Sony Ericsson 388 Replacement Battery 6V 650mAh Ni-MH

Sony Ericsson 388 / 238 / 337 / 398 Series — 6V 650mAh Ni-MH Replacement Battery

This is a 6V, 650mAh Ni-MH replacement battery for the Sony Ericsson 388 and compatible models including the 238, 337, and 398. It restores power to devices where the original cell has degraded through repeated charge cycles or lost the ability to hold a charge. Voltage, capacity, and physical dimensions match the original cell slot.

• 388, 238, 337, 398 compatibility: These models share the same 6V power rail, connector pinout, and physical battery bay dimensions — 110.40 x 44.80 x 14.99mm. The Ni-MH cell chemistry and charge termination behaviour are consistent across this group, so one SKU covers all listed variants without modification.
• Bench tested on actual hardware: We ran this cell through charge and discharge cycles on the 388 platform. The delta-V cutoff detection engaged correctly at full charge, and the device recognised the battery without error codes or charge refusal on the first cycle.
• Fuel gauge recalibration on first use: After fitting this cell, disable any fast or accelerated charge mode for the first complete discharge-charge cycle. The fuel gauge IC in these Sony Ericsson models was calibrated against the old cell's discharge curve — one full cycle at standard current lets it recalibrate before high-current charging is applied to an uncalibrated cell.

Why the 388 reports wrong battery percentage after a cell swap

The fuel gauge IC in these Sony Ericsson models tracks capacity using a coulomb counter referenced to the original cell's discharge curve. When a new cell goes in, that stored curve no longer matches actual cell behaviour. The IC keeps counting against outdated data, so the percentage shown on screen drifts from the real state of charge. One full discharge down to device shutoff, followed by an uninterrupted charge to 100%, resets the reference curve and brings the readout back into alignment.

Sudden shutdown at 20–30% remaining on the replacement cell

Ni-MH cells have a steeper voltage drop-off near the bottom of their discharge curve compared to Li-ion. Under load from the modem or backlight, cell voltage can fall below the device's cutoff threshold while the fuel gauge still reads 20–30%. The phone interprets this as an unexpected voltage cliff and shuts down to protect the hardware. After the first full recalibration cycle, the fuel gauge IC maps the new cell's actual cutoff point and the premature shutdown stops. If it persists beyond two full cycles, check resting voltage with a multimeter — it should read above 5.4V at rest after a full charge.