               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]



