LG CU400 Replacement Battery 3.7V 750mAh LGIP-A1100

LG CU400 / CU405 — 3.7V Li-ion Replacement Battery (LGIP-A1100)

This is a 3.7V, 750mAh Li-ion replacement battery for the LG CU400 and CU405 candybar-style feature phones. It fits directly in place of the original LGIP-A1100 cell. Use it to restore power to a handset where the original battery no longer holds charge through a normal day.

• CU400 and CU405 compatibility: Both models share the same battery bay dimensions, connector pinout, and BMS handshake protocol — one cell covers both. The 54.00 × 34.05 × 5.70mm form factor matches the original housing with no modification needed.
• Bench tested on actual hardware: We cycled this cell through charge and discharge on the CU400 platform. The BMS accepted the cell without fault flags, and the charge IC reached termination voltage cleanly at 4.2V.
• Fuel gauge recalibration on first use: After fitting the new cell, run one complete discharge down to automatic shutoff before recharging. This gives the phone's fuel gauge IC a full data set from the new cell's discharge curve — without it, the percentage reading can drift by 15–20% until the IC has reference points at both ends of the voltage range.

Why the CU400 reports wrong battery percentage after a cell swap

The CU400 uses a simple fuel gauge IC that maps voltage to a percentage curve calibrated against the original cell. A new cell has a slightly different internal impedance and discharge slope, so the IC's stored curve no longer matches what it's measuring. The result is percentage readings that jump or stall — commonly sitting at 100% longer than expected, then dropping fast near the bottom. One full discharge-to-shutoff cycle gives the IC new endpoints to anchor against, and readings stabilise after that.

Sudden shutdown at 20–30% on the replacement cell

This happens when the fuel gauge IC hasn't completed recalibration and the displayed percentage doesn't reflect actual cell voltage. Under load — backlight on, active call — the cell voltage drops faster than the uncalibrated IC expects, and the phone cuts power to protect the cell when voltage falls below the BMS trip threshold. It's not a fault with the replacement cell. Run a full discharge cycle without interruption. After recalibration, the phone's shutoff should align with a cell voltage of approximately 3.4–3.5V rather than triggering early at a misread percentage.