About this replacement
Sony VAIO A12 — 7.6V Li-Polymer Replacement Battery (VJ8BPS55)
This is a 7.6V, 3100mAh (23.56Wh) lithium-polymer battery for the Sony VAIO A12 ultraportable laptop. It replaces OEM part number VJ8BPS55 and fits the VAIO A12 series directly. Cell degradation and capacity fade are the most common reasons owners replace this battery.
- VAIO A12 fitment: The A12 uses a flat Li-Polymer pack with a specific connector pitch and BMS handshake tied to the VJ8BPS55 part number. Swapping to a cell that matches this spec keeps the firmware communication intact and prevents false fault codes on boot.
- Bench tested on actual hardware: We cycled this cell through charge and discharge on VAIO A12 hardware and confirmed the BMS negotiation completed without error. The protection circuit held cutoff voltage correctly at both ends of the cycle.
- First-cycle recalibration after install: After fitting this battery, run the laptop down to hibernate-cutoff under normal use, then charge uninterrupted to 100%. This forces the BIOS battery learn cycle to reset against the new cell and clears the inaccurate health warning that appears after every cell swap.
Why the VAIO A12 shuts down at 20–30% after a battery swap
The VAIO A12 fuel gauge IC calibrates against the previous cell's charge curve. When a new cell goes in, the stored curve no longer matches actual cell voltage under load. Under full CPU and display draw, the new cell hits a voltage cliff the old curve doesn't account for, and the system interprets it as empty. Running two to three full discharge-to-hibernate cycles recalibrates the IC against the new cell's actual curve and stops premature shutdowns.
BIOS reporting battery health as poor or unknown after replacement
The VAIO A12 BIOS reads health data from EEPROM stored in the original battery pack. A replacement cell carries different EEPROM values, which the BIOS flags as degraded or unrecognised on first boot. This is a firmware mismatch, not a fault with the new cell. Run the battery learn cycle — discharge fully to hibernate, then charge to 100% without interruption — and the health status will update to normal within one to two cycles.