate for a higher-voltage pack may not be suitable for a lower-voltage architecture carrying the same power.

For low-voltage products, the allowable voltage drop should be expressed as both:

• an absolute voltage value;

• a percentage of the operating voltage.

A fixed 0.2V loss has a much greater effect on a 3.7V system than on a 14.8V system.

9. Size the Wire for Continuous and Peak Current

Wire selection should consider both thermal and electrical effects.

For continuous current, check:

• conductor temperature;

• insulation rating;

• ambient temperature;

• bundled-cable derating;

• enclosure heat accumulation;

• sustained voltage drop.

For peak current, check:

• peak voltage drop;

• pulse duration;

• repetition rate;

• device undervoltage threshold;

• BMS overcurrent threshold;

• connector contact resistance;

• battery internal resistance.

A short peak may not require the same thermal rating as a continuous current, but it can still require a larger wire if the device is sensitive to voltage sag.

The peak current profile should therefore include:

Peak current + peak duration + repetition frequency + device response

Do not use the battery’s maximum discharge current as the automatic wire-size value. The battery rating, BMS rating, connector rating and wire requirement are related but separate design values.

10. Account for Temperature and Installation Conditions

Wire resistance increases as conductor temperature rises. The cable’s ability to dissipate heat also changes according to its installation.

Review:

• ambient temperature;

• charging temperature;

• enclosure temperature;

• cable bundle size;

• contact with heat-producing components;

• insulation temperature rating;

• airflow or heat conduction;

• exposure to vibration;

• repeated flexing;

• moisture and contamination.

A cable installed in open air may dissipate heat differently from the same cable installed inside a tight enclosure.

Flexible products may also need fine-stranded wire, a suitable insulation material and a controlled bend radius. A larger conductor that cannot be routed without mechanical stress may create a different reliability problem.

The wire specification should include both electrical and mechanical requirements.

11. Check the Connector, Crimp and Fuse

The cable is only one part of the current path.

The complete path may include:

• cell tabs;

• BMS terminals;

• fuse;

• wire;

• crimp terminal;

• connector contact;

• host-side connector;

• PCB terminal;

• device power trace.

Each interface introduces resistance.

When specifying the battery wire, confirm:

• connector current rating under the actual temperature;

• contact resistance;

• crimp quality;

• terminal size;

• wire-stripping length;

• strain relief;

• fuse rating;

• fuse voltage drop;

• connector retention;

• polarity and keying.

The connector should be sized for the actual continuous and peak current, not only the average current. The related guide on choosing a connector for a custom lithium battery pack can support the interface-selection stage.

12. Separate Charging and Discharge Requirements

Some battery packs use the same leads for charging and discharging. Others use separate paths.

If one wire set carries both directions of current, size it for the higher relevant requirement.

For separate paths, document:

• discharge current;

• charge current;

• balance current;

• BMS path;

• connector pinout;

• fuse location;

• temperature-sensor wiring;

• charger-side interface.

Charging current may be much lower than discharge current, but the charging wires still need adequate insulation, connector retention and protection.

The wire size should be considered together with the charger and BMS rather than selected independently after the battery pack is finished.

13. Prepare an OEM Battery Wire Specification

A clear wire specification helps the battery manufacturer prepare an accurate sample.

Provide:

If the required wire size is not yet known, provide the current profile and mechanical envelope so the manufacturer can recommend a suitable design.

14. Validate the Finished Battery Cable Assembly

The preliminary calculation must be followed by testing in the final device.

Electrical validation

Measure:

• battery-side voltage;

• device-side voltage;

• voltage drop at continuous current;

• voltage drop during peak current;

• current at startup;

• current during normal operation;

• charging-path voltage drop;

• connector and terminal resistance.

The measurement should be taken at the relevant electrical points. Measuring only at the battery terminals may hide losses in the cable and connector.

Thermal validation

Check temperatures at:

• wire insulation;

• crimp terminals;

• connector contacts;

• fuse;

• BMS terminals;

• battery tabs;

• nearby enclosure surfaces.

Test under:

• continuous maximum load;

• repeated peak load;

• charging;

• simultaneous charging and operation;

• high ambient temperature where required.

Mechanical validation

Inspect:

• cable bend radius;

• strain relief;

• connector retention;

• movement during vibration;

• abrasion points;

• enclosure interference;

• battery restraint;

• repeated insertion and removal where applicable.

The final pack should be validated with its actual wire length, connector, insulation, BMS and enclosure.

A broader battery pack testing checklist can be used to organize electrical, thermal, mechanical and system-level validation.

15. Common Battery Pack Wire Sizing Mistakes

Selecting wire size by mAh

Capacity does not define the current profile or voltage-drop limit.

Using only continuous current

Short peak loads can cause device resets or BMS protection even when continuous heating is acceptable.

Measuring only the one-way distance

The positive and negative conductors form a complete loop. Both lengths contribute to resistance.

Ignoring connector and crimp resistance

A large cable cannot compensate for a poor terminal or high-resistance contact.

Choosing from an ampacity chart without checking voltage drop

A cable may remain thermally acceptable but still cause too much voltage loss for the device.

Using the battery’s maximum discharge current as the wire rating

The cell specification does not automatically define the cable, connector or BMS requirement.

Ignoring low-voltage operation

The same power demand creates higher current at a lower battery voltage.

Selecting a cable that cannot fit the enclosure

Electrical performance is only one part of an OEM cable design. Routing, flexibility and strain relief also affect reliability.

Testing only the loose battery pack

The cable must be tested in the final product under representative load conditions.

FAQ

How do I calculate battery pack wire size?

Start with the total loop length, expected current and allowable voltage drop. A preliminary conductor area can be estimated using A ≥ ρ × L × I ÷ Vdrop. Then verify temperature, connector, crimp, insulation and installation conditions.

Is AWG or mm² better for battery pack wires?

Both can be used. AWG is common in many cable specifications, while mm² directly expresses conductor cross-sectional area. The important point is to confirm the actual conductor area and manufacturer rating.

Should battery wire size be based on continuous or peak current?

Both should be checked. Continuous current affects heating and sustained voltage drop. Peak current affects instantaneous voltage sag, device operation and protection response.

Does a longer battery cable need a larger wire?

Usually, yes. A longer cable has greater resistance and therefore greater voltage drop at the same current. The complete positive-and-negative route should be included in the calculation.

Can I use a connector’s current rating to choose the wire size?

No. The wire, connector, crimp, fuse, BMS and battery terminals should be evaluated as one current path. A high-rated connector does not compensate for an undersized wire.

How much voltage drop is acceptable?

The allowable value depends on the device’s minimum input voltage, battery voltage, converter requirements and peak-load behavior. It should be defined in the OEM electrical specification rather than chosen as a universal number.

Does a higher-capacity battery need thicker wires?

Not necessarily. Wire size depends mainly on current, cable length, allowable voltage drop, temperature and installation. A higher-capacity pack may operate at either low or high current depending on the device.

Should the charging wires and discharge wires be the same size?

Not always. If they are separate, each path can be sized for its own current and thermal requirements. If the same wires carry both charging and discharging current, size them for the higher applicable requirement.

Conclusion

Choosing the correct battery wire size for a battery pack requires a system-level calculation.

OEM engineers should evaluate continuous current, peak current, total loop length, allowable voltage drop, conductor area, temperature, insulation, connector resistance, crimp quality and mechanical routing together.

AWG and mm² provide useful starting points, but neither number alone proves that the cable is suitable. The final design should be tested under the actual device load, including startup, peak current, charging, temperature and enclosure conditions.

For projects requiring a battery pack with defined wire size, connector, cable length, BMS and mechanical structure, ZERNE’s custom lithium battery pack solutions can be evaluated from the complete OEM specification.