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CMOS battery voltage for external battery (486)

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First post, by digistorm

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Hi, I replaced the CMOS battery from my 486 board (Soyo SY-25K2) with a battery box with 3 AA lithium batteries. It had originally a Varta NiCD battery of 4.5 V. I connected the battery box to the designated connector.
At first everything looked fine, but yesterday I noticed that the clock was running behind, and tonight the BIOS lost all it's settings. Is it possible that I need a 4 x AA battery box delivering 6 volts? Or could something else be wrong? It did retain BIOS information for a short time, because the PC had been off before (but maybe not as long?). I also connected it as described in the motherboard manual, pin 1 positive, pin 4 negative.
If 4.5 V is correct, I should look into bad connections or whatever.

Reply 1 of 7, by Horun

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The picture from the web shows a standard 3.6v Nicad barrel battery on the same SY-25K2 board, not a 4.5v. Yes pin 1 = +, pin 4 = -. Are your Lithium batteries 1.2v or 1.8v ? If they are 1.2v then yes you need 4 (for 4.8v). If they are the very new 1.8v type then you only need 3. The external battery should be about 0.5v to 1.0v more than the old onboard. added: Perhaps someone replaced some parts (diodes, battery) and it now requires a higher voltage but that would be rare. Can you post a good close picture of the battery area please ?

Hate posting a reply and then have to edit it because it made no sense 😁 First computer was an IBM 3270 workstation with CGA monitor. Stuff: https://archive.org/details/@horun

Reply 2 of 7, by digistorm

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This is the area around the CMOS battery:

cmos battery.jpg
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And these are the batteries that I used:

IMG_7514.JPG
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1.05 MiB
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Reply 3 of 7, by digistorm

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Addition: After power off, the clock only seems to have run for about 5 or 10 minutes. After about half an hour of power down it run behind 25 minutes or so. The settings have stuck for now, but they were last this morning.
Also, the board was never modified in any way, I owned it ever since I bought it 25 years ago (this was my first pc bought from my summer holiday job).

Reply 6 of 7, by digistorm

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maxtherabbit wrote on 2020-03-08, 13:55:

you need 4, I had the same issues when I tried to use less of them on my board the clock just didn't' tick

Thanks, I’ll order a different box and give it a try.

Reply 7 of 7, by digistorm

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Quick addendum:

Using a 4xAA box didn't work either, and when I measured the voltage across 4 lithium cells, it was 6.9V! With 3, I got 5.2V, seems more than enough. I then measured if there was any voltage on the mainboard… and nope 😐 Seems like the Arduino jumper connectors I bought don't fit or are just bad junk. I don't know. Because I don't know how to obtain a proper 4-pin battery connector from a local supplier I just brutally soldered the leads of the battery box to the header pins. And well, that works! 😁

So for all the other owners of a Soyo SY-025K2 mainboard: it works fine with 3 Lithium cells on the external battery connector. Just make sure you have it properly connected.

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