Legacy System CMOS Battery Dead

DeskRabbit · May 26, 2020

I have a motherboard that will not boot due to a dead cmos due to it not holding any bios settings as a result. Unfortunately it is not a cr2032 it's an old lithium block battery that appears to be directly soldered to the motherboard. Anyone have any ideas to work around this without having to solder the battery/motherboard? I am going to leave it on over lunch to see if it will charge up/hold a charge.

You can see the CMOS battery in the second picture.

mariushm · May 26, 2020

I suppose you could carefully drill into the top plastic or use a sharp blade to cut the case and hopefully the internals aren't full of epoxy (black goo) ... then either inject 3v from a separate power supply or replace the old lithium cell.

You could use a battery holder with 2 AA batteries and a diode in series to replace the lithium battery but you'd have to solder the battery holder wires...

fordy_rounds · May 26, 2020

I googled the part number (M48T86PC1)—it's got an integrated battery with the RTC. In other words, that black module isn't just the battery, it's also the clock, so you can't just replace it with a new battery. The only way to replace it, sadly, is to replace the entire module, which means desoldering the old one and soldering on a new one. It appears to be through-hole, so it shouldn't be too hard to do, though I don't know your soldering skill level.

Alternatively, a look at the datasheet indicates that Vcc is pin 24 and GND is pin 12 (unless you have the 28-pin version, then it's 27 and 13/14, but I don't think you do), so you could probably get an external 3V supply and connect it to those two pins, as @mariushm said. That's kind of jerry-rigging it though, if you can you'd be better off to just replace the whole chip.