Neffos C9 Replacement Battery NBL-40A3730 3.8V 3650mAh

Neffos C9 / TP707A — 3.8V Li-Polymer Replacement Battery (NBL-40A3730)

This is a 3.8V, 3650mAh Li-Polymer battery for the Neffos C9 and TP707A smartphones. It replaces OEM part number NBL-40A3730 when the original cell has degraded, fails to hold charge, or causes the phone to shut down unexpectedly. Dimensions are 87.26 × 64.52 × 3.80mm — verify clearance before ordering if your unit has been repaired before.

• C9 and TP707A compatibility: Both models share the same battery bay geometry, connector pinout, and BMS handshake protocol — that is why one cell covers both. The fuel gauge IC communicates over the same single-wire interface on each board revision.
• Bench tested on actual hardware: We ran this cell through full charge and discharge cycles on the C9 platform. The BMS accepted the charge IC handshake without a fault flag, and the protection circuit tripped correctly at the low-voltage cutoff threshold.
• First-cycle fast charge tip: On first use after installation, disable fast charging in Android settings and complete one full discharge-charge cycle at standard current. This gives the fuel gauge IC time to map the new cell's discharge curve before high-current charging begins pushing amperage into an uncalibrated cell.

Sudden shutdown at 20–30% on the Neffos C9 after a cell swap

The C9's fuel gauge IC holds a discharge curve learned from the original cell. When a new cell goes in, that stored curve no longer matches the actual voltage-to-capacity relationship of the replacement. The phone reads 25% remaining but the cell voltage has already dropped below what the modem and display need under load. The SOC estimate collapses and the phone shuts off to protect the circuit. One full discharge to auto-off followed by a complete charge to 100% forces the coulomb counter to resync — do not interrupt that first cycle.

Phone not powering on after the replacement battery sat in storage

Li-Polymer cells self-discharge during storage. If the cell voltage dropped below approximately 2.5V per cell before installation, the BMS will have entered lockout mode and will block current flow even when a charger is connected. Plug the phone into a wall adapter — not a computer USB port — and leave it for 20–30 minutes without attempting to power on. The charge IC on the C9 board will trickle-charge the cell from around 50mA until voltage rises above the BMS re-enable threshold, at which point normal charging resumes.