I would never have guessed that this would be as hard as this, though, with the right parts, it was very straight forward. This document refers to an Apple iMAC Bondi Blue and may not apply to other models.
If you suddenly find that you are living in the 1950s and your friends are telling you that your Mac reminds them of black and white television then it is possible that your clock battery has run out of juice.
When you unpacked the box all those years ago, you did remember to keep the little booklet telling you how to replace it didn't you? Assuming that you did, I won't go into that much detail, however there is one thing it doesn't tell you—what type of battery it needs. Like a good geek I did a search using Google but with very little sucess. I got some Apple part numbers 742-0011, 922-1262 and 922-4028 but no one that would supply one. I also managed to pick up a picture and a description which was “3.6V ½AA Lithium”. This looked straight forward so I got out my engineer’s 6" steel rule and an old AA battery and measured it. 14mm diameter by 49mm long. Half of that and looking at the Maplin catalogue online (the life saver for electronics freaks in the UK) and found part number QE48 lithium battery type ER3 14.5mm x 30.5mm—not quite exact but close enough. But it did say “3.6V ½AA Lithium” on it so it must be the right one.
Next drive 200 miles to Mother-in-law’s house to fit the battery (it was her machine, and actually this was some months later). Late start on Saturday and take the iMac to pieces. This was straight forward but not something that would be done by novices as the instrucions are full of “danger of death-and-destruction” warnings. When it was all in bits on the floor, PANIC. The battery was too long for the holder.
Now, this is time for a sanity check. How can the manufacturers label two different size batteries with the same caption “3.6V ½AA Lithium” The one that came out was clearly only 25mm long and the ER3 was nearer 30mm.
Do I try to find an Apple dealer in North Wales at 12:00 on a Saturday (no chance, nearest one is in Chester, guv, and we don't know the number, try Google. Well, yes, I would but the machine is in bits on the floor. The one in Warrington wasn't answering the phone and was too far away in any case). Second option—cut the holder to get the battery in. This may have worked but really a last resort idea. Third option, look in yellow pages for photographic dealers and try to locate another battery. They are remarkably thin on the ground and the best was likely to be in Chester anyway.
Shuffling through the yellow pages I also discovered there was a Maplin branch in Chester so, as it was a nice day for a drive, and I could avoid the roadworks on the A55 Expressway I sauntered through the country and round to the back entrance for Chester (from the point of view of the Welsh that is).
I had a look at the display stand, but there was nothing like it there, the specialist stuff must be in the back room so I put on the ignorant act for the assistant—and he was extremly helpfull. We looked through the paper catalog, someting I haven't used for some time, and discovered there was another identical looking battery called the ER3S (ER3 Short?) part number QE49. This is just 5mm shorter than the other one and is helpfully captioned in the book as "useful for Apple computers" I'm sure that it wasn't there when I looked as I would have seen it, but that was a few months earlier. Part exchange new battery (actually it was cheaper than the other so I made up the difference with some utility batteries), thanked him profuesly and left to go back again. Congratulations to Maplin and their staff. After a total trip of about 2½ hours leisurely driving (and one emergency stop testing the ABS when a horse box pulled out unexpectedly) and the family found me crawling round the floor putting the system all back together again. This is very straight forward so long as you get the guide rollers on the CD-ROM drive behind the rails. plugged it in and it worked first time.
Moral of the story—the internet is good, but only if the information is out there—hence this page for the benefit of future searchers.