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FLEX 445894 18V Cordless Saw Replacement Battery 5000mAh

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From $81.99 USD $100.99
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Fits FLEX 517674 CS 45 18.0-EC C Compact cordless circular saw, replaces OEM part 445894.
18V, 5000mAh lithium-ion cell delivers sustained power through cross-cuts and rip work without voltage sag.
Connector slides onto the FLEX tool base with a quarter-turn locking tab; seats flush with housing.
We ran this pack through motor-start inrush cycles on the circular saw platform—BMS accepted the trigger-pull surge without premature cutoff.
On first use with this circular saw, run half-throttle cuts for two cycles before pushing the motor at full torque to allow the BMS to establish its overcurrent protection threshold for the motor's inrush profile.
Delivery time

This product ships directly from our Manufacturer’s Warehouse and is usually delivered within 5 – 8 business days to your doorstep.

Discount: As a thank you for your patience, enjoy 5% off on your order
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Battery Care Tips

🔹 Getting Started

Charge your new battery fully before you use it for the first time. Over the next few charge cycles, run your device down to around 20% before you recharge—this helps the battery perform its best. After that, charge whenever you need to.

🔹 Keep It Healthy

Avoid letting your battery completely drain or staying plugged in constantly. Both extremes wear it out faster. Store the battery in a cool, dry place when you're not using it, since heat damages batteries quickly.

Delivery and Shipping

🔹 Most orders ship the next day, and we use FedEx, UPS, Purolator and other carriers to get them to you. Lithium batteries have to ship by ground only, not air or USPS. Make sure your address is right before you order, because if we have to send it back, you pay for shipping again.

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⚠️ Disclaimer: All product names, trademarks, and registered trademarks belong to their respective owners.

🔹 We use these names, brands, or model numbers only for identification and compatibility purposes.

Voltage

18V

Amp

5000mAh

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.

Compatible Models

517674 CS 45 18.0-EC C Compact cordless circular saw CS 45 18.0-EC C Compact cordless circular saw RFE 40 18.0-EC Cordless gutter bracket slot cutter

Replaces Part Numbers

445894

Technical Specifications

Voltage18V
Amp Hours5000mAh
Capacity5000mAh
Rate90Wh
Net Weight645g /22.75 oz
Gross Weight835g /29.45 oz
Approximate Weight835g /29.45 oz
Dimension 119.20 x 75.00 x 67.80mm

Product Highlights

  • Brand: FLEX
  • Manufacturer: CS
  • Series: Standard
  • Color: Black
  • Product Type: Li-ion
  • Battery Type: Li-ion
  • Warranty: 12 Months
  • Bulk Orders: sales@batteryweb.com

Frequently Asked Questions

My FLEX CS 45 cuts out the instant I pull the trigger on a thick board — why does it stop immediately rather than bogging down first?

That's a BMS overcurrent trip, not a motor fault. The trigger-pull inrush current on a hard start into dense material spikes sharply — if the BMS threshold is set conservatively on a new or cold pack, it reads that spike as a short and cuts power before the blade has even loaded. Let the pack warm to room temperature, then do two light-load passes before attempting full-depth cuts. This allows the BMS to see normal inrush current before setting its overcurrent window.

The saw runs fine for the first few seconds then progressively loses power mid-cut without fully shutting off — what's causing the weak performance?

This is voltage sag under sustained load, not a BMS trip. As the motor draws current through a long cut, internal cell resistance causes the pack voltage to drop — the saw's motor controller sees a lower rail voltage and reduces power output to compensate. Check the battery contacts and tool contact rails for corrosion or debris; even slight resistance at the connection point amplifies sag. If contacts are clean and the problem persists on a pack with many cycles, cell capacity has faded — resting voltage should read 20V or above on a fully charged 18V Li-ion pack.

The FLEX circular saw battery works fine indoors but cuts out quickly when I'm working outside in winter — is the pack damaged?

No damage — Li-ion internal resistance rises significantly below 5°C, which reduces the current the cells can deliver before the BMS trips on undervoltage. Store the pack indoors and bring it out just before use; a warm pack can handle motor-start inrush that a cold pack cannot. If you must work in sub-zero conditions for extended periods, keep a second pack in a jacket pocket to rotate in. A pack that recovers fully once back at room temperature is functioning normally.

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FLEX 517674 CS 45 18.0-EC C Compact cordless circular saw Replacement Battery, 18V, 5000mAh, Li-ion 1

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