Question Laptop CMOS battery non-existant?

[Myronazz]

Hello,

I've managed to get myself a nice and cute MS-DOS laptop from somewhere around the early 90s or late 80s, unfortunately as someone would expect, the CMOS battery is dead, and the worst part? The laptop does not remember CMOS values even if plugged in at all times!!!! And the even worst part? The laptop requieres HDD parameters to be entered in manually through the BIOS, without those, it doesn't see the HDD, so essentially if it can't remember CMOS, it ain't booting! which is such a shame

I have soldering experience, and know how to replace a battery even if soldered to the board, however that laptop even though I took it apart has none, the only place I didn't look was under the board, which needed a special tool to remove a screw, but looking under it via a rough angle, I don't really see anything other than a bunch of ICs, will open it later on this week to confirm

So then how was the laptop supposed to remember CMOS values? My guess is that it was using the actual main laptop battery itself, which was a NIMH based one, and is long dead, so that really sucks, and there is no way im gonna find a replacement battery online, there is barely any information on the laptop to begin with, it's a Mitac, ever heard of them before?

So here is my question, do you think it would be a good idea, to simply solder in a 5 volt battery to the battery contacts? Well actually, as im writing this I realized something, that would be a very stupid idea, because if I plug in the laptop to use it, it would try to charge the 5v battery with 16 volts, and explosions would happen really fast, unless I use a MOFSET somehow maybe?

I don't really know where else to turn to, so i'll just post this here, maybe a magical genious will help me

 

[jsmithepa]

U should just break down and remove the Mobo to look for the batt underneath, what u gotta lose, break the relic?

 

[Myronazz]

Sorry for the late reply, but you are probably right, although I wanna make sure the thing doesn't get destroyed

Anyway, i've got some idea, it's kind of a long shot, but does make sense, assuming the CMOS chip isn't proprietary, then theoretically, I can have a look at every IC in the board and attempt to find the one related to CMOS, and find which pins accept power and inject there by soldering a 4v battery to it, even if it indeed relies on the main battery, it probably splits that battery's volts somewhere in the board's circuit and regulates it to just 4 volts for the CMOS, so in theory, this is very much possible, the hard part? finding which IC it is and then being lucky enough to find a datasheet so we can identify the correct pins

In the meantime, the only IC I can identify as potentially the one releated to the CMOS part of the circuit is this one

Anyone knows anything about it? It's the closest thing I can think of, but I sure hope this thing ain't surface mounted to the board