Old CMOS Batteries

An odd observation surfaced while adventuring with vintage computers. The CMOS battery on the 486 computer is not dead.

When continually tinkering with the system the date and time never needs correction. That implies the battery is still recharging.

How long does the battery last without recharging?

The first test was about three to four days. After powering on the date and time was accurate to within several seconds.

Next was a 14 day stretch. No meaningful change.

Next was a monthly stretch. The date was correct but the time had drifted by about four minutes slower.

Two months before testing again. The date was correct but the time had drifted by about nine minutes slower.

A three month wait to test again found the date had drifted slower by 12 days.

How often does the system need to be powered on to keep the battery charged? Until the day when the battery fully depletes, seems the battery is dependable up to one month but can be pushed to between two and three months.

Common with 486 systems, the battery is a barrel battery and soldered to the mainboard. There is a 3-pin jumper next to the battery to use an external battery. The on-board battery is disconnected when the jumper is configured to use an external battery.

Usually these batteries have a nominal rating of 3.6 VDC. Inside the “barrel” wrapping are three 1.2V nickel-cadmium (NiCad) cells. At full charge the measured voltage is about 4.2 VDC (1.4V/cell).

A common recommendation is to replace barrel batteries because historically they tend to leak corrosively.

There were many years when the computer was powered on only once or twice per year, if at all. That might explain the long life. Yet these tests reveal the battery has an unusual life span.

Much like finding remarkable that 20 to 30 year old floppy disks could be archived, remarkable too is this battery remains ticking after more than 30 years.

Posted: Category: Usability Tagged: General

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