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CMOS Battery Replacement
The original IBM PC 5150 did not have any on-board RTC (real time clock). ISA expansion cards could be added that did have this feature (and those cards would have their own on-board battery, that would also eventually need to be replaced). So, on very original systems, with no fixed disks and booted straight from floppy – we were forever entering the current time during startup.
But by the late 1980s, the inclusion of a RTC was generally standard practice. Like wristwatches, these typically used “button” style lithium batteries (but alternative options were also possible). These were intended to last only 5-10 years (depending on its typical power on/off usage). Some have lasted 20 years, but after 30+ years it is a good bet that original CMOS batteries need to be replaced.
For some systems, their BIOS will be programmed such that they will refuse to start if there is no CMOS battery. Other systems are forgiving and will start with a default configuration that is typically suitable (or let you manually set those settings, each time the system is booted). This CMOS battery stores both CMOS-pertinent settings, as well as the current date/time.
Luckily my 386 here has one of those “DALLAS” style RTC chips and it’s socketed. The battery is built directly into this chip. There is a way to cut into the IC and wire in a replacement battery. But if the chip is socketed: these DALLAS IC’s are still available (as of 2023/2024, the year is two digits prefixed on the label – so you may want to avoid the ones older than about 2012). But if the chip is socketed, it’s a better choice to replace it with a new IC that has an external battery.
Here are two common modern-day choices:
- Search “Dallas DS12887A, DS1287 Replacement W/ Battery” by retrotroniks_com (despite the photo, they do come assembled)
- Search “DS12887 RTC Drop-In Replacement” by nz-tdm
It can be difficult to tell which way is PIN1. But all I can say is, I tried both ways and didn’t blow up the system. One way works, the other doesn’t. Still, it would be more prudent to try to verify first. These generally use easy-to-find CR2032 3V cell batteries.
If the 286 or 386 board you’ve found doesn’t have a socketed DALLAS RTC chip – you can consider using an external serial I/O card. Consult the motherboard manual to disable the onboard serial, parallel, and RTC (if any), and the serial expansion card should seamlessly take over those ports and IRQ. Because I damaged the onboard parallel port of this 386, I had to resort to using an external 8-bit ISA card (that is, external to the motherboard and inserted into one of the ISA expansion slots). But I did disable the RTC of the serial card and kept on using the DALLAS RTC on the mainboard.