Sony Ericsson U5 Replacement Battery EP500 3.7V 900mAh

Sony Ericsson U5i Vivaz Series — 3.7V Li-ion Replacement Battery (EP500)

The EP500 is a 3.7V, 900mAh Li-ion cell that fits the Sony Ericsson U5, U5i Vivaz, U5i Cosmic, U5a Vivaz, and 27 additional models. It slots into the same battery bay as the original and connects to the same charging circuit. Dimensions are 42.30 × 45.00 × 6.35mm — verify yours before ordering if the back cover has been replaced.

• U5 series fit explained: The U5 family shares a common battery bay and BMS handshake across the Vivaz, Cosmic, and base U5 variants. All use the same connector pitch and the same EP500 form factor, so one cell covers the entire line without adapter modifications.
• Bench tested on actual hardware: We cycled this cell through charge and discharge on a U5i Vivaz to confirm BMS communication. The charge IC accepted the cell cleanly, the protection circuit triggered correctly at low-voltage cutoff, and the fuel gauge did not flag an unrecognised cell error.
• Fuel gauge recalibration on first use: On the first cycle after installation, disable any fast-charge mode and run one full discharge to near-zero before charging to 100%. This gives the fuel gauge IC one clean reference cycle against the new cell's discharge curve before high-current charging begins.

Sudden shutdown at 20–30% on the EP500 replacement cell

This is a voltage cliff problem, not a capacity problem. Under peak load — camera shutter, active data connection, or screen at full brightness — the U5's modem and display draw a combined current spike the new cell cannot sustain if the fuel gauge IC is still calibrated to the old degraded cell curve. The voltage drops faster than the gauge predicts, and the phone shuts down to protect the circuit before the percentage display catches up. Run one full discharge-charge cycle without fast charging, and the fuel gauge recalibrates its discharge curve to the new cell. After that cycle, shutdowns at 20–30% stop in nearly all cases.

Phone won't power on after EP500 sat in storage for months

Li-ion cells left uncharged in storage self-discharge slowly over weeks. If the cell drops below approximately 2.5V, the BMS enters lockout mode and refuses to pass current to the phone — the device appears completely dead. Plug the phone into a wall charger, not a PC USB port, and leave it for 20–30 minutes without pressing the power button. A wall adapter delivers enough trickle current to push the cell above the BMS re-enable threshold; once it crosses that floor, the phone will boot. If the charge indicator still does not appear after 30 minutes on a wall adapter, check that the charger output is at least 5V 850mA.