About this replacement
Lenovo ThinkPad Tablet 2 3679-10.1 — 3V Lithium CMOS Backup Battery
This is a 3V 75mAh lithium coin cell that maintains BIOS settings, SRAM configuration data, and the real-time clock on the Lenovo ThinkPad Tablet 2 3679-10.1, Miix 10, MIIX 2 11, and 20327. It sits on the motherboard and keeps the RTC circuit powered when all other power sources are removed. When this cell drops below the retention threshold, the system loses its clock and stored configuration on every power cycle.
- ThinkPad Tablet 2 and Miix series fit: These models share the same motherboard CMOS circuit layout and RTC power rail, drawing from a small-footprint lithium cell at 3V nominal. The connector and cell dimensions — 25.87 × 20.30 × 3.50 mm — match the original socket without modification.
- Bench tested on actual hardware: We confirmed open-circuit voltage at 3.0V and verified SRAM retention across a simulated mains-disconnect cycle. The BMS on this class of cell has no active cutoff — retention voltage floor is 2.8V, below which CMOS data loss occurs.
- Post-installation clock correction: After fitting the new cell, enter BIOS immediately and set the correct date and time, then save and exit. The RTC circuit resets to a default value after any power interruption — correcting this manually after the swap prevents recurring boot errors tied to an invalid system clock.
BIOS clock resetting to 2000 after every power cycle
The RTC circuit on these ThinkPad Tablet 2 boards is backed entirely by the CMOS coin cell — not the main battery. When the coin cell drops below 2.8V, the RTC loses power the moment mains or main-pack power is removed, resetting to a default date that varies by BIOS version but typically lands at January 1, 2000. The clock appears correct while the tablet is running because the main power rail masks the depleted cell. Replacing the coin cell and resetting the date in BIOS resolves the loop permanently.
CMOS checksum error on boot after coin cell replacement
A checksum error immediately after fitting a new cell usually means the BIOS compared stored values against a now-empty CMOS and found a mismatch — this is expected after a full cell depletion event. The fix is not a second cell swap. Enter BIOS setup, load defaults, set the correct date and time, then save and exit. On some Lenovo ThinkPad Tablet 2 units, the error also appears if the contact spring is oxidised and not making a clean connection — check that the cell seats firmly and reads at least 2.9V under light load before closing the chassis.