About this replacement
Ryobi RP4400 Series — 4V Li-ion Replacement Battery (AP4001)
This is a 4V 2000mAh Li-ion replacement battery with OEM part number AP4001. It fits the Ryobi RP4400, RP4410, RP4530, RP4550, and additional models in the same compact rotary tool family. Use it for drilling, grinding, cutting, and polishing tasks where the original pack has degraded.
- RP4400 series compatibility: These rotary tool models share the same 4V voltage rail, physical form factor, and BMS handshake protocol. The AP4001 pack seats and communicates identically across the full model range.
- Bench tested on actual hardware: We ran this pack on an RP4400 under repeated load cycles. The BMS held overcurrent protection at motor start and resumed cleanly after inrush settled. Cell voltage stayed within spec throughout.
- Motor inrush conditioning: On first use, run the tool at half load for two cycles before full-torque applications. This lets the BMS profile the motor's inrush current draw before locking in its overcurrent protection thresholds.
BMS cutoff on motor-start inrush in the RP4400
The RP4400's brushed motor pulls a brief current spike the moment the trigger engages. On a new or recently stored pack, the BMS may read that spike as an overcurrent fault and cut power instantly. This is a protection response, not a defective battery. Two half-load conditioning cycles train the BMS to distinguish a normal inrush spike from a genuine fault event.
Charger not recognising the AP4001 pack after storage
If the RP4400 charger shows a blinking red light and never transitions to charging, the cell voltage has likely dropped below the charger's acceptance threshold — typically around 2.5V per cell. A deeply discharged Li-ion pack triggers the charger's safety lockout. Some Ryobi chargers support a recovery or "wake" mode — hold the pack on the charger for 30 minutes to see if it enters trickle charge. If the indicator stays red past that point, check cell voltage directly at the battery terminals; a reading below 2.8V total indicates the pack cannot be recovered safely.