About this replacement
FLEX 517674 CS 45 18.0-EC C — 18V Li-ion Replacement Battery (445894)
This is an 18V, 5000mAh (90Wh) Li-ion replacement battery for the FLEX 517674 CS 45 18.0-EC C Compact cordless circular saw and the RFE 40 18.0-EC Cordless gutter bracket slot cutter. It matches the OEM part number 445894 and fits the same battery bay and connector as the original pack. Voltage, cell count, and BMS communication protocol all match the FLEX 18V platform.
- FLEX 18V platform fit: The CS 45 and RFE 40 share the same 18V rail, connector pinout, and BMS handshake protocol — one battery services both tools without any adaptor or modification.
- Bench tested on actual hardware: We ran this pack through motor-start inrush cycles on the CS 45 saw. The BMS held the overcurrent threshold correctly on trigger pull and did not trip under repeated hard starts.
- First-use load cycling on the circular saw: Run the saw at half load for two cycles before full-depth cuts. This lets the BMS profile the motor's inrush current draw before locking in its overcurrent protection thresholds — prevents nuisance trips on your first heavy cut.
Thermal cutoff under sustained circular saw load
The CS 45 runs a compact motor housing that traps heat during long ripping cuts. When cell temperature climbs past the BMS thermal threshold — typically around 60°C — the pack shuts down to protect the cells. This is not a fault; it is the BMS doing its job. Let the pack rest for 10–15 minutes and surface temperature drops enough to re-enable discharge. For sustained work, swap between two packs to keep both within safe operating temperature.
Charger not recognising the pack after extended storage
Li-ion cells sitting unused for several months can drop below the charger's acceptance voltage — usually around 2.5V per cell. The FLEX charger reads this as a fault and refuses to begin the charge cycle. Place the pack on the charger and leave it for up to 30 minutes; most FLEX chargers include a trickle pre-charge mode that slowly brings cells back above the acceptance threshold before switching to full charge. If the charger still shows a fault after 30 minutes, check resting voltage across the pack terminals — a healthy pack in recovery should read above 15V before the charger handshake completes.