Hi Pat;
Funny you should ask . Last week someone stumbled on my website by chance and emailed me about that (this was before I joined this group, suggested to me by an eBay buyer).
PRAM (parameter RAM) stores things like date and time on a Mac, and the mobo has a battery or cell to save them when the computer is unplugged (just like a PC and its BIOS settings).
Yes, as you know, most Macs (from Classic, LC II, III, 475 to G5) use a 3.6V Lithium ion half-AA size cell. These are still freely available from electronics stores (such as Jaycar online), so I can get them ok, for $6 to $9 each, though you might find a better deal shopping around or buying in bulk. I don't try to make these, but maybe a battery holder, 3x1.2V NiMH cells and attachments (like alligator clips) to the mobo connector terminals might do fine.
However, I'm talking about the 4.5V rectangular block "battery" consists of 3 x 1.5V cells, and is really like a half-9V transistor battery. When I found out how rare these are ($15US or more to order), I decided to try something homemade, though I'm not good with electronics or soldering. There are a select range of 040 to G2 (mainly the all-in-one models [LC 575, 580, Performa 5000 series] but also the 4400-7220 (which plays "dead" when the pram battery is flat), LC 630 and others) Macs which use this battery.
My first approach was to dissect a 9V transistor battery and see if I could "re-pack" an existing 4.5V battery with 3 cells from one of those..... too messy, I decided. Then I went for the option of carefully soldering 3 x AA or AAA cells in series, and soldering the connecting wires from the Mac battery (just snip them off an old battery) on, red to the +ve, black to the -ve. If you wrap the three AAs together with good tape, then use the sticky velcro off the old battery too, it will sit in place on the motherboard ok. Have done this with about 10 Macs now, and if you use good alkaline AAs, it will last 1-2 years.
I have been cautioned about taking too long to solder the wires onto the AA terminals, as it can heat up the battery a fair bit. So far, I've had no problems, but light abrassion of the ends of the AA cell can help the solder "take" better. Short bits of tinned copper wire work well, but insulated copper wire is ok. I could send you a photo of a finished product later. The battery-holder option for this might be better, I should try it, that would eliminate the need to solder onto the AA cells themselves, you'd only need to solder the + and - wires to the battery holder, and it would be reusable. I'll look out for a 3xAA battery holder on Jaycar etc.
I'm just trial and erroring with this, and am no electronics expert, so some advice and suggestions would be appreciated.
cheers, -Daniel
Hi Daniel, "Home-made" PRAM battery? vas ist das? Regards, PatC.
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