Repair guide

Deal with a leaked CMOS or RTC battery on an old PC board

286 to early Pentium PCs often carry a barrel-style rechargeable cell or a soldered lithium cell that has since died or leaked onto the board. Clean the residue before it spreads, then fit a battery arrangement that is easy to swap.

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Battery · Careful · about 90 minutes. Worth doing. Reports say this fix helps on many machines.

You are in the right place if: The BIOS says the CMOS is invalid or forgets its settings, and you may see crust or green stains next to the battery.

Before you start

Leaked residue irritates skin and eyes. Wear gloves and glasses.
Some boards charge the battery. Do not put a non-rechargeable cell in a charging circuit without checking your board revision.
If several chips are corroded, stop and have the board assessed rather than powering it up.

Board revisions and models

Barrel NiCd cells, soldered coin cells and RTC modules (with built-in batteries) are all used across this era. Identify yours before choosing a fix.

Parts you may need

Battery holder or off-board battery pack. Keeps a leak off the board Amazon · eBay
Replacement lithium coin cell. Common modern cell Amazon · eBay
Thin jumper wire. Repair damaged traces Amazon · eBay
Isopropyl alcohol. Clean-up Amazon · eBay

Tools

Soldering iron and braid. Remove the old cell Amazon · eBay
Brush and swabs. Clean residue Amazon · eBay
Magnifier. Spot trace damage Amazon · eBay
Nitrile gloves and glasses. Protect yourself Amazon · eBay

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Step by step

Disconnect and open the PC Unplug the PC and open the case, ground yourself and remove cards that block access to the battery.
Find the battery and photograph it Locate the battery (barrel cell, coin cell or an RTC module). Take a photo of its orientation and surroundings.
Check for corrosion Look for crust, green tracks and damaged solder near it. Pay attention to keyboard controller and BIOS chips if they are close.
Remove the old cell Desolder it carefully, or cut it away from its legs if stuck, and clear the pad holes.
Neutralise and clean Gloves and eye protection on: dab white vinegar on alkaline residue, scrub gently with isopropyl alcohol and a soft brush, and dry fully.
Repair traces Under magnification, check traces and vias and bridge any breaks with thin wire.
Add a safer battery arrangement Fit a socket or an off-board holder so the new cell cannot leak onto the board, matching polarity and voltage to what the board expects.
Test the clock and setup Power up, enter setup, set date and time and any drive settings, save and power off for a while to check it keeps them.
Recheck after a few days Look at the area again later and confirm the new cell is fine and the board is dry.

Tips, tales and maybes

Found in write-ups and forum threads. These are other people's reports, not tested by us.

Story (Medium confidence). An original Compaq Portable and a 386 were brought back by replacing failed tantalum capacitors and cleaning up battery corrosion. Source
Maybe (Low confidence). If the battery is part of a sealed RTC module, the common approach is to replace the module or bypass its battery; confirm the method for your exact chip before cutting anything. General technique, no source.

Guides we used

We wrote the steps in our own words from these. They have the photos and the full detail, so please read them and support their authors.

Reviving some 80's electronics (Forum) by PhilA and EEVblog members. An original Compaq Portable and a 386 recovered by replacing failed tantalums and cleaning battery corrosion.

Boards and models vary, so check what is in front of you. A repair can involve hot tools, chemicals and, on some machines, dangerous voltages. You do it at your own risk.