Sony Ericsson BA700 Xperia Neo Replacement Battery 3.7V 1000mAh

The Xperia Neo won't turn on at all after the replacement battery sat in a drawer for months — is it dead?

A BA700 cell stored uncharged for an extended period can drop below 2.5V, triggering BMS lockout — the protection circuit disables output to prevent cell damage. Plug the phone into a wall charger and leave it connected for 30–45 minutes without pressing the power button; most charger ICs trickle current into a locked-out cell to bring it above the recovery threshold before switching to normal charge mode. If the charge LED doesn't light within an hour, try a different cable and charger brick rated at 5V/1A — USB ports on PCs often lack the sustained current to push a locked-out cell past the threshold. Once the LED activates, let it reach 100% before powering on.

The phone feels warm near the battery compartment during the first few charges after fitting the BA700 — is something wrong?

Mild warmth on the first few charge cycles is normal with a new high-impedance cell. A freshly manufactured Li-ion cell has slightly elevated internal resistance compared to a broken-in cell, so the charge IC dissipates a little more energy as heat during the constant-current phase. Temperature should drop to near-ambient within three to four charge cycles as internal resistance settles. If the back of the phone becomes hot to the touch — above roughly 45°C — or the charge stops prematurely, swap to a 5V/1A charger and avoid charging on a soft surface that traps heat.

The battery percentage jumps erratically — skipping from 45% to 12% in minutes — after installing the new cell. What's causing it?

The fuel gauge IC is recalibrating its coulomb counter against a cell discharge curve it doesn't recognise yet, so percentage readings are unstable until it completes a full reference cycle. Run the phone down until it shuts itself off automatically — do not force a shutdown — then charge it in one uninterrupted session to 100% with the screen off or on standby. After that single complete cycle the IC writes a new baseline and erratic jumps stop. If jumping continues past two full cycles, check that the battery connector is fully seated — a partial contact causes intermittent voltage readings that mimic calibration errors.