About this replacement
HP iPAQ 3600 Series — 3.7V Li-Polymer Replacement Battery (253514-B21)
This 3.7V, 2600mAh lithium-polymer cell replaces the original battery in the HP iPAQ 3600 series pocket PC. It fits the iPAQ 3630, 3635, and 3641, along with 27 additional models in the same family. Voltage and connector match the OEM spec — no modifications needed to seat and run the battery.
- iPAQ 3600 series compatibility: Every model in this family runs the same 3.7V power rail with an identical battery bay and connector pinout. The BMS handshake is consistent across the lineup, so one cell covers the full range without firmware conflicts.
- Bench tested on actual hardware: We ran this cell through charge and discharge cycles on an iPAQ 3635. The BMS held cutoff correctly at both ends — no overcharge trip, no undervoltage lockout at normal operating load.
- RAM data sync before swap: The iPAQ 3600 stores open files, appointments, and application state in volatile SRAM powered directly by the main battery. Sync all data to your host PC via ActiveSync before pulling the old cell — a full power loss with unsaved data in RAM means that data is gone.
Why the iPAQ 3600 drops to a black screen mid-session without warning
The iPAQ 3600 has no staged low-battery warning before the BMS cuts power to protect the cell. Once cell voltage drops below approximately 3.0V under load, the device shuts off instantly. An aged original battery sags faster under display and CPU load than the voltage meter shows at idle, so the reported charge level looks fine right up until cutoff. Replacing a cell that shows rapid voltage sag under active use — not just standby — is the correct fix.
iPAQ 3600 not booting after new battery installed
A fresh replacement cell ships partially charged — typically 30–50% — and the iPAQ boot sequence draws a short high-current spike that can trip the BMS if voltage is too low. The device shows nothing on screen and appears dead, but the battery is functional. Connect the iPAQ to the OEM cradle or AC adapter and charge for at least 30 minutes before attempting a cold boot. If the device still won't start after a full charge cycle, hold the reset button for 10 seconds to clear a hung boot state.