Editing Battery Refurbishment (section)

=== Coin cells, CMOS and RTC batteries ===

Many vintage machines keep their configuration in CMOS RAM backed by a coin cell soldered or "pigtailed" to the board. When the OEM part is long gone, the fix is to rebuild the assembly around a '''tabbed''' coin cell.<ref name="ifixit">iFixit community guide, [https://www.ifixit.com/Guide/How+to+rebuild+a+laptop+CMOS+battery+-+pre+tabbed+cells/218330 "How to rebuild a laptop CMOS battery — pre tabbed cells"]. Source for the tabbed-cell approach, the CR2016/CR2025/CR2032 substitution note, the heatshrink sizing for CR20xx cells, and the warning about bending tabs.</ref>

* '''Buy pre-tabbed cells.''' A tabbed cell lets you solder to the tab, well away from the cell body. Soldering to a bare coin cell can make it vent or explode.
* '''Coin cell numbers are dimensional too.''' In a four-digit code the first two digits are the diameter in millimetres and the last two are the height in tenths of a millimetre, so '''CR2016, CR2025 and CR2032 are all 20 mm across''' and differ only in thickness — 1.6, 2.5 and 3.2 mm.<ref name="iec" /> The '''CR''' prefix denotes lithium manganese dioxide chemistry at 3 V nominal.<ref name="iec" />
* '''A CR2032 replaces a CR2016 or CR2025 if there is physical room''', and gives the longest life; a CR2025 substitutes for a CR2032 where space is tight, at the cost of runtime.<ref name="ifixit" />
* '''Use vertical tabs where possible, and never bend a tab to fit''' — a folded tab can short across the cell body.
* '''Re-sleeve the finished cell.''' Heatshrink of 19.1–19.2 mm inner diameter fits a CR20xx cell. All bare metal must end up covered.
* A dead CMOS cell reads a few millivolts or 0.00 V on a meter. On many machines a working main battery masks a dead CMOS cell, so remove the main pack and hold the power button for 30 seconds before testing.