POWER4.100 file by R. V. Getsla 74405,1177 Last update: 17 MAY 85 Rev. 1.0 ----------------------------------------------- This file is to clear up a lot of things which have gone by on the message board, and to assemble it all in one place. Please add your input by leaving me a message if you know of something of use which I have not included. There have been quite a few questions about NiCd batteries and how to go about using them. They are pretty rugged cells, but a little care is necessary in their use. 1. Charging NiCd cells - care and feeding: Each cell has a recommended charging current. There are 2 types available. The 1st are known as "fast charge" and can take higher charging currents than the more common 2nd type. The cost is also higher for the fast charge type of cell. During charging, the cell generates heat proportional to the charging current. The rating of the cell is such that the cell does not get too hot and vaporise the electrolyte enough to lift the built in relief valve. The problem with lifting the relief valve is that liquid is lost and the cell loses capacity early in life. So, follow the charge rate recommended on the cell by the manufacturer. The usual charging current for fast charge cells is several hundred milliamperes, while the more common slow type charge at around 100 ma or so. The AA cells I use routinely are rated at 80 ma charge. The manufacturer will usually suggest a time for the charge in hours. My cells charge in 18 hours at 80 ma. If the cell is fully charged, and the charger does not reduce the charge current, it will result in heating the cell, but supposedly not enough to do any damage, thus, the manufacturers say you can leave them on charge all of the time. Not a good practice in my opinion. One of the causes of early death of NiCd cells is heat. Ideally, the charger should sense the cell voltage and reduce the rate of charge when the cell voltage says it is done. There are "smart" chargers out there, but the price is pretty steep. I built one, but I am handy with a soldering iron, and it wasn't easy, so I do not think putting a circuit up here is the way to go. Now, for other problems. NiCd cells are prone to a unique "memory" effect. This is what happens. The cell appears to lose capacity as it is used and recharged. The cell "remembers" how much you discharged it last time, and the more times you do it, the "harder" it remembers. So, if you always charge the cell after only a 50% charge, then eventually, it will remember, and only let you take 50% out! The cure for this effect is to run the cell "into the ground" so to speak, and then PROMPTLY recharge for the full charge rating of the cell. It will take a few times before the cell capacity is restored, and it will never get back to the 100% it had, but it will come close, and is better than buying a new cell. Another problem. Sometimes a cell will just NOT charge. You put it into the charger, and all it does is get warm. The problem here is internal shorts as a result of a crystal "tree" growing between the plates inside. There is no permanent fix for this, but there is a temporary one. There are several ways to "zap" the cell, and "blow the fuse" which is the crystals. The gist is to hit the cell for a very short time with a really high current, several amps for part of a second is usually enough. The easy way to do this and not risk exploding the cell is to hit up a surplus store and get some filter capacitors used for power supplies rated at about 50,000 micro farads or so, up to about 500,000. Observing correct polarity for the cap, hook it to a 6v lantern battery or equivalent. After a few seconds, disconnect the cap and hook the cap across the dead cell. Expect a spark as the leads touch and do not be alarmed. What you are doing is charging the capacitor from the battery, then shorting the power stored in the capacitor into the dead cell, hopefully blowing the crystal "fuse". Recharge the cap and try it again if the cell still measures zero volts. If the cell measures around a volt or better, you did it. Charge the cell in the usual manner, and use it. It will unfortunately die again if you do not keep the cell in almost constant use. Voltage considerations: The full charge voltage of a single cell is about 1.25 volts to 1.3 volts, a bit lower than an alkaline or carbon-zinc cell, so you have to take that into consideration in what you are doing. In the Model 100, it senses voltage of the battery pack and turns on the "low battery" LED at around 4.1 +/- 0.1 volts. The computer shuts down if battery voltage drops to 3.7 +/- 0.1 volts. During the shutdown process, RAM is protected. in other words, it does NOT hurt the Model 100 to run the batteries down until not only the LED is on, but the whole thing goes away as well! Turn the on/off switch off, replace the used batteries, turn it on, and you will see where it was when the power sense circuit did the equivalent of you turning the on/off switch to "off". The problem with replacing the 4 AA cells with NiCd cells is the initial lower voltage, even with a full charge in the NiCd cells, of around 5 volts. There are ways around this by adding a 5th cell internally to the Model 100, but I do not recommend this as it will VOID your warranty, and Radio Shack has been known to refuse to service equipment modified by anyone other than themselves. You have to weigh the risk for yourself. Personally, I would rather keep my Model 100 in an unmodified state so that if it ever needs fixing (heaven forbid) it will be fixable by RS, probably at some exorbitant (sp?) charge. Discharge characteristics: NiCd cells are somewhat unique in that they will hold output voltage well untill just before they are completely discharged. What this means to you is that the cell voltage will not drop off as rapidly as an alkaline cell, but when it does start to drop off rapidly, you will only have a few minutes of use before the cell is gone. The rate of drop off for my cells, measured experimentally, is a steady drop from 1.25 down to 1.1 over about 90% of the capacity. Then it drops to less than 1 volt in a matter of minutes and hits zero, or very close to it faster than my digital multimeter can keep up with. The moral of the story is that the voltage of the cell is not proportional to the state of charge and should NOT be used to determine when to do a charge, rather, rely on time used versus the capacity of the cell, or better yet, run 'em till they drop, and do an immediate recharge. This is the way to keep up the capacity of the cell and avoid the memory effect I discussed earlier. Periodically, more often is better, run the cells untill they can do no more, then charge for the recommended full charge at the rate given by the manufacturer. The cell will still "remember", but now it is "remember"ing that you took out 100%. Make sense? I hope so. On to bigger and better things. 