Motherboard of a computer usually has an small cell or battery that keeps powered the BIOS chip, which stores the configuration parameters of our machines, the time and date.
Throughout the years, such a cell or battery, has been evolved making possible to find several types of them, such as:
1. Nickel / Cadmium Ni-Cd Batteries, Nickel Metal Hydride Ni-MH, especially onto the older motherboards from 8086 to some 486 microprocessors and Pentiums
2. Lithium battery, commonly known as button cell, usually placed on a socket (one), or several grouped and soldered on the motherboard.
These batteries are classified by size according to a numbering. Actually, the most widespread is CR2032, 3.0 v.
These batteries can be rechargables (so,you do not need replacing them, although if the PC hasn´t been running for too long, they may need some time to re-take the minimum load that causes the PC to boot) or non rechargables, which are the most usual.
When we have a dead battery (no charge on it), can be the case that the PC will not boot, or if it does, you get the time, date or any settings in the BIOS changed.
In this case, it should be replaced as soon as possible to avoid misconfigurations in the BIOS or not to start the PC.
The Ni / Cd and NI / MH battery, may even leaks acid on the motherboard on which is soldered, if not used for long time. Its replacement way is to unsolder it, clean the residual liquid (if any), and solder a new one.
The batteries that go into a socket, are easier to replace.
However, in motherboards of the first Pentiums, 486´s and older ones, this battery comes in a encapsulated package, something that makes it hard to locate, as this package is often confused with one of the many chips of the motherboard
These encapsulated (RTC,Real Time Clock) are usually composed of a chip, a quartz and a lithium cell or battery and can be different: soldered to the motherboard or inserted into sockets
Replacing the RTC (Real Time Clock) can be a problem, if soldered to the motherboard, as we need to unsolder it. If inserted into a socket, is easy to replace it, removing it carefully and replacing with one of the same type or compatible.
The problem comes in finding these encapsulated actually. Years ago, there were several manufacturers of these RTC:
Dallas, Houston Tech, Benchmarq, Odin and ST, although nowdays, I think the only one that still makes them is Dallas.
The most common models of RTC (also known as NVRAM) of some of these manufacturers are or were:
-DS12887A Dallas (which is compatible with models ODIN and ST below this lines),
-Tech Houston using the same nomenclature as Dallas
-Benchmarq BQ3287, compatible with the Dallas DS12887
-Odin OEC12C887A
-ST M48T86
Replacement in these situations is to find a compatible model and change it, taking care of the position of the pins (Pin 1 is usually marked as a small circle or an small chamfer close to it). Normally, this RTCs (Dallas and ODIN) are pin compatibles (within same models, of course) so I think this is the correct pinout for this chips:
Generally, these chips are available online with an estimated retail price of around 13 euros (16 dollars plus shipping). However, there is another way to fix our old pcs, when this battery is dead.
As I said before, a RTC consists of a chip, a crystal and a lithium battery. Generally, this battery is connected between pins 16 and 20 of the IC so, once located the battery, you can desolder it, put a new equivalent one, and your pc will work again.
EXAMPLES:
NOT SOLID RTC
If the RTC has a lid, that is not solid, you can simply lift the lid up, so the cell will be accessible for changing, as happened with this mobo of a 486.
If we have a Houston Tech RTC, it´s really simply modding it, just pull the lid up and you´ll see the battery exposed . Just unsolder the old battery and replace it with a new one
However, although in this case would have been more advisable to desolder and replace this battery with a new one, I took a little imagination and added a CR2032 socket, so, in the future,replacement will be much easier.
On the top of the RTC plastic lid I made two holes for the pins of the new socket with a cutter and carefully, I glued the socket on the lid and soldered two wires (+ and -), in that way:
a)Working on the plastic lid:
b)Working on the RTC:
SOLID RTC
If the RTC (NVRAM) is solid, I mean, you can not take apart the top of the chip, the only way to remove the top is, for example, with a mini Dremel, taking care of the height at which we will make the cut to avoid damaging the chip (usually you can see an small chamfer that makes the box not to be totally square so, cutting above the notch, there should be no major problem). Also we need to take care of the depth of cut near 16 and 20 pins, and near 1-2 and 23-24, as they tend to be bent upward to be soldered to the battery and the quartz, as we could cut them.
Afterward, with a flat screwdriver slowly remove the top (in my case, the top cover left easily, not the part surrounding the cell) to to see the battery.
As we can see in this case, the pin 16 (-) go inside the resin to
hollow core of the cell, while the pin 20 (+) is soldered to a metal contact. Carefully lift the metal contact up and took the battery out with a small flat screwdriver (in this case it is a CR1220 battery) replacing the new one.
Once the old battery was replaced, just use some fixig tape to hold the positive metal contact to the RTC and reassemble the whole chip in its socket on the motherboard
I found this type of RTC on an Pentium 166 motherboard ( EXP8661) and the RTC, in this case, was a OEC12C887A ODIN, whose replacement, if nedded, would be the Dallas DS12887A.
By the way, it seems that the “A” character at the end of the name, refers to those that are erase-password protected and can not be manipulated by normal users, while those who do not have the “A”, can be completely deleted by any user without a password .
Well, I hope this post is usefull for you and help on locating and replacing the battery on an old mobos.
Thanks ( And sorry if I made a mistake with my English).
