The NVRAM battery is a little black box that contains the clock circuit and a small lithium battery. The most common manufacturers of that circuit they are Dallas, Houston Tech, Benchmarq, Odin and ST. That circuit is usually connected to the motherboard through a socket, facilitating its replacement. To replace that circuit, you have to buy a new one first. Dallas is the only manufacturer that still sells that type of circuit, and you can buy it over the Internet, at http://www.maxim-ic.com/. And here comes the big trick. Odin only manufactured one circuit, the OEC12C887A. If your motherboard has that type of circuit, buy one called Dallas DS12887A, which is 100% compatible. The same holds true for the M48T86 circuit from ST, which is 100% compatible with the Dallas DS12887A one. As for Benchmarq, that company was bought by Texas Instruments and at the site from Dallas there is a complete table of the compatibility, at http://www.maxim-ic.com/alternatives.cfm/show/TEXAS_INSTRUMENTS. In that table you will find which Dallas circuit corresponds to the Benchmarq one. For instance, if your motherboard uses the Benchmarq BQ3287 circuit, you can replace it directly with the Dallas DS12887 one, which is 100% compatible. The Houston Tech circuits use the same nomenclature as the Dallas ones. After buying the chip, all you have to do is to replace it (while your computer is off). You should carefully remove the old circuit using a small screwdriver or integrated circuit extractor. When installing the new circuit, pay attention to the marking of pin 1, that is, the side of the circuit that has a small ball or a chamfered on it it should coincide with the side of the socket that has a similar mark.
Figure 6: NVRAM.
The NVRAM may be directly soldered to the motherboard instead of being held on to it through a socket. In this case, you will have of de-solder the old circuit and solder the new one. That task is only recommended for people who really have experience de-soldering and soldering electronic components.
There is a trick to "recondition" an NVRAM. The NVRAM is a circuit that contains a memory circuit, a crystal, and a lithium battery in a single chip. Depending on the brand of the circuit on your motherboard, you can easily remove the upper part of the NVRAM – which is a rectangular plastic cover – using a small screwdriver. After removing that cover you will find the crystal and the lithium battery. Now, since the NVRAM also uses a lithium battery, all you have to do is to de-solder the old battery and replace it with a new one, paying attention to follow the correct polarity. If you are not skilled with the soldering iron, ask for the help of a friend who is or of an electronics technician.
If the cover can not be easily removed, you will have to force it, in other words, to break it using a blade and a small screwdriver. You will have to be careful not to break the crystal. Since the crystal is located near pin 1, at the end of the NVRAM, and the lithium battery is located in the middle of the circuit, we suggest that, in this operation, you only force the area in the middle of the circuit.
Another possibility is soldering the new battery outside the circuit. The positive pole of the new battery (which may be a 3 V CR2032 lithium battery) should be connected to pin 20 of the NVRAM, while the negative pole of the new battery should be connected to pin 16. See the figure to know how to identify such pins. Pin 1 is the pin where there is a small white ball (or a bas-relief) marked on the upper part of the NVRAM.
Figure 7: How to "save" the NVRAM circuit.
After reconditioning the NVRAM, you will have to make sure the new battery and wires you may have used are isolated, without any contact with other pins or components of the NVRAM or the motherboard.