2. Gel cells: information Gel cells are essentially like your car battery in that they are a lead-acid type of cell. The major differences are that they come in a smaller package, the electrolyte is "gelled" sort of like Jello, and the cells are not adversely affected by long idle times of many days without a recharge. The same care applies to charging gel cells as it does to NiCd cells. Overcharging them has the same result, subsequent heating and loss of electrolyte after full charge is reached. What is better, though, is that cell voltage is a fairly good indicator of the state of charge, the same way as it is in your car battery. The usual voltage is around 2.2 volts per cell. The voltage regulator in your car knows this, and reduces the charge rate put out by your generator to prevent boiling out the electrolyte. Think of the punishment you are putting your car battery through! You go to start your car, and that battery is called upon to deliver a few HUNDRED amps to the starter motor. Which, it does without too much complaint. Then your regulator senses the lower voltage, and jams current in as fast as the generator can put it out until the voltage is back up again. And, this goes on for YEARS! Now I do not recommend doing this type of thing with smaller gel cells, but the point is that they can take it and come back for more. Isn't it nice to know that there are thing like this that will forgive you? The only drawback is the weight. Lead is pretty heavy, after all. I use a 6v gel cell purchased at an electronics surplus place. Cost: $3.00 plus parts to plug it into the AC adapter connection. I use the AC adapter to charge it once a week or so, overnight. Actually, any adapter which puts out about 6v DC will do as long as the voltage is higher than the cell voltage. The AC adapter puts out a respectable 0.5 amps at about 7.5 volts. The other way to charge this type of cell is to use a regulated supply which has an adjustable voltage output. Set the voltage regulator to the full charge voltage, in my case, 6.6 volts, and it will start reducing the charge current as the voltage in the cell comes up to the full charge voltage. For the most part all automatically. Essentially, a "smart" car battery charger is doing the same to avoid overcharging in much the same manner. Gel cells are pretty rugged, but they can be hurt by overcharging. They can also deliver much the same current into a short as a car battery can, so beware! Discharging a gel cell, or any other type for that matter, into a short causes heating and subsequent vaporising of the electrolyte which raises internal cell pressure. If the relief valve in the cell fails, it can explode, big time! And take part of your body with it, and scatter acid all over your favorite computer hideout with all the bad things associated with corrosives!!! If a cell ever gets warm to the touch, it is about to do bad things. Adjust the charge current, or voltage so it is less. The idea is to have a few mils charge current when the gel cell is fully charged, about 5-10 is about all that is needed to keep the cell topped off and happy. 3. Portable power options: There are a number of ways that you can power your portable and not run your AA cells down. The easiest way is to make up a cable with a plug like the AC adapter on one end, and clip leads on the other. I did this and used the lantern battery in my flashlight at the bottom of the Grand Canyon. Color code the clips so that the polarity and voltage is correct!!! The POSITIVE side of the battery goes to the OUTSIDE of the plug. The NEGATIVE side of the battery goes to the CENTER of the plug. The battery has to be at least 5 volts. A 6 volt lantern battery is just fine for this. Just plug it into the connector on the side where the AC adapter goes, and you have power to spare. There are devices commercially available that already do this using "D" cells in a holder, or rechargeable cells in a pack of some kind. The advantage is more power, but unfortunately, something else to carry around, which is why I used the battery in something I was already going to have along with me. Solar power: I have not tried this myself, but herewith is some info on solar cells and the Model 100. The current drain on your poor old AA cells at the worst case is about 150 milliamps. This occurs when driving the accoustic couplers. At all other times the drain is less, but it never drops to zero. That is why your Model 100 stays "alive" even when it is turned off. The only advice I have on this is that the cell rating must be larger than the worst case, and then it will work under most circumstances I can imagine, including a cloudy day. Silicon solar cells typically put out .45 volts per cell. Literally connect enough in series to get over 6 volts, get the polarity right and plug in your solar power the same way as I plugged in the lantern battery. Be careful of the cells, though, as most are built onto a glass substrate and are fragile. Put the cells behind the glass in a picture frame with a bit of rubber cement under each cell to hold it in place, and wa la! Put a rechargeable battery in parallel with the solar panel, and charge it at the same time, remember, the current rating of the cells will limit what you can actually do. If you do put a battery in parallel, you MUST put a diode in series with the panel to prevent damage to the solar cells from reverse power. The diode acts like a check valve in that it only allows current flow in one direction. You will have to add at least 1 more solar cell to the array to compensate for the voltage drop across the diode, which is typically around 0.5 volts when it is conducting. --------------------------------------------------------- | | | Model 100 battery solar panel | | | ----------------------------diode here------------------- Hopefully this diagram makes sense. CHECK POLARITY CAREFULLY!!!!! The Model 100 IS NOT protected against reverse polarity, and you will do your Model 100 in quicker than you can shake a stick!!!!! And it will be your fault, and you will have to pay RS an arm and a leg to fix it since it will be obvious to them how your machine died. All in all, there is one rule to remember on doing things, be careful. I have tried with this file to answer some of the questions which I have seen come up over and over again regarding the various options available on powering your portable computer from other than the internal batteries. Drop me a line and let me know of your experiences, and I will incorporate whatever I can into future revisions of this file, or maybe even another file all together. If you have any questions, let me know via the message board or by EMAIL. I am more than willing to help anyone over the rough spots. Ralph V. Getsla [74405,1177]