POWER4.100 file by R. V. Getsla 74405,1177
Last update: 17 MAY 85 Rev. 1.0
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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]