This procedure uses thermal imaging to compare ONYX battery surface temperatures before and after real load. It supports investigation of wiring, BMS, and connection losses; a surface image cannot prove an internal cell, weld, or magnet temperature.

Keep the battery enclosure closed. Do not remove insulation, expose conductors, or recreate a high-current test on an unsupported battery. Compare equivalent surfaces and loading conditions; reflective metal can distort apparent temperatures.

Thermal Imaging and Hotspot Detection

Thermal imaging is one of the most effective ways to evaluate a battery under real load conditions.

A thermal imaging camera (e.g. FLIR) allows you to visualize how heat is distributed across the pack, revealing resistance, imbalance, and inefficiencies that are not visible through voltage or current measurements alone.

However, thermal imaging is only useful when applied correctly.

When to Use Thermal Imaging

Thermal behavior changes over time. A single snapshot is not enough.

To get meaningful data, observe the pack at multiple stages:

  • Cold state (before use)

    • establishes baseline temperature
    • reveals ambient conditions

  • Early load (first few minutes of riding or testing)

    • shows initial current distribution
    • identifies immediate imbalance

  • Sustained load (extended riding, hill climbs, or racing conditions)

    • reveals true system limitations
    • exposes weak interconnects, BMS heating, and wiring losses

  • Post-load (immediately after stopping)

    • highlights retained heat
    • makes hotspots easier to identify

Thermal issues often do not appear instantly. They develop as the system approaches thermal saturation.

Preparing for Inspection

For accurate inspection, the battery should be removed from the bike after testing.

  • this allows full access to all surfaces
  • eliminates heat influence from surrounding components
  • makes hotspot patterns easier to isolate

Inspection should be done immediately after load while the pack is still warm.

Delays reduce temperature contrast and can hide problem areas.

How to Scan the Pack

For accurate results:

  • scan under real load conditions (not idle testing alone)
  • observe temperature changes over time, not just peak values
  • compare similar regions (parallel groups, busbars, connections)
  • look for asymmetry, not just absolute temperature

Key areas to monitor:

  • busbar paths
  • weld points
  • parallel group transitions
  • BMS region
  • wiring exits and connectors

Heat should be distributed relatively evenly across equivalent sections.

Interpreting Hotspots

Heat is a direct indicator of resistance.

Localized hotspots typically indicate:

  • increased resistance
  • poor connections or welds
  • uneven current distribution
  • undersized conductors or bottlenecks

A well-designed pack:

  • heats gradually and evenly
  • stabilizes over time
  • shows minimal localized hotspots

A poorly designed pack:

  • develops concentrated hot areas
  • shows uneven temperature distribution
  • continues increasing in temperature under sustained load

Temperature difference between similar areas is more important than absolute temperature.

Metal Enclosure Considerations

A metal enclosure changes how heat appears in thermal imaging.

  • the enclosure spreads heat across its surface
  • hotspots may appear diffused rather than sharply defined
  • external temperature may lag behind internal heating

However:

  • heat still originates from internal resistance points
  • localized warming of the enclosure often corresponds to internal hotspots
  • repeated patterns or asymmetry still indicate underlying issues

Even with a sealed metal case, thermal imaging can reveal:

  • where heat is concentrating
  • whether one side of the pack is working harder than the other
  • whether heat is building unevenly over time

Surface temperature is useful, but it does not always represent the hottest internal point.

System-Level Insight

Thermal imaging evaluates the entire system:

  • cells
  • interconnects
  • BMS
  • wiring
  • connectors

It shows how current actually flows through the pack under load.

Voltage and current measurements tell you what is happening.

Thermal imaging shows you where it is happening.

Practical Use

Thermal imaging is most useful for:

  • validating new builds
  • comparing different pack designs
  • diagnosing performance issues
  • identifying early failure points

It provides a direct view of where energy is being lost and where the system is constrained.

Heat is not just a byproduct.

It is a map of resistance across the system.

All of these constraints converge inside the enclosure.

For construction context, see ONYX Battery Design.