Coolpad 8021 Replacement Battery CPLD-139 3.7V 1150mAh

Coolpad 8021 — 3.7V Li-ion Replacement Battery (CPLD-139)

This is a 3.7V, 1150mAh Li-ion replacement battery for the Coolpad 8021 smartphone. It carries OEM part number CPLD-139 and fits directly into the 8021's battery bay. Use this when the original cell no longer holds adequate charge or has failed due to age and cycle wear.

• Coolpad 8021 fit: The 8021 uses a compact 52.50 × 39.00 × 6.00mm cell with a low 3.7V nominal rail. This replacement matches that footprint and voltage exactly, so the connector seats correctly and the charge IC receives the expected voltage reference on boot.
• Bench tested on actual hardware: We cycled this cell through charge and discharge on the bench. The BMS accepted charge without triggering an overcurrent fault, and the phone's charge IC negotiated correctly without throwing a "charging paused" alert.
• Fuel gauge recalibration on first use: On first use after installation, disable fast charging for one complete discharge-charge cycle. This gives the fuel gauge IC time to map the new cell's discharge curve before high-current charging pushes current into an uncalibrated cell — reducing erratic percentage readings early on.

Sudden shutdown at 20–30% on the Coolpad 8021 after a cell swap

The 8021's fuel gauge IC stores a discharge curve calibrated to the original cell. A new cell has a different internal impedance, so the gauge misreads remaining capacity. Under modem or screen load, the actual cell voltage drops faster than the gauge predicts, and the phone cuts out before it reports zero percent. One full discharge-to-shutdown followed by a complete charge resets the coulomb counter against the new cell's actual curve. After that cycle, shutdowns at 20–30% typically stop.

Phone won't power on after the replacement cell sat in storage

Li-ion cells self-discharge during storage. If the CPLD-139 cell drops below approximately 2.5V per cell, the BMS enters a lockout state to prevent damage and the phone will not respond to a normal power-on press. Connect the phone to a charger and leave it for 15–20 minutes without pressing anything — the charge IC will trickle current into the cell at a low rate until voltage rises above the BMS recovery threshold. Once voltage climbs back above roughly 3.0V, the BMS releases its lockout and normal charging resumes.