Please Note: The information in this document has just been translated over from the printed version (14-Aug-98) and is being reviewed. There are errors! This notice will go away once all the obvious errors are corrected. From there, this document will be upgraded over time.
The "memory backup battery" in the model 100, 102 and 200 is a ni-CAD battery soldered to the main circuit board. If you need to replace it, it is readily available. The part number and sources are listed at the end of this article.
First, though, you should be sure that it is the ni-CAD that is causing whatever problem you are having. Perhaps your computer suffers a memory loss or other aberrant behavior, particularly after having lain off and idle for a period of time?
Or perhaps you are just uneasy about having your memory backed up by a part that might unexpectedly go bad at the most inconvenient moment. I'd guess that most of the time the ni-CAD is not to blame, and not much to worry about.
If the computer has AA cells installed, and/or if the power adaptor is plugged into an active wall outlet, it is those batteries or the AC power that provides the energy both to recharge the ni-CAD "and" to back up the memory. Fresh alkaline AAs can be counted on to hold the memory for a long, long time. Do not leave ordinary carbon cells installed, because they are more than likely to leak. Do not store the computer at high temperatures, because under hot conditions even alkalines might leak. If the computer has been stored in a wet, humid environment or has fallen into a lake (...it happens!) open the case and inspect the battery terminals for tell-tale green corrosion. That could be a serious problem, with corrosion of both the battery and the circuit board.
You have to realize that the ni-CAD is hardly ever called upon to deliver energy. The only times it discharges are:
You are replacing the AA cells. This is usually a matter of minutes or less, and even severely depleted ni-CADs should be able to handle that.
The computer is stored for a long period with no AA cells installed.
The computer is left with the AC adaptor plugged into the side, but the AC adaptor is not plugged into an active wall outlet. (Even if AAs are installed, the AC power plug disconnects them from the circuit inside the computer)
The computer is stored for years, and whatever AA cells that were in it have turned green at the ends... leaking noxious substances all over. Oh no!
The computer is stored for long periods at exceptionally high temperatures (the ni-CAD or any other batteries leak current internally), or under exceptionally humid or wet environments (moisture on the internal circuit board would cause corrosion, green spots, and current leakage especially around the battery terminals.)
To test the ni-CADs in a machine that is acting normally, just back up your files, but leave them in the memory, turn off the main power (not memory power), remove the AAs and the power adaptor, and wait one week. Reinstall the batteries or adaptor, turn on the machine and check that all the files are there and that any programs you have operate normally. If so, the battery meets the specifications that Radio Shack set for it.
On the other hand, if files disappear or become garbled or programs crash, try the experiment again for a shorter period of time. Cold start the computer, reload some files and programs, recharge the ni-CAD from some AAs or from the power adaptor for a couple of days. Now try the experiment again, for one day or even for 15 minutes. If you still lose things, you have a problem. Don't immediately jump to the conclusion that the problem is the battery. Maybe the battery is being pulled down by some component in the computer that is drawing too much current. It's like a car battery -- is it the battery, or is it the regulator?
The ni-CAD "should" hold the memory a lot longer than one week. Early 32K model 100s drew about 30 microamps standby current, which would give a battery life projection of more than a month on a fresh 50 milliamp hour ni-CAD (.050/.00003 = 1667 hours = 69 days). More recent 102s have a better standby current, about 10 microamps, so the fresh ni-CAD on those should hold the memory for more than 200 days! In either case, Radio Shack's one week specification is conservative. Remember that the ni-CAD doesn't ever come into play unless you are replacing the AAs without a power adaptor plugged in. It usually doesn't take a week to replace the batteries, unless you're waiting for a battery sale!
If the computer has a bad attitude, the memory chips themselves may be suspect. This is especially true if you have one of the older model 100s. The old memory technology was not as good at packing all those transistors close together. I have seen memory modules that were drawing tens of milliamps in standby (instead of a couple of microamps), and also they were causing bizarre behavior from the computer.
As a quick check, measure the current drain from the AA cells or power adaptor when the computer is turned off. This drain should be less than 0.0005 amp, most of which goes into charging the ni-CAD.
If you do not have a micro-ammeter or technical skills, try going back to your basic 8K computer. Turn off memory power, remove all the memory modules you can, except for the original one. Turn the memory power back on, install some files, then see if the aberrant behavior or high current consumption goes away. If so, you will have to decide which memory module is bad. Make yourself a 16K computer using one module at a time and see which one causes the problem.
If you do have the technical equipment and skills, you can make some direct, conclusive tests. If you have a voltmeter, first go in and measure the voltage across the ni-cad terminals. The voltage definitely should stay above 3 volts for a while after the AA cells and power adaptor are removed. If the ni-cad voltage stays at or near zero even with the AAs installed, then you almost certainly have a ni-cad replacement in store. If the voltage from the ni-CAD rises when the AAs are installed, only to fall quickly when they are removed, you should proceed to check the standby current drain before concluding that the ni-cad is bad.
It is better to measure the standby current drain directly. You have to break into the circuit strategically to splice the ammeter between the ni-cad and the computer. The best place to do this is by cutting or unsoldering one lead of diode D22. You can resolder it when you are finished. You can do this test with the AAs installed or not, it shouldn't matter. The standby current drain with the computer turned off should be between 8 microamps for more recent model 102s, up to 30 microamps for older model 100s. If there is an excessive current problem, you can continue to monitor this standby current as you swap out the different memory modules. There is little besides the memory modules that could cause the excessive standby current consumption, except for the standby steering circuit itself. If the standby current is normal and the ni-cad won't hold a charge, you will in fact have to replace the ni-cad.
If you decide that the ni-CAD needs to be replaced, here is where to get it: 3.6 volt, 50 milliamp-hour ni-cad, the manufacturer is Yuasa, part #3-51FT/A.
Radio Shack/Tandy part number ACS-0100, costs $14.18Call Tandy National Replacement Parts at 1-800-442-2425 (keep trying!)
Newark Electronics part number 87F644, costs $6.85Find Newark in phone directory of any large metropolitan area. Newark may have a minimum order.
The battery is easy to find on the circuit board. Take care in unsoldering and soldering it, not to overheat it. Be sure to put in the new one with the correct orientation!
ni-CAD battery article
EME Systems, 2229 Fifth St., Berkeley CA 94710