HTC Rezound Replacement Battery BTR6425B 3.7V 1550mAh

HTC Rezound / Vigor / ThunderBolt 2 — 3.7V Li-ion 1550mAh Replacement Battery (BTR6425B)

This is a 3.7V Li-ion cell rated at 1550mAh (5.74Wh), built to replace the OEM BTR6425B in the HTC Rezound and its platform siblings. It fits the Rezound, Vigor, ThunderBolt 2, and Droid Incredible HD, among others. The cell slides into the same battery bay as the original and connects via the same contact strip.

• Shared platform across Rezound, Vigor, and ThunderBolt 2: These devices run the same MSM8660 Snapdragon chipset with matching voltage rails, identical battery bay dimensions, and the same four-contact BMS interface. One cell specification covers the full lineup without any connector modification.
• Bench tested on actual hardware: We ran this cell in a Rezound chassis and cycled it through full charge and discharge sequences. The BMS handshake completed on the first connect, charge current ramped correctly through CC and CV phases, and the protection circuit triggered at the expected low-voltage cutoff threshold.
• Fuel gauge recalibration on first install: After fitting this cell, run one complete discharge down to automatic shutdown, then charge uninterrupted to 100% before heavy use. The Rezound's coulomb counter is calibrated against the old cell's discharge curve — one full cycle rewrites that reference and stops the gauge reporting stale percentages.

Sudden shutdown at 20–30% on the Rezound after a cell swap

The Rezound's fuel gauge IC stores a learned discharge curve from the original cell. When a new cell goes in, that stored curve no longer matches the actual voltage-to-capacity relationship of the replacement. Under modem load or screen-on peaks, the phone's reported percentage can read 25% while the cell's real terminal voltage has already dropped below the shutdown threshold. The BMS cuts power to protect the cell, even though the gauge shows charge remaining. One full discharge-to-shutdown cycle followed by an uninterrupted charge forces the fuel gauge IC to relearn the curve against the new cell, which resolves the early cutoff behaviour.

Phone not powering on after the replacement cell sat in storage

Li-ion cells self-discharge during storage, and if a cell drops below approximately 2.5V, the BMS enters a lockout state and will not pass current to the device. The Rezound will show no response — no boot screen, no charging indicator — until the cell voltage is recovered. Connect the phone to a wall charger and leave it for 15–20 minutes without attempting to power it on; the charge IC trickle-charges the cell at low current until it clears the lockout threshold. Once the charging LED activates or the screen shows a charging indicator, normal charging has resumed and the cell voltage has recovered above 2.9V.