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Most absurd CMOS battery ever?

September 6th, 2010 (12:16 pm)

I have a Performa 6400 PowerMac, which I bought off a friend several years ago. It runs Linux and I use it for portability testing (as it's a big endian machine). Considering its age, it's not surprising that the battery for the onboard clock ran out years ago. It's not a huge problem, but it's certainly annoying to have make complain about modification dates in the future, and be confronted with "filesystem has not been checked for 19370 days" on boot-up. So I decided to replace the battery.


This is the battery that is supposed to go in it. Almost £15 for a battery! This seemed far too expensive to me, so I set out to find an alternative. Unfortunately 4.5 volt batteries are rather uncommon. However, I managed to find this, which was listed as a "lantern battery" used for torches, bike lamps and doorbells. I suspect that there are probably 3 AA batteries inside:




- Greencell 312G 4.5V lantern battery alongside the original (depleted) CMOS battery.


I did a quick sanity check of the old battery to make sure I had the polarity right. The voltage is down to 1V:




All that needs to be done now is to connect the connector from the old battery to the new one. I've always been hopeless at soldering, but fortunately this is a job that is simple enough for sellotape.




This is the logic board from the Mac. I've maxed out the RAM and added PCI Ethernet and USB cards. The black square at the bottom left is where the battery is supposed to be attached (it has a velcro strip at the back).




Obviously this is far too small for the new battery, and the card slides into the back of the machine, mounted vertically. It has to be attached to the board somehow. Where can it go?




Fortunately, the engineers at Apple were apparently smart enough to anticipate this very problem, and designed a convenient space on the riser card to put the battery in.