BlackBerry Q5 Replacement Battery BAT-51585-003 3.8V 2100mAh

BlackBerry Q5 Series — 3.8V Li-Polymer Replacement Battery (BAT-51585-003)

This 3.8V, 2100mAh Li-Polymer cell replaces the original battery in the BlackBerry Q5, Q5 LTE, Q5 LTE SQR100-1, and Q5 SQR100-3. It matches OEM part numbers BAT-51585-003, BAT-51585-103, and PTSM1. Capacity is rated at 7.98Wh — identical to the factory specification.

• Q5 series fit: All Q5 variants share the same battery bay dimensions, connector pinout, and BMS handshake protocol. The 75.69 x 48.72 x 4.35mm cell slots into each model without modification, and the fuel gauge IC on each board reads the same charge curve from this cell.
• Bench tested on actual hardware: We cycled this cell on a Q5 SQR100-1 board and monitored BMS communication through two full charge-discharge cycles. The charge IC accepted the cell on the first connection, and the fuel gauge IC stabilised percentage reporting by the end of the second cycle.
• First-cycle fuel gauge calibration: On first use after installation, disable fast charging for one complete discharge-charge cycle. This lets the fuel gauge IC map the new cell's discharge curve before high-current charging pushes power into an uncalibrated state — preventing early erratic percentage jumps.

Sudden shutdown at 20–30% on the Q5 after a cell swap

The Q5's fuel gauge IC retains the discharge curve of the original cell in its coulomb counter memory. When a new cell is installed, the IC still references the old curve, causing it to misjudge the actual remaining voltage. Under modem load or screen brightness spikes, the new cell's voltage drops below what the IC expects for that percentage, and the BMS triggers a protective shutdown. Running one complete discharge down to automatic shutoff — then a full uninterrupted charge — forces the coulomb counter to remap against the new cell. After that cycle, shutdowns at 20–30% stop.

Device not powering on after the replacement cell sat in storage

Li-Polymer cells in storage self-discharge slowly. If the cell drops below approximately 2.5V per cell, the BMS enters a lockout state and refuses to deliver current to the device — the phone appears completely dead. Plugging into a charger often shows no response because the BMS blocks charge input until voltage is recovered above the reinitialisation threshold. Use a USB charger with trickle or recovery mode, or leave the phone connected for 20–30 minutes before attempting to power on. If the BMS reinitialises, the charge LED will activate and the phone will boot normally once voltage clears 3.0V.