A battery cable can pass continuity inspection and still lose too much voltage during a high-current event. Buyers need a defined circuit, current profile and measurement method before two suppliers' voltage-drop claims can be compared. “Heavy-duty cable” and a low-current resistance reading taken without the operating load do not provide that evidence.
This guide covers purchasing records for low-voltage battery cable assemblies used in starting and other high-current circuits. The equipment designer must set the allowed voltage at the load and all safety requirements. Use the AWG versus mm² battery cable sizing guide for conductor-size identity; here the central question is loss across the complete path during the agreed load event.
Define the circuit before calculating loss
Draw the positive and return paths from the battery posts to the load terminals. Include cable assemblies, lugs, battery posts, disconnects, fuses, shunts, busbars and intermediate joints. Name every boundary included in the voltage-drop budget and every element excluded from it.
Use total conductor length, not the one-way routing distance, when both positive and negative conductors form the measured path. Blue Sea Systems' official DC wire-selection guidance defines circuit length as the route from source to load and back. Its percentages and examples are marine application guidance; the transferable RFQ lesson is to state the complete path and application basis. Blue Sea Systems DC wire-size guidance.
Record whether the return uses a dedicated cable, a chassis or a mixed path. A chassis return still has joints and test boundaries; do not replace its resistance with zero. If optional routing creates several possible lengths, ask suppliers to quote the controlled variants or state the maximum built length used in their calculation.
Cable length alone is not the complete resistance. The Victron Wiring Unlimited material distinguishes conductor loss from contact losses and explains that voltage drop becomes larger during high-current events. It also shows positive- and negative-side measurements as separate checks. Apply those principles within the relevant equipment manual and design authority, rather than treating Victron's product guidance as a universal acceptance limit. Victron Wiring Unlimited, theory and voltage-drop sections.
Specify the current profile and load condition
Cranking current changes during engagement and rotation. An inverter, pump or winch can also have an inrush followed by a different running current. A single ampere value is therefore ambiguous unless the RFQ says what it represents.
Define the event, starting state, source condition, load configuration, current measurement location and review window. State whether the buyer needs maximum observed drop, a value over a defined interval, a stable-load value or several checkpoints. Name the data-recording rate or instrument response needed to capture the event, but let the responsible test authority approve the final method.
Fluke's starter-circuit guidance places the meter across the portion being evaluated and captures voltage loss while the engine is cranked. Its probe locations and numeric guidance apply to that automotive troubleshooting context. An OEM RFQ should use the equipment manufacturer's limits and agreed method, while retaining the useful principle that current must flow during the measurement. Fluke starter-circuit voltage-drop guide.
Ask suppliers to state battery terminal voltage during the event as well as the drop across the cable path. Load-terminal voltage equals the source-terminal voltage at that instant minus losses between the chosen boundaries. A cable-drop result cannot by itself separate a weak source from excessive path resistance unless both sets of measurements are available.
Calculate conductor and connection losses separately
The basic relationship is V = I × R. Keep the units visible: amperes multiplied by ohms gives volts. If resistance is stated in milliohms, divide by 1,000 before multiplying. Blue Sea Systems also publishes this relationship in its circuit-protection reference and separates voltage-drop sizing from allowable ampacity. Blue Sea Systems circuit-protection reference.
For the conductor term, request resistance per unit length, the reference temperature, conductor material and construction, and the maximum built length. Then add the agreed resistance or measured drop for terminations and series components. Do not silently count a component twice when a supplier's end-to-end test already includes it.
Temperature matters because conductor resistance changes with temperature, while contacts and components may heat differently during repeated events. Define whether the calculation uses a specified reference, a worst-case design temperature or another documented condition. If a correction is applied, request its source and arithmetic.
Keep voltage-drop acceptance separate from ampacity, short-circuit protection and insulation temperature. A large conductor may meet a drop target but still require a reviewed fuse and installation. A low calculated drop does not approve a lug crimp; use the battery cable crimp-tooling and evidence guide for that process.
