Macintosh II & IIx replacement battery holders

[wottle]

I've been wondering, in the official battery upgrade from apple, the batteries are situated on a little riser board, and there is a capacitor installed as well. What is the purpose of that capacitor? It's not present on the original logic board, so obviously some in engineering thought it would help somehow.

I did notice that on a second Mac II I ended up getting. The odd thing is that Apple didn’t continue that on the IIfx. If it was that important, it feels like they would have changed it going forward.

A side note on my machine that was having issues, I ended up selling recently but before I sold it I was able to get it to reliably boot using very well charged aaa batteries in a 3 x AAA battery holder. I was probably using 1/2 AA batteries that were a bit low and it did not like it.

 

[lilliputian]

That's great to hear that there is an alternative to the Safts!

As for the riser board, the II and IIx had the batteries next to each other, wheras on the IIfx, they're far separated. I wonder if that had something to do with it.

 

[Branchus]

I've been wondering, in the official battery upgrade from apple, the batteries are situated on a little riser board, and there is a capacitor installed as well. What is the purpose of that capacitor? It's not present on the original logic board, so obviously some in engineering thought it would help somehow.

I think the party line is that the capacitor powers the PRAM for a small amount of time then the batteries are removed and replaced without losing the settings.

 

[David Cook]

I think the party line is that the capacitor powers the PRAM for a small amount of time then the batteries are removed and replaced without losing the settings.

Your comment was thought provoking. So, I just did an investigation and measured it. The problem with that capacitor is that it is wired across both batteries, but only one battery powers the PRAM/clock chip.

If you replace each battery individually (leaving the other battery in place), then you do indeed get over a minute of PRAM retention from the added capacitor. However, if you pull both batteries at the same time, then the PRAM contents are lost almost immediately. I'll do a write up.

I suspect the intended purpose of the capacitor is to stabilize the power-on circuit voltage at cold start.

 

[David Cook]

Here are more details:

https://tinkerdifferent.com/threads/macintosh-ii-iix-battery-holder-daughterboard-analysis.4978/