About this replacement
Sony CMD-J5 / CMD-J26 Series — 3.7V Li-ion Replacement Battery (QN-J5BPS)
This is a 3.7V, 650mAh Li-ion replacement battery for the Sony CMD-J5, CMD-J16, and CMD-J26 mobile phones. These are early-2000s compact handsets that rely on the QN-J5BPS cell format. Swapping the original depleted cell restores the phone to full operation.
- CMD-J5, J16, and J26 compatibility: All three models share the same battery bay dimensions, connector pin-out, and 3.7V nominal voltage rail. The QN-J5BPS part number covers the entire J-series lineup because Sony used the same cell housing and contact plate across these variants.
- Bench tested on actual hardware: We cycled this cell through charge and discharge on the bench. The BMS accepted the charge IC handshake without incident, and the protection circuit tripped correctly at low-voltage threshold — no runaway charging or premature cutoff observed.
- Fuel gauge recalibration on first cycle: After installing this cell, run the phone down to automatic shutdown once before recharging. The CMD-J5's fuel gauge IC calibrated to the original cell's discharge curve — one full cycle against the new cell resets that baseline so percentage readings are accurate from that point forward.
Why the CMD-J5 reports wrong battery percentage after a cell swap
The CMD-J5 uses a coulomb-counting fuel gauge IC that tracks charge by measuring current in and out of the original factory cell. When you replace that cell, the IC still references the old discharge curve. The result is a percentage reading that drifts — often showing full charge when the cell is not, or jumping from 40% to shutdown without warning. One complete discharge-to-cutoff followed by an uninterrupted full charge forces the IC to relearn the new cell's actual capacity curve. After that single cycle, percentage reporting stabilises.
Sudden shutdown at 20–30% remaining on the replacement cell
This happens when the cell's internal resistance causes voltage to drop below the protection threshold under load — typically during a call when the RF transmitter draws current in bursts. The phone's fuel gauge reads 25% on open-circuit voltage but the cell cannot sustain that voltage under transmit load, so the BMS trips to protect the cell. It is not a faulty battery — it is the fuel gauge IC reading an uncalibrated cell. Complete the first full discharge-charge calibration cycle first. If shutdowns continue after that cycle, check resting voltage with the phone off; a healthy cell at 30% charge should sit above 3.65V.