Work through an invented calculation
This is a fictional, recalculable procurement example, not a customer case, recommended cable size, product rating or test result. Drawing V-50 defines 2.4 m of positive cable and 2.1 m of negative cable. The fictional design inputs assign 0.40 mΩ/m to the conductor at the stated reference condition, 0.24 mΩ total to four terminal interfaces, and 0.60 mΩ to the named switch-and-protection path.
The conductor term is 4.5 m × 0.40 mΩ/m = 1.80 mΩ. The total path is therefore 1.80 + 0.24 + 0.60 = 2.64 mΩ, or 0.00264 Ω. At a fictional 220 A review point, the calculated drop is 220 × 0.00264 = 0.5808 V. At a fictional 420 A event point, it is 1.1088 V.
These two results are not competing wire ratings. They demonstrate why the same physical path has a different voltage drop at a different current. The design reviewer must decide which points, source voltage and receiving-equipment requirement govern. The supplier must show where every resistance input came from and whether tolerances or temperature corrections are already included.
Request comparable calculation and test evidence
Give bidders one controlled input sheet and require them to return the assumptions rather than only a pass/fail label.
On small screens, swipe or scroll sideways to read every column. Keyboard users can focus the table and use the arrow keys.
| Evidence item | Required supplier record | Buyer check |
|---|---|---|
| Circuit identity | Drawing revision, positive/return boundaries, built lengths, conductor and every series component | Same path and maximum configuration used across bids |
| Calculation | Current checkpoints, resistance sources, temperatures, tolerances and visible V = I × R arithmetic | No missing joint, duplicate loss or one-way-length error |
| Loaded verification | Sample identity, source/load state, current trace, voltage probe points, instrument record and exceptions | Event and capture method match the approved plan |
For a sample or first-article check, record cable serial or lot, drawing revision, conductor length, terminal part numbers and crimp process reference. Identify the battery or source state, starting temperature, load event and measurement timestamps. Preserve raw data where the project requires it; a screenshot of one displayed value may hide the current at which it occurred.
Ask the qualified test team to report voltage drop across defined segments when troubleshooting or allocating the budget: positive assembly, return assembly, disconnect, fuse path and selected joints. The segment drops should be reconciled with the end-to-end drop within the agreed measurement method. If they do not reconcile, investigate timing, probe locations, event repeatability and data alignment before changing the design.
Do not use a loaded voltage-drop check as an improvised acceptance test without a safe procedure and competent personnel. High-current circuits can create heat, arcing and moving-equipment hazards. The RFQ should request an approved method and evidence package, not direct an unqualified operator to reproduce a test.
Compare quotations without hiding tolerances
Ask each bidder whether conductor resistance is a nominal or maximum value, whether length tolerance is included, and whether terminal and component losses are guaranteed, characterized or merely estimated. A lower nominal calculation can be less useful than a slightly higher result with controlled worst-case inputs.
Quote the same deliverables: calculations, first-article records, raw traces if required, exception handling and change notification. Link the electrical result to the battery cable first-article and change-control guide. A conductor, terminal, crimp tool, routing length or switch substitution can invalidate the reviewed loss model even when the sales part number stays unchanged.
Send a battery-cable voltage-drop RFQ
Send the circuit drawing, maximum lengths, current profile, source/load conditions, allowed loss, component list and evidence plan. Ask suppliers to identify assumptions and exclusions before pricing.
Buyer FAQ
Can a continuity test prove acceptable voltage drop?
No. Continuity can find an open circuit but does not reproduce the defined high-current event. Request a controlled calculation and, where required, loaded measurement across the specified boundaries.
Should cable length mean one-way distance or round trip?
State the convention explicitly. For a two-conductor circuit, include the positive and return conductor lengths in the resistance calculation. Identify any chassis or shared return separately.
Can we use a universal cranking voltage-drop limit?
Use the equipment manufacturer, system designer and applicable installation requirements for the actual circuit. Published troubleshooting guidance may inform the method but does not automatically become an OEM acceptance limit.
What changes require the voltage-drop evidence to be reviewed again?
Review changes to conductor material or size, maximum length, terminals, crimp process, joints, fuses, switches, routing temperature, source/load profile or test method. Define the approval authority in the change-control plan.