Temperature Sensing Placement in Makita Chargers — Why It Matters

Accurate temperature sensing in Makita-style chargers is critical for safe fast-charging and reliable charger → pack interaction. Where you place thermistors/temperature ICs affects how quickly the charger detects pack or charger hardware heat, how it balances charge current vs thermal risk, and whether protective thresholds trigger correctly. This guide explains placement principles, sensor types, measurement & calibration practices, reproducible tests, common failure modes and immediate engineering actions — practical, non-marketing, and suitable for engineering teams.

Safety first  

- Work on unplugged chargers when doing physical placement work. When powering a charger for tests use an isolation transformer, differential probes, and RCD/GFCI.  
- Thermistors and wiring near mains/SMPS must be insulated and routed to avoid accidental shorts.  
- Never ignore unexpected temperature rises during tests — stop immediately and quarantine suspect units.  
- Do not bypass thermal protections to “force” charge behavior — that creates safety risk.

Why placement matters (practical effects)  

- Accuracy of protection: a sensor far from the heat source underestimates real temperature and delays protective action.  
- Latency: sensor thermal time constant (attachment quality, mass) determines how quickly a transient (inrush, thermal coupling) is seen.  
- Thermal gradients: transformers, MOSFETs, and battery cells heat differently; a single poorly positioned sensor gives misleading aggregate info.  
- Control strategy consequences: misplaced sensors cause inappropriate derate/shutdown or false trips, degrade battery life, and affect charge times.  
- Regulatory & safety: standards often require temperature monitoring at specific points (e.g., near cell thermistor interface) for certification.

Common sensor types & trade-offs

- Negative Temperature Coefficient (NTC) thermistors: high sensitivity, low cost, compact. Good for surface and pack-contact measurements. Non-linear — needs calibration/lookup table.  
- Positive Temperature Coefficient (PTC) thermistors: used sometimes for over-temperature trip as self-limiting devices; less accurate for measurement.  
- Semiconductor temperature ICs (TMP36, LM75, digital I²C sensors): linear-ish output, easy digital readout, good calibration; require thermal coupling and may be larger.  
- Thermocouples (Type K): suitable for very fast or high-temperature probes, but need cold-junction compensation and amplification.  
- RTDs (PT100/1000): high accuracy and stability for lab-grade measurement; typically overkill in consumer chargers but useful in bench/qualification fixtures.

Practical choice: NTCs or digital temperature ICs on charger PCB and at battery contact points balance cost, response and accuracy.

Recommended sensing locations (and why)

1. Battery-contact / cradle thermistor (or thermistor harness detection): primary pack temperature proxy; essential to monitor pack surface where the charger mates — catches pack pre-heating and cell-level heat that the pack BMS may not report. Mount so it contacts the pack housing firmly.  
2. Charger heatsink / MOSFET mounting pad: MOSFETs and switcher components are major heat sources in the charger; sensor here detects internal stress and prevents charger damage. Mount on or near MOSFET tab or heatsink base.  
3. Transformer / inductor surface or winding hotspot: high-core losses or wire heating show here; sensor placed on winding bobbin or core buttresses captures this.  
4. Ambient-inlet / outlet sensors (small 1–2 sensors): measure intake and exit air to monitor cooling efficiency; helpful for derating when ambient is high or airflow is blocked.  
5. Connector/terminal local sensor: measures contact heating due to high resistance at terminals; useful when connectors are known failure points.  
6. Optional pack thermistor verification contact (secondary): many packs expose thermistors — the charger should read the pack thermistor as primary pack temp; an independent cradle sensor is a cross-check.

Rule: monitor both the charger internal hot spots (MOSFET, transformer) and the pack-contact zone. Use at least two sensors for critical designs (pack and charger).

Mounting & thermal coupling best practices

- Surface contact: use a flat mating surface; attach NTC with thermally conductive adhesive or a tight mechanical clamp — avoid loose placement.  
- Avoid air gaps: use thermal adhesive or thermal tape to reduce thermal resistance between sensor and measured surface.  
- Protect from mechanical stress: route leads to avoid flexing the sensor or cracking adhesive.  
- Thermal mass & time constant trade-off: small, well-coupled sensors respond faster but are more sensitive to transients; a slightly larger thermal mass smooths noise but increases latency. Choose per control philosophy.  
- Placement geometry: avoid placing sensors directly on varnished or insulated surfaces that slow thermal transfer; prefer metallic or thin plastic housings with known conduction paths.  
- EMC & safety: keep sensor wiring away from mains switching nodes; use grounded shielding if necessary and keep traces short for analog sensors.

Sensing strategy & control logic (single vs multi-sensor)

- Single-sensor designs: simpler but risky—position must be chosen conservatively (near hottest expected point).  
- Dual-sensor (pack + charger) minimum: use max(pack_temp, charger_temp) logic to trigger derates.  
- Multi-sensor aggregation: weighted averaging, moving max, or per-sensor thresholds (e.g., MOSFET triggers immediate derate while ambient raises general derate).  
- Hysteresis & filtering: implement debounce/hysteresis to avoid oscillatory behavior on borderline temps. Use exponential filtering on ADC readings but maintain responsiveness to fast rises.  
- Priority rules: safety-critical sensors (MOSFET junction proxy, transformer hotspot) should take precedence; pack-contact sensor guides charge current under normal operation.

Electrical & firmware considerations

- ADC resolution & sampling: ensure ADC LSB temperature resolution maps to meaningful °C steps (e.g., 0.5–1 °C). Use proper pull-up networks for NTC divider linearization and accurate LUT.  
- Reference stability: use stable voltage reference; ADC drift impacts temperature accuracy.  
- Sampling rate & anti-aliasing: sample sufficiently fast for expected thermal transients; apply digital filtering but maintain worst-case response time guarantees.  
- Cold-junction compensation (thermocouples): implement if TC used.  
- Error detection: detect open/short sensor conditions and fallback to safe mode (reduce charge or refuse).  
- Calibration constants & LUTs: store calibration offsets in NVM and allow in-field recalibration under lab conditions.

Calibration & validation procedures (reproducible)

1. Factory calibration: measure sensor vs calibrated lab thermometer at 0 °C, 25 °C, 50 °C and create per-sensor LUT or polynomial compensation. Record offset and slope.  
2. On-line verification: at startup perform self-test — read sensors and validate they are within expected ambient ranges; if not, flag sensor fault.  
3. Thermal step test (qualification): apply localized heating (heater pad) at sensor mounting points and compare sensor reading to reference IR camera and a secondary probe; log time-to-plateau (time constant).  
4. Environmental soak: validate sensor behavior in chamber across operating range (−10 to +50 °C or product spec) and ensure hysteresis avoids nuisance trips.  
5. Periodic recalibration: schedule calibration checks on production fixtures (e.g., yearly) or after significant mechanical work/repair.

Reproducible tests & acceptance checks (shop-floor)

- Sensor contact integrity check: cold-start reading vs known ambient — outliers may indicate poor coupling.  
- Thermal transient response test: apply a small heater near hotspot and measure time-to-threshold; compare against golden unit.  
- IR cross-check: use IR camera to spot thermal gradients and verify sensor reads near IR-measured hotspot within ±2–3 °C.  
- Open/short sensor test: resistive checks at power-off to verify sensor wiring continuity.  
- Firmware watchdog test: force simulated over-temp via calibration offset to ensure derate/shutdown logic works.

Common problems & troubleshooting

- Slow response / delayed trip: loose adhesive, high thermal interface resistance, or too-large sensor thermal mass. Fix: re-mount with thermal adhesive or reposition.  
- False trips / oscillation: insufficient hysteresis, noisy ADC, hot air drafts near sensor. Fix: add filtering, hysteresis or change sensor placement away from transient airflows.  
- Drift over time: sensor aging, adhesive creep, or reference voltage drift. Fix: add periodic calibration and check reference.  
- Open/short sensor: detect via ADC range checks and fail-safe to conservative derate.  
- Mismatch between charger & pack thermistor readings: ensure pack thermistor location (on pack) and cradle sensor are both read and reconciled; prefer using pack thermistor when available and cross-check with cradle sensor.

Maintenance & serviceability

- Inspect sensor mounts and adhesives after mechanical service.  
- Include sensor continuity and response test in post-repair verification.  
- Log sensor replacement with serial traceability.  
- Keep spare calibrated sensors and documented mounting jigs for consistent rework.

Summary — one-line takeaway + 3 immediate actions  

Proper thermal-sensor placement (pack-contact + charger internal hotspots + ambient) with solid thermal coupling, redundancy, calibration and conservative control logic is essential to safe, fast and reliable charging; misplacement causes delayed protection, false trips or degraded lifetime.  

Immediate actions:
1. Add or verify a cradle-pack contact sensor (firm thermal coupling) if your charger lacks one.  
2. Place or validate a sensor on the MOSFET/heatsink and on the transformer winding area; implement `max()` logic between pack and charger temps.  
3. Run a quick IR vs sensor cross-check on a pilot charger under a 30–60 s charge load and confirm sensor reads hotspot within ±3 °C.

FAQ

Q — Should the charger rely only on the pack-provided thermistor?
A — No. Use the pack thermistor as primary for pack temp but cross-check with a cradle sensor and internal charger sensors (MOSFET/transformer). Pack thermistors can be missing, miswired, or faulty.

Q — How fast must a sensor respond to be effective?
A — Design for response times shorter than the fastest damaging thermal transient you expect (tens of seconds for charger hotspots; seconds for localized contact heating). Validate time constant in qualification.

Q — Can poor sensor placement be corrected in the field?  
A — Sometimes — improving thermal coupling (thermal tape/adhesive) or relocating a sensor is feasible. If PCB redesign is needed, perform lab verification before redeploying.

Q — Digital sensors vs NTC — which is better?
A — Digital sensors simplify interfacing and calibration; NTCs are compact and cost-effective. Choose based on required accuracy, response time, cost and available ADC resources.

Q — What is a safe fallback if a sensor fails?  
A — Implement a conservative fallback: reduce charge current significantly or refuse charge and flag for service; do not continue high-current charging with sensor failures.