About this replacement
Coolpad 7251 / 5311 — 3.7V Li-ion Replacement Battery (CPLD-121)
This is a 3.7V, 1650mAh (6.11Wh) Li-ion replacement battery for the Coolpad 7251 and 5311 smartphones. It replaces OEM part number CPLD-121. Fit it when the original cell no longer holds a usable charge or the device shuts down unexpectedly under load.
- Coolpad 7251 and 5311 compatibility: Both models run on the same 3.7V battery rail and use the same CPLD-121 footprint — identical connector pinout and BMS handshake protocol — so one cell covers both devices without modification.
- Bench tested on actual hardware: We cycled this cell through full charge and discharge passes on compatible hardware. The BMS accepted the charge current without tripping, and the protection circuit responded correctly to low-voltage cutoff at the expected threshold.
- Fuel gauge recalibration on first use: On first use after installation, disable fast charging for one complete discharge-charge cycle. This lets the fuel gauge IC recalibrate against the new cell's discharge curve before high-current charging pushes current into an uncalibrated cell.
Sudden shutdown at 20–30% on the Coolpad 7251 after a cell swap
A new cell has a slightly different voltage-versus-capacity curve than the worn cell the fuel gauge IC was calibrated to. The phone reads 25% remaining but the actual cell voltage under modem or screen load drops below the cutoff threshold the processor enforces. The device shuts down even though the percentage display suggests charge remains. Run one full discharge from 100% to automatic shutdown, then charge uninterrupted to 100% — this forces the coulomb counter to re-anchor its calibration points to the new cell.
Device not powering on after the replacement battery sat in storage
Li-ion cells self-discharge in storage. If this battery spent time on a shelf before installation and dropped below approximately 2.5V per cell, the BMS enters a lockout state to prevent damage and blocks normal charge current. The phone appears completely dead and won't respond to the power button. Connect the device to a wall charger — not a USB port — and leave it for 20–30 minutes before attempting to power on; wall adapters supply enough trickle current to nudge the BMS out of lockout and allow normal charging to resume.