Re: [NTLK] Misc. replies regarding battery tray thread

From: Johannes Wolf (jwolf_at_xe.estec.esa.nl)
Date: Wed Jan 23 2002 - 03:27:01 EST


Let me add two things:

1) I am not really happy with the terminology for "thermistor" what is in
my opinion most likely a thermo-fuse and for "resistor" which should read
thermistor. Of course every resistor will change its resistance with
temperature, but while this is a side effect for resistors that may cause
trouble in your circuit, this effect is especially distinctive and intended
for thermistors, because they are used for sensing temperature.

2) Regarding the thermo-fuse: I saw a Newton bursted at the seams due to
overheating and blowing up of the battery pack during charging with a wrong
(too powerful) power supply. Thus the thermo-fuse seems not to contribute
very much to safety and in my opinion you can leave it out with no doubt as
long as you use the original power supply or a power supply with less then
9W power.

Johannes

-----Original Message-----
From: Gruendel, Frank 3837 S-WF-RD6 [SMTP:Frank.Gruendel_at_de.heidelberg.com]
Sent: Tuesday, January 22, 2002 5:58 PM
To: 'newtontalk_at_newtontalk.net'
Subject: [NTLK] Misc. replies regarding battery tray thread

Hi folks,

I'm answering a couple of mails at the same time here to save some
bandwidth.

> From: Jbjng_at_aol.com

> So, one could feasibly create a rechargeable tray setup with new cells,
> incorporating the original extra components from a worn out rechargeable
pack.
> Thus allowing slightly easier replacement of cells upon their demise at
some
> point in the distant future. Right?

Theoretically and electrically, yes.
It will be difficult (although not impossible I think), though,
to come up with a version that is reliable enough in terms of mechanical
stability. Remember, the battery contacts in the Newton are fairly fragile
and get bent easily. Of course "reliable" is a function of one's own
definition,
mine is pretty high. Keep in mind that the battery tray lacks the negative
power supply terminal, if my memory doesn't plays tricks on me, there's
just
a
hole in the battery holder and the spring contacts the battery directly.
This
is where you would have to connect the resistor to.
If someone can spare a battery tray and a dead accu pack,
I would gladly give this a try and publish the result on my website. If I'm
successful, you get the modified tray back at my expense.
That said, let me assure you that once you have refurbished your dