Why Milwaukee M18 Packs Get Hot on Certain Tools

Safety First

Allow packs to cool before handling if surface temperature exceeds ~50 °C (122 °F).
Avoid touching terminals while the pack is under load.
Place hot packs on a non-combustible, ventilated surface.
Do not open packs — risk of short circuits, fire, or chemical exposure.

Internal Resistance: High-current draws produce I²R losses inside cells; older or high-cycle packs have higher DCIR.
Tool Electronics: Motor type, PWM motor controllers, and rapid torque pulses generate localized heat.
Ambient Temperature: High workspace temperature exacerbates pack heating; add 5–10 °C to surface readings.
BMS Intervention: Thermal throttling or over-current protection can cause hotspots near MOSFETs or contact points.

High-torque intermittent tools (impact drivers, hammer drills) → short bursts of 50–100 A peaks → rapid temperature rise within 30–60 s.
Continuous high-current tools (circular saws, grinders) → sustained 20–40 A draw → cumulative heating over several minutes.
Voltage sag under load correlates with internal heat generation: a 0.5–1.0 V drop per cell often corresponds to significant thermal buildup.

Cell Type & Chemistry: Older 18650 cells vs. newer 21700; higher-energy-density cells typically run warmer.
Series/Parallel Layout: Uneven current distribution can produce hotspots.
Thermal Path & Housing: Busbars, insulation, and venting determine how heat dissipates.
BMS Behavior: Allows brief over-current pulses before triggering thermal cutoff; repeated spikes increase surface temperature.

Surface Temperature Measurement: Use an IR thermometer or thermocouple during use; mark >50 °C as high.
Voltage Monitoring Under Load: Excessive sag indicates high internal resistance or heavy current draw.
Cross-Tool Comparison: Test the same battery on multiple tools to isolate tool vs. pack contribution.
Runtime & Temperature Monitoring: Early heat spikes relative to runtime indicate potential stress on the pack.

Rotate packs to allow cooling between heavy-use cycles.
Use higher-capacity packs on high-current or continuous-duty tools.
Ensure proper workspace ventilation; avoid confined or hot enclosures.
Replace older packs with low internal resistance, low-cycle packs when performance drops.
Avoid operation beyond the manufacturer-specified duty cycles.

Heat generation is normal but tool-dependent.
Main factors: internal resistance, tool load profile, ambient conditions, and BMS intervention.
Safe measurement and load tests identify high-risk packs.
Regular rotation, cooling, and timely replacement extend pack life and ensure safety.