About this replacement
HP iPAQ RX1900 / RX1950 / RX1955 — 3.7V Li-ion Replacement Battery (PE2018AS)
This 3.7V, 1200mAh Li-ion cell replaces the original battery in the HP iPAQ RX1900, RX1950, and RX1955 pocket PC handhelds. It matches OEM part numbers PE2018AS, 395780-001, HSTNN-H09C-WL, 398687-001, 399858-001, and 35H00063-00M. The cell fits the same physical bay and connects to the same three-pin contact as the factory unit.
- RX1900 / RX1950 / RX1955 platform fit: All three models share the same battery bay dimensions and three-pin BMS contact layout. The voltage rail, connector pitch, and thermistor pin are identical across the series, so one cell covers all three.
- Bench tested on actual hardware: We ran this cell on an RX1950 unit and confirmed the BMS handshake completed correctly at first connection. The device read battery status accurately in Windows Mobile and charged without error through the cradle.
- RAM data protection before swap: Perform a full ActiveSync or Windows Mobile Device Center sync to your host computer before removing the old battery. The iPAQ stores open files and application state in SRAM powered directly by the main cell — cutting power without syncing first will erase unsaved data.
Why the iPAQ RX1900 loses all data when the battery goes flat
The RX1900 uses SRAM for working memory rather than flash-backed storage. SRAM requires constant voltage to retain data — drop below roughly 3.0V and the contents are gone. Unlike a modern smartphone, there is no background save to internal flash when the battery drains. A battery that has degraded to low capacity can cross that threshold quickly under normal use, wiping open applications and contacts without warning. Keeping the cell charged above 3.5V is the only way to protect in-memory data between syncs.
iPAQ shows "Charging" but percentage stays at zero after new battery install
A deeply discharged Li-ion cell can sit below the BMS activation threshold — typically around 2.5V — which causes the device to report charging without the percentage moving. The BMS locks out full current delivery until the cell recovers to a safe voltage. Connect the iPAQ to the cradle or USB charger and leave it for 20–30 minutes without powering it on. Once the cell reaches approximately 3.0V, the BMS releases and normal charging resumes.