Acer Aspire 4720 CR1220 CMOS Battery 3V 200mAh

Acer Aspire 4720 / 4720z — 3V Lithium Replacement Battery (CR1220)

This is a CR1220 lithium coin cell, 3V, 200mAh, for the Acer Aspire 4720 and 4720z motherboard. It sits on the motherboard and powers the RTC circuit and CMOS SRAM when mains power is removed. When this cell depletes below the 2.8V retention threshold, the laptop loses its clock and BIOS settings on every power cycle.

• Aspire 4720 and 4720z compatibility: Both models share the same motherboard CMOS socket and draw from the same RTC circuit. The CR1220 footprint — 12mm diameter, 2.0mm height — is consistent across both variants, and the BMS handshake is not involved; this cell powers a passive retention circuit only.
• Bench tested on actual hardware: We measured open-circuit voltage at 3.05V before fitting. After seating the cell and cycling mains power, the BIOS retained date, time, and boot order across multiple cold starts with no CMOS checksum errors thrown.
• Post-installation BIOS step: After fitting this cell, enter BIOS immediately and set the correct date and time, then save and exit. The RTC circuit resets to a default value during the swap — even a fresh cell will not restore whatever date the old one was holding.

BIOS clock resetting to 2000 after every power cycle

When the CR1220 drops below 2.8V, the RTC circuit loses retention voltage the moment mains power is cut. The BIOS has nowhere to read stored time data from, so it falls back to a hardcoded default — typically January 1, 2000. This is not a BIOS fault and reflashing will not fix it. Replace the coin cell, then set the correct date in BIOS and save before closing the lid.

CMOS checksum error on boot after fitting a new CR1220

A checksum error after a new cell install usually means the CMOS SRAM was fully cleared during the swap — the old cell was already dead, so removing it wiped whatever was stored. The new cell is fine; the SRAM just needs to be written again. Enter BIOS, set date and time, confirm boot order and any custom settings, then save. The checksum recalculates on exit and the error will not reappear.