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Key points
• 10 kΩ ± 1 % at 25 °C, β25/85 ≈ 3950 K
• Serves charge-inhibit, discharge cut-off, and cell balancing functions inside the battery
• Typical trip temperature ≈ 60–70 °C (discharge), 45–50 °C (charge)
• If you experience early shut-down, suspect cell imbalance or a drifting NTC in the battery, not the saw.
Tool architecture
• R18CS is a brushed DC circular saw with a simple MOSFET trigger.
• No micro-controller or temperature feedback loop is fitted in the tool housing; therefore no NTC is mounted near the motor windings or the PCB.
Battery-based temperature sensing
• Ryobi’s ONE+ packs embed a 10 kΩ EPCOS / Murata-class NTC epoxy-bonded to the cell cluster.
• The thermistor forms a voltage divider with a precision pull-up in the charger/BMS.
• Voltage \(V_T\) is digitised; temperature is calculated via the Steinhart-Hart equation:
\[
\frac{1}{T(K)} = A + B \ln(R_T) + C\,[\ln(R_T)]^3
\]
• The charger uses this value to obey IEC 62133 / UL 2054 Li-ion safety limits (0 – 45 °C charge, –20 – 60 °C discharge).
Protection sequence during cutting
• Heavy cut → rapid current draw → internal cell heating → NTC drops below ≈ 3.5 kΩ (≈ 70 °C) → BMS opens MOSFET stack → saw switches off.
• Cool-down restores \(R_T\) to ~5–6 kΩ → MOSFETs re-close → saw restarts.
Typical failure modes
a) Weak cell(s) → local hot spot → genuine over-temperature → nuisance trip.
b) NTC detachment (epoxy cracks) → reads ambient, pack overheats unsafely.
c) NTC drift / open circuit → charger shows red/green fault, will not start charge.
Measuring the NTC
• Remove battery casing (Torx T10 security).
• At 25 °C you should read 9.5–10.5 kΩ between B– and T.
• Warm pack with hair-dryer: resistance should fall smoothly to ≈ 4 kΩ at 60 °C.
• Any discontinuity or > ±5 % deviation indicates replacement.
• Newer “ONE+ HP” / brushless tools (e.g. R18CS7, PBLCS300) add an on-board NTC on the MOSFET heatsink for dual-domain monitoring.
• Smart packs (Ryobi “Lithium+” and “HP” cells) expose temperature and current data via I²C/SMBus; legacy packs remain analog.
• Industry moving toward NTC-plus-NTC or NTC + NTC + NTFET arrays for finer control and ageing prediction.
• Why the saw keeps working with a “battery adapter”: most adapters wire only B+ and B–; because standard Ryobi tools do not read the “T” line, they still operate (chargers will not).
• Analogy: the battery/NTC acts as the coolant thermostat in an engine; the saw is the load, unaware of coolant temperature, relying on the thermostat to open/close.
• Bypassing or replacing the NTC with a fixed resistor is unsafe and voids UL/CE approvals.
• Lithium-ion packs without thermal feedback can enter thermal runaway at ≈ 120 °C, posing fire and injury risk.
• Values above are typical for Ryobi-branded 1.3–5.0 Ah packs manufactured since 2014; some earliest Ni-Cd adapters used 100 Ω PTC (not NTC).
• Because Ryobi does not publish schematics, minor variations may exist among factories (TTI CN vs. VN).
• Correlate IR-camera data with NTC readings during rip-cut to refine thermal models.
• Explore CAN/I²C smart-pack retrofits for legacy tools to allow user-visible temperature telemetry.
• Study NTC ageing drift over 500 / 1000 cycles to predict preventive replacement intervals.
The Ryobi R18CS relies exclusively on the 10 kΩ NTC housed in the ONE+ battery pack for all temperature-related safety functions. The saw itself contains no thermistor; premature shut-downs, charging faults, or over-temperature indications almost always trace back to the battery’s NTC or to deteriorated cells, not to components inside the saw. Handle the NTC circuit with care—never bypass it—and replace the battery or thermistor if resistance/temperature behaviour deviates from specification.
