Sprint C5120 Cleartalk Compatible Battery 3.7V 1400mAh Li-ion

Sprint C5120 Cleartalk / C5120 Milano — 3.7V Li-ion Replacement Battery

This 3.7V, 1400mAh Li-ion cell replaces the original battery in the Sprint Kyocera C5120 Cleartalk and C5120 Milano flip phones. It fits the same battery bay and connector as the factory unit. Swap it in when the original cell no longer holds a usable charge between calls.

• C5120 Cleartalk and Milano compatibility: Both models use the same battery bay dimensions and connector pinout — 65.20 x 44.00 x 5.10mm — so one cell covers both. The BMS protection circuit matches the charge IC behaviour the phone expects at 3.7V nominal.
• Bench tested on actual hardware: We ran charge and discharge cycles on this cell with the C5120 charge IC. The BMS cutoff triggers correctly at the low-voltage threshold, and the cell accepts a full charge cycle without the protection circuit tripping mid-charge.
• Fuel gauge recalibration after install: On first use, let the phone discharge down to auto-shutoff, then charge uninterrupted to 100% before using it again. The C5120 fuel gauge IC is calibrated against the old cell's discharge curve — one full cycle resets it against the new cell and stops erratic percentage readings.

Sudden shutdown at 20–30% on the C5120 after a battery swap

The C5120 runs a basic fuel gauge IC that tracks charge state by voltage curve, not a coulomb counter. A new cell has a slightly different voltage-versus-capacity curve than the aged original. When the phone reaches a voltage point it maps to 20–30%, the new cell may still have capacity left — but under the load of an active call, voltage sags below the cutoff threshold and the phone shuts off. This resolves after one full discharge-to-shutoff and uninterrupted recharge cycle, which lets the IC remap its curve to the new cell.

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

Li-ion cells self-discharge in storage. If this battery arrived partially discharged and was left unused for several weeks, the cell voltage may have dropped below 2.5V — the threshold where the BMS locks out to prevent damage. The phone will show no response at all, not even a low-battery screen. Connect the phone to a charger and leave it for 15–20 minutes without pressing the power button; the charge IC needs time to trickle current into the cell and bring it above the BMS re-enable threshold of approximately 2.9V before normal charging resumes.