About this replacement
Sony Clie PEG-UX40 / PEG-UX50 — 3.7V Li-Polymer Replacement Battery (UP553)
This is a 3.7V, 850mAh Li-Polymer replacement for the Sony Clie PEG-UX40 and PEG-UX50 PDAs. Both units share the same battery footprint and BMS handshake, making this a direct fit across the pair. Original UP553 cells in these early-2000s devices have typically lost 40–60% of their rated capacity by now.
- PEG-UX40 and PEG-UX50 compatibility: Both Clie models use the same flat Li-Polymer cell format, the same flex connector, and the same Palm OS power management stack. The BMS negotiation is identical across both, so one cell covers the pair without modification.
- Bench tested on actual hardware: We ran this cell through the Clie's charge cycle and confirmed BMS handshake at 3.7V nominal. The protection circuit tripped correctly at low-voltage cutoff, and charge acceptance reached full 850mAh capacity within spec.
- Sync before you swap: The Clie PEG-UX40 and UX50 hold application data, contacts, and open files in volatile RAM powered directly by the main battery. Pull the battery without first syncing to the desktop HotSync software and that data is gone — there is no secondary backup cell in these units.
Why the Clie PEG-UX40 loses all data when the battery goes flat
The PEG-UX40 runs Palm OS from ROM, but all user data — contacts, calendar entries, memos, installed applications — lives in SRAM. That SRAM needs continuous battery voltage to hold its contents. When the cell drops below the BMS cutoff threshold, even briefly, the SRAM loses power and wipes. There is no onboard flash storage backing up that data between syncs. The fix is a full HotSync to your desktop before voltage drops that low — not after.
Clie PEG-UX40 stuck on boot screen after new battery install
A freshly shipped Li-Polymer cell typically arrives at storage voltage — around 3.6V — which is below the minimum the Clie's Palm OS boot sequence needs to complete. The device starts initialising, hits the power floor, and stalls on the boot screen or resets in a loop. Do not assume the battery is faulty. Connect to the AC adapter and let the cell charge for at least 30 minutes before attempting to power on. A stable 3.7V or above clears the stall and lets the boot sequence finish normally.