About this replacement
Samsung SGH-A400 / SGH-A408 / SGH-A401 — 3.7V Li-ion Replacement Battery
This is a 3.7V, 750mAh Li-ion replacement battery for the Samsung SGH-A400, SGH-A408, and SGH-A401 mobile phones. These early-2000s handsets use a slim Li-ion cell with a low-current BMS suited to basic call and messaging loads. Total energy is 2.78Wh.
- SGH-A400 / A408 / A401 platform fit: All three models share the same battery bay dimensions, connector pinout, and charge IC voltage rail at 3.7V nominal. The BMS communication on this generation is passive — no cryptographic handshake — so the cell slots in without firmware rejection.
- Bench tested on actual hardware: We cycled this battery through charge and discharge on the SGH-A400 platform. The BMS held the cutoff at the correct lower threshold and the charge IC terminated cleanly at full capacity without thermal anomalies.
- Fuel gauge recalibration on first use: After fitting this cell, run one complete discharge down to automatic shutdown, then charge uninterrupted to 100% before regular use. The phone's fuel gauge IC was calibrated to the old cell's discharge curve — one full cycle writes the new curve and stops erratic percentage readings.
Sudden shutdown at 20–30% on the SGH-A400 after a cell swap
This happens because the fuel gauge IC still holds the discharge curve of the degraded original cell. The new cell has a steeper voltage cliff at low state-of-charge than the old worn cell did. When the phone draws current for a call or backlight burst, the cell voltage dips sharply and the BMS trips the cutoff — even though the reported percentage looks safe. One full discharge-to-shutdown cycle followed by a full uninterrupted charge resets the coulomb counter and maps the new cliff correctly. After that cycle, shutdowns at 20–30% stop.
Phone shows 100% immediately after fitting the new battery but drops fast
The SGH-A400 reads state-of-charge from a simple voltage lookup table stored in the fuel gauge IC. A new cell sitting at storage voltage — typically around 3.7–3.8V — can map to a high percentage on that table before the IC has any real usage data. The reading is not accurate until the IC has tracked at least one full discharge and charge cycle. Charge the phone to 100%, use it until it shuts itself off, then recharge fully. After that single cycle the percentage tracking stabilises.