J.E.Lawrie HINTS-21.
Batteries - a constant topic.
To understand various matters in connection with batteries on the Z88 it is
necessary to define certain things :-
1. The BATT LOW indicator is virtually useless. It will show only when the
internal battery is in use and its voltage is below 4.8/5.0 volts. According
to "Z88 Magic" you should change the batteries "as soon as your `battery low'
comes up..." That other excellent book on the Z88, "Z88 Portable Computing"
says "this indication is a warning that the batteries need replacing quickly.
They are both very wrong ! Tests with Duracells in and the Z88 running
continuously, resulted in BATT LOW showing after 4.5 hours but the screen not
breaking up for another 9 hours. The voltage when the screen broke up (a very
sure sign to switch off and change the battery !) was 3.2 volts - note that
with another Z88 at a later date this occurred at 3.6 volts.
Note also that the battery was a Duracell - without doubt, the best. Also
note I say "battery", singular. The Z88 battery consists of four cells, not
four batteries, an important distinction.
One very important point - its significance is apparent later when we talk
about rechargeables - is that the alkaline battery can be used until the
screen fails and even though the battery is not removed immediately the Z88
will retain its files and work-in-progress for some hours yet.
2. Is it essential to use alkaline cells? and are Duracell the best? No
question; "yes" to both. Ordinary cells are virtually useless, having
insufficient capacity; nothing like the 1500 milliampere-hours of the
Duracells. There are other alkalines which give a good performance but not
better than Duracells.
Fortunately, I have always been able to buy Duracells at around 50p each -
buying multiple packs at Xmas time or on local markets and in small hardware
shops. I have to say at this point that I only use rechargeable batteries in
the Z88 - Duracells are for other things.
3. Rechargeables can be used inside the Z88 but they do have a fairly
limited life if the computer is being used for more than an hour or so each
day on battery only. The tests with the Duracells showed a load current of
around 130 mA when they were new but as the Z88 operates at constant power,
this increases proportionately as the voltage falls - almost 200 mA in my case
before failure. Now rechargeables have a starting voltage of only 1.35 each,
dropping to 1.25 after only 30 minutes by which time the current will be 20%
more than with Duracells.
The biggest problem with rechargeables is yet to come. After another two
hours (admittedly, on continuous use) they are almost exhausted. The voltage
drops from 5 to 4.5 in only 2 1/2 hours, and the computer shuts down
completely in another 3-4 minutes. If the cells are not removed and replaced
fairly quickly all files and work-in-progress will be lost.
A bigger worry still ! - BATT LOW indication came on shortly after you put
them in and now there is nothing to warn you when failure is imminent. If you
were to stop work shortly before this point and leave the machine idle for
some hours, perhaps overnight, complete failure is more than likely.
4. So what to do ? Well, you could always use the PSU from the mains supply
but presumably that is not always possible or convenient otherwise the
questions regarding batteries would not arise in the first place ! Or forego
rechargeables and put up with the cost of alkaline batteries.
Or you could use an external battery supply, plugged in to the socket. The
most-favoured one (though not the only one) is the 5-cell rechargeable
package. This has one positive advantage over the internal rechargeables.
- the voltage is higher so the current is reduced.
A positive bonus is that BATT LOW at last means something ! As soon as
this indicator appears the external battery MUST be discontinued. Why? -
because the volts per cell is down to 1.0, at which point there is a real risk
of getting cell reversal - not catastrophic as re-charging usually corrects
this, but it should be avoided where possible.
5. What is happening to the internal battery when an external supply is
connected ? If the external supply is from the mains then the internal
battery, of any type, is only supplying about 1 mA, negligible. BUT, if the
external supply is a battery then as its voltage falls the internal battery
will begin to share the load.
A test I did on this showed that, with Duracells inside, after 2.5 hours the
internal was delivering 4 mA but after another hour this had risen to half the
total load of 140mA ! There isn't much point in this set-up ! I concluded
that if I were to use a 5-cell rechargeable pack externally I would only have
three cells inside. The voltage of these being 4.5 it would not be used since
at this stage the external rechargeables have been taken off for re-charging -
BATT LOW is showing.
It is necessary of course to have a "dummy" cell with the three inside. When
the external battery is removed the fourth cell can be put back inside - this
will avoid having a very heavy drain on the three cells.
Extra tips on Rechargeables
Unlike alkalines the rechargeable cells lose capacity if left on one side.
At room temperature this can be 40% in 40 days. If stored in the refrigerator
at 0% C. they will only lose 10% in the same period.
A better way is to make a slight modification to your charger so that,
after charging, you can switch over to a "trickle" charge of 5 mA. My charger
needed a 1k resistor in circuit to do this.
Re-charging Alkalines !
This used to be possible, in spite of words to the contrary printed on the
cells - this warning was illegal in Japan where re-charging was encouraged as
an environmental matter. I made a special charger (not my design) which kept
a battery going for a few years but now it is not possible - a change in the
chemical make-up of the electrolyte ?
Battery wasters
Consider altering the "timeout" at the Panel to 1 minute. Maybe yours is
still at 5 - what a waste! No use setting it to 0, it will never switch
itself off. Don't forget that if the clock is left on screen the timeout is
not effective. You must not think that just having the screen on with nothing
happening it is not consuming power - it is, the screen is being renewed
continually.
Is the power coming from the PSU?
Well if it is plugged in and switched on can there be a doubt ? Oh yes, a
big one! A broken connection in the lead or a bad connection at the input
socket (this should be cleaned, carefully, from time to time).
So how can you check? You could have an LED on the PSU and glance at it
from time to time; or you could have the LED on the Z88 itself. And you could
have another LED to indicate that re-chargeable batteries inside the Z88 need
to be changed. See below and HINTS-CT. Sheet 2.
A red LED is part of a simple electronic circuit to indicate that the battery
voltage has dropped to a pre-determined value. The other, a green LED, will
indicate that the supply from the PSU is getting to the internal circuits.
If you have no experience in making circuits, soldering small components
and/or fitting and soldering in a confined space then get someone else to
carry out the work. REMOVE THE BATTERY - THIS MEANS THAT YOU WILL LOSE
ALL FILES AND W.I.P SO TAKE THE NECESSARY ACTION.
"Mains on" Indicator - the simplest of the two. Required - One green, 3mm
LED, one 3.3k, 1/6th watt resistor and some wire. Open up the Z88, ease
out the two strips from the keyboard connectors and remove the keyboard.
The connections are made as follows :- the positive wire is soldered to one
end of the resistor and the other end of this to the centre pin of the input
socket. The negative wire goes to the top of the fourth resistor from the
left, below the input socket - a 1.5M resistor; one brown and two green bands.
Lead these two wires along to the right-hand end of the battery compartment,
then go under the main PCB, exiting next to slot 3. then under the pcb to the
right-hand side of slot 3, loop around the fixing hole in the pcb and down to
the front edge of the pcb, near the flap switch
There is just enough room to position the LED through the front face of the
Z88, right next to the slot side and 4mm from the top edge. DON'T attempt to
make the hole other than with a 3mm drill held in a small tap wrench or
similar tool. Soldering of the LED must be done before it is placed into the
hole - TAKE CARE not to splash solder over the pcb ! and be sure to connect
the positive wire to the longest lead on the LED. If your hole is slightly
too large then use a tiny drop of glue - NOT the `instant' type.
Replace the keyboard connecting strips, pushing them home, one at a time, by
squeezing them with the middle finger on to the resistors in front of the
strip housings. Don't screw back the keyboard until you test that the light
is on when the power from the mains is applied and goes off when the plug is
withdrawn.
---------------------------------
" Battery Low" Indicator - refer to the accompanying circuit diagram.
The transistor is any switching or general-purpose type. All fixed
resistors are 1/6th watt type; the potentiometer to be as small as possible
consistent with the ability to adjust the slider. Do not mount the parts
on a board, just solder them together in the smallest possible space. It
will be positioned to the right and possibly partly under the screen. Use a
piece of insulation (paper will do) over it to prevent any "shorting" on the
back of the screen.
The incoming +ve and -ve leads are connected directly to the battery spring
supports. The leads to the red LED are taken under the pcb as far as
possible, to the right-hand corner of the unit. Make a 3mm hole 4 mm down
from the top face and 15 mm in from the right-hand edge. Do it carefully !
The positive wire is taken to the longest lead on the LED. It only remains to
find some cells (including possibly a "dummy") to make up 4.4 volts. Adjust
the potentiometer so that the LED just comes on or just goes off. Q.E,D.
SEE ` HINTS - CT '
END
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