Replacing the Motherboard Battery
By Gabriel Torres on November 24, 2004
Every PC motherboard has a battery. That battery serves two purposes: to feed the configuration memory (also called CMOS) and to feed the real time clock of the computer (the one that show the date and the time).
An indication it is time to change the motherboard battery is receiving one of the following error messages when you turn on your computer: CMOS CHECKSUM FAILURE, CMOS BATTERY STATE LOW, CMOS SYSTEM OPTIONS NOT SET and CMOS TIME AND DATE NOT SET. Another indicator of a low battery is a clock that, after being set, runs well while the computer is on, but shows the wrong time when the computer is turned on the following day (it is late).
The motherboard battery can be built using three different technologies: Nickel-cadmium (NiCd), NVRAM (Non-Volatile RAM) and Lithium (Li). The Lithium battery, which is a round one (the size of a coin) and can be easily found at watchmaker's and computer parts stores, has long been the most used type. To buy one of these batteries, all you have to do is to look for a model CR2032 one.
Replacing the motherboard lithium battery demands some care. It seems to be a simple task, but it is not.
The lithium battery may use basically three socket types: socket with upper tab (Figure 1), socket with lateral tab (Figure 2), and the socket into which the battery stands instead of lying (Figure 3).
Figure 1: Socket with upper tab.
Figure 2: Socket with lateral tab.
Figure 3: Another kind of battery socket.
While the replacement of the battery in the socket with the lateral tab or of the one that stands is simple (Figures 2 and 3) – all you have to do is draw back the tab using your finger or a small screwdriver and replace it – the replacement of the batter that has an upper tab covering it (Figure 1) demands extra care. In this type of socket, if you raise the metallic tab to replace the battery, it will lose its pressure and will not make contact with the battery anymore, damaging the socket. In this case, the correct replacement of the battery in done pressing a small plastic lock at the side of the socket with the fingers or using a small screwdriver. That will allow the battery "to slide" laterally, not damaging the upper tab.
Whatever the socket used, don't forget that the replacement of the battery should be done while the computer is off.
If the motherboard battery is not lithium, that is, if you do not find the round flat battery the size of a coin we spoke about last week, that means your motherboard battery uses either nickel-cadmium (NiCd) battery or an NVRAM one.
The nickel-cadmium battery, unlike the lithium or the NVRAM ones, is a rechargeable battery, which means that in theory it should never need to be replaced. However, if your motherboard has a battery of that type and the computer shows the defects described last week (such as the loss of the hour and date), that means you will have to replace this battery. The problem is that replacing that type of battery requires some knowledge of electronics and some expertise, because you have to de-solder the old battery from the motherboard and solder the new one. That is why we recommend you seek the help of an electronics technician to do it, in case you don't know how to use a soldering iron.
Figure 4: Nickel-cadmium battery.
The great problem of the nickel-cadmium battery is that it is very likely to leak, and that may even corrode the motherboard. In case the battery of your motherboard has leaked, you will have to clean the affected area using an old toothbrush and isopropyl alcohol. You will have to see if the battery acid did not corrode any of the motherboard tracks. If that has happened, the affected tracks will have to be remade using a wire. If you don't know how to do that, look for an electronics technician.
Figure 5: Leaked nickel-cadmium battery.
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.