About this replacement
FLEX 517674 CS 45 18.0-EC C — 18V Li-ion Replacement Battery (445894)
This is an 18V, 3000mAh Li-ion replacement battery for the FLEX 517674 CS 45 18.0-EC C Compact cordless circular saw. It also fits the RFE 40 18.0-EC Cordless gutter bracket slot cutter. The OEM part number is 445894.
- CS 45 and RFE 40 platform fit: Both tools share the same 18V rail, slide-in connector, and BMS handshake protocol — one battery covers either tool without modification or adapters.
- Bench tested on actual hardware: We ran this pack through full charge and discharge cycles on a CS 45 platform. The BMS correctly handled motor-start inrush current at trigger pull and held cell voltage within the expected 18V nominal range under sustained load.
- Motor inrush conditioning on first use: Run the saw at half load — shallow crosscuts through softwood — for two cycles before tackling full-depth hardwood rips. This lets the BMS profile the motor's inrush draw and set overcurrent thresholds accurately before maximum torque applications.
Thermal cutoff under sustained circular saw load
The CS 45's enclosed battery housing traps heat from both the motor and the cells during extended cuts. When cell temperature exceeds the BMS upper threshold — typically around 60°C on Li-ion packs — the BMS shuts the pack down to protect the cells. This is not a fault; it is the protection circuit doing its job. Let the pack rest off the tool for 10–15 minutes and allow both the housing and cells to cool before resuming.
Saw bogs and loses power mid-cut under load
A bogging saw that slows under load but does not fully cut out usually points to voltage sag — rail voltage drops during high current draw because contact resistance at the battery terminals is too high. Pull the pack, inspect the slide-in contacts for sawdust compaction or corrosion, and clean them with a dry brush. Reseat the pack firmly and retest; rail voltage under load should recover to above 16V on an 18V nominal pack.