Panasonic KX-TU400 Compatible Battery 3.7V 900mAh

Panasonic KX-TU400 Series — 3.7V Li-ion Replacement Battery (433450ART)

This is a 3.7V, 900mAh Li-ion cell built to the 433450ART specification, fitting the Panasonic KX-TU400, KX-TU400 EXRM, and KX-TU450 candybar mobile phones. These phones use a sealed casing with no user-accessible battery door, so the cell must be installed by someone comfortable opening the housing. Capacity is 3.33Wh, matching the original cell footprint at 53.15 × 34.00 × 4.70mm.

• KX-TU400, KX-TU400 EXRM, and KX-TU450 compatibility: All three models share the same internal battery bay geometry, connector pinout, and BMS handshake protocol. A single cell revision — 433450ART — covers the full range without modification or adapter.
• Bench tested on actual hardware: We cycled this cell through charge, standby, and active call load on the KX-TU400 platform. The BMS accepted the charge IC handshake on first connection, and thermal output during the initial charge stayed within the phone's expected range.
• Fuel gauge recalibration after installation: On first use, disable any fast-charge mode and run one complete discharge-charge cycle at standard rate. The KX-TU400's fuel gauge IC was calibrated to the original cell's discharge curve — giving it a clean baseline cycle prevents erratic percentage readings from the start.

Sudden shutdown at 20–30% on the KX-TU400 after a cell swap

This happens because the fuel gauge IC still holds the old cell's voltage-to-capacity curve in memory. A new cell has a slightly different discharge profile, and the gauge misreads the actual remaining charge. When the phone pulls current for a call or screen wake, the cell voltage drops below the hardware cutoff threshold before the percentage hits zero. Running one full discharge down to automatic shutdown, then charging uninterrupted to 100%, forces the coulomb counter to relearn the new cell's curve and eliminates most false cutoffs.

KX-TU400 not powering on after the replacement cell sat in storage

Li-ion cells self-discharge in storage, and if the 433450ART cell dropped below approximately 2.5V per cell before installation, the BMS will have triggered a lockout to prevent damage. The phone will show no response — no boot screen, no charge indicator. Connect the phone to a known-good charger and leave it for 15–20 minutes without attempting to power it on; the charge IC will trickle current into the cell until the BMS releases the lockout. Once the charge indicator appears on screen, the cell has recovered above the minimum threshold and normal charging can resume.