HP Omnibook 500 CMOS Replacement Battery 7.2V 20mAh Ni-MH

HP Omnibook 500 — 7.2V Ni-MH CMOS Backup Battery

This is the 7.2V Ni-MH CMOS backup battery for the HP Omnibook 500 laptop. It runs the RTC circuit and holds SRAM-based BIOS settings when main power is absent. Capacity is 20mAh (0.14Wh) — matched to the low-drain retention load this circuit draws.

• Omnibook 500 motherboard fit: The Omnibook 500 uses a rechargeable Ni-MH backup cell rather than a standard coin cell. The motherboard trickle-charges this cell from the main power rail. Voltage, connector, and physical dimensions — 41.32 x 12.42 x 7.52 mm — must match exactly for the charging circuit to function correctly.
• Bench tested on actual hardware: We ran this cell against the Omnibook 500 retention load and confirmed the BMS held settings across full main-power removal. RTC stayed locked through repeated cold-boot cycles with no CMOS error thrown.
• Post-install clock correction: After swapping the backup cell, enter BIOS immediately, set the correct date and time, then save and exit. The CMOS cell powers the RTC circuit — any interruption resets the clock to a default value. Until you correct it in BIOS, every timestamp and scheduled task on the system will be wrong.

BIOS clock resetting to a default date after every power cycle

The Omnibook 500 RTC circuit requires the backup cell to hold a minimum retention voltage to keep the clock running when mains power is removed. When the cell drops below that threshold, the RTC loses power and resets to a hardcoded default date on every cold boot. A cell that reads low on a multimeter but still shows some voltage is not enough — retention requires sustained voltage across an extended discharge. Replacing the backup cell and resetting the date in BIOS is the only fix.

CMOS checksum error on boot after the backup cell is replaced

A CMOS checksum error means the settings stored in SRAM no longer match the checksum the BIOS calculated when they were last saved — typically because the SRAM lost power during the cell swap. This is expected behaviour after any backup cell replacement on this platform. Enter BIOS setup immediately after the first post-swap boot, re-enter all settings including date, time, and boot order, then save and exit. The error will not reappear as long as the new cell maintains retention voltage.