A Simple Self-Charger for the Model 100

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The Model 100 is easy to use, powerful and lightweight. It also eats batteries.

If you're tired of hand-feeding it every few hours, you can modify its dietary habits to self-charge. All you need are a resistor, a power line adapter (current retail cost $5.95) and a quartet of rechargeable batteries.

After you've owned your machine for 90 days, its warranty expires and this modification won't affect it. If your warranty is still in force, we point out that Radio Shack does not recommend what we're about to describe. However, we've had no problems with our two machines over several years of use.


Rechargeable nickel-cadmium (nicad) batteries have a nominal voltage of 1.2 volts (V). This remains virtually constant as the cell discharges, then drops quickly to near zero. Four such batteries give the Model 100 a combined voltage of 4.8 volts, which is lower than the recommended 6.0V. The 100 will shut itself off if voltage drops slightly below 4.8V. In spite of this relatively narrow voltage band, we have never experienced problems from low power shutdown.

We have tried several brands of nicads. All give about 8-10 hours of use when fully Charged. Makers recommend occasionally letting the cells run all the way down before recharging. Our experience is that repeated partial discharge (or extensive use of trickle charging) will reduce charge life to 4-5 hours. One or two cycles of full discharge/recharge restores the 8-10 hour useful life.

We have used the same nicads for hundreds of cycles over two years without evident degradation. Aside from the relatively limited life of each recharge, the only noticeable effect is a very short warning time after the lowbattery light comes on, before the 100 shuts itself off.

Different brands of nicads have slightly different dimensions. The positive tips of Radio Shack nicads are somewhat shorter than those of other makers such as Lynacharge, so use of the Radio Shack cell can lead to poor positive contact and intermittent operation. If you experience this, the best solution is to switch to a different brand of nicad. However, it's possible to use needlenose pliers and partially uncoil the 100's positive spring terminals for better contact.


You probably wonder, as we do, why all portable computers don't have built in rechargeable power supplies. Radio Shack has been consistently guilty of this fault, and even compounds it with new battery-powered products that lack automatic shutoff.

However, if you have ten minutes and twenty cents, you can wire in a resistor (without soldering) that will recharge your IOU overnight from a power line adapter.

We repeat: This change may invalidate your 90-day warranty if still in effect--but there is virtually no risk from adding a resistor, nor will it make troubleshooting difficult if your machine should ever need repair. Simply removing the resistor before sending the 100 for service will avoid any issue being raised. .

The resistor you need for self-charging is a 47 ohm, 1/4 watt resistor which you can find at any electronics or TV repair store. A resistor twenty percent above or below those values will still work.

With the 100 turned off, place it face down on a smooth surface to avoid scratching the screen. The screen should be facing away from you. Using a Phillips screwdriver, fully loosen the four recessed corner screws. Then turn the computer over to the left. As you do so, the screws will fall out - don't lose them. Clips holding the front and back together can be loosened with your fingernail. Now lift the left side of the front and fold it part way over to the right. Look at all the neat parts inside, but don't touch!

With the accompanying photograph as a guide, find the plug location for insertion of your 47 ohm resistor. Using cutting pliers or scissors, trim the two wire leads of the resistor down to about 1/2 inch. Now bend the wire leads into a U-shape with needle nose pliers, and insert the leads into the recesses along with the orange and red wires as shown. (Yes, we are cheating by using the existing terminals without asking you to solder in new ones) Make sure the resistor is firmly mounted and will not touch or interfere with other parts.

Now fold the 100's front into place, make sure it seats all around, insert the screws, and tighten them snugly in the recessed holes. That's all there is to it.


When you first put in your rechargeable nicads, it will take overnight to charge them fully. The 100 can be in use or not while charging.

With a little practice, you will learn how often your batteries need a charge. In an emergency, you can always replace them with regular batteries. But don't use the power line adapter with regular batteries except in dire emergency, because they don't charge properly.

IMPORTANT: Don't leave the power line adapter plugged into the computer without power at the other end. That will discharge the nicads. However, there's no need to worry about the memory backup battery. This change has no effect on it at all.

Now it's time to enjoy all the benefits you've gained from ten minutes of work. You've seen the guts of your 100, and you won't have to change batteries for years to come. Freedom! D

The arrow points to the location of your 47 ohm resistor. Trim the resistor's two wire leads down to about l” inch and insert into recesses along with the orange and red wires.