Repair guide

Replace the Mac PRAM battery and clean up a leak

A flat PRAM battery makes older Macs forget their settings, and a leaking one can damage the logic board. Replacing it and cleaning any residue early is among the cheapest ways to protect a compact Mac or a II-series.

Open the interactive checklist Tick off each step as you go. It saves in your browser.

Battery · Medium · about 60 minutes. Worth doing. Reports say this fix helps on many machines.

You are in the right place if: The Mac shows a flashing question mark or wrong date and settings after being switched off, or there is crust around the battery holder.

Before you start

Compact Macs contain a CRT with dangerous stored voltage. Never open the CRT or touch the anode area; stop at the logic board.
Leaked residue irritates skin and eyes. Wear gloves and eye protection.
If the leak has reached the logic board connectors or chips, stop and consult a repair specialist.

Board revisions and models

Battery type and holder differ between the 128K to Plus, SE/SE 30, Classic and the II series. Check your model's service notes for the right cell.

Parts you may need

Replacement PRAM battery for your model. Check the type your logic board uses Amazon · eBay
Battery holder. Useful on boards where the holder is damaged Amazon · eBay
Isopropyl alcohol. Clean-up Amazon · eBay

Tools

Small screwdriver set (Torx T15 for compact Macs). Open the case Amazon · eBay
Soldering iron and desoldering braid. If the battery is soldered Amazon · eBay
Soft brush and swabs. Clean residue Amazon · eBay
Nitrile gloves and safety glasses. Leak residue irritates skin and eyes Amazon · eBay

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Step by step

Unplug and open the Mac Disconnect power and everything else, then open the case following a guide for your model. Compact Macs need care around the CRT, so only go in as far as the logic board.
Locate the battery Find the battery holder on the logic board and take a photo of its position and polarity before removing anything.
Check for leak damage Look under and beside the battery for white or green crust, dull solder and eaten traces. If there is damage, plan a clean-up before refitting anything.
Remove the old battery Take it out of the holder or desolder it if it is soldered in. Do not short its terminals.
Clean any residue With gloves on, dab a mild acid such as white vinegar on residue, scrub with isopropyl alcohol and a soft brush, and dry fully.
Inspect and repair Look at traces and pads under magnification and bridge any breaks with fine wire.
Fit a new battery Fit the correct replacement for your model (check the battery type your Mac uses) and a holder where one is missing.
Close up and test Reassemble, boot, set date and time in the Control Panel and restart the Mac to check the settings stay.
Zap the PRAM if needed If the Mac misbehaves after a battery swap, reset the PRAM with the key combination for your system version.

Tips, tales and maybes

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

Maybe (Low confidence). Some people add a socket or remote holder so future swaps need no soldering. General technique, no source.
Tip (Medium confidence). A compact-Mac recap write-up shows how to deal with leaked logic board areas once the battery has been removed. 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.

Macintosh Classic logic board recap (Blog) by Matt Sephton (gingerbeardman). A clear logic board walkthrough on a Mac Classic (eight capacitors and a tantalum swap), no value list.
Macintosh Classic II (Blog) by Retro.Engineer. Classic II project page with photos of a leaked logic board (photos only).

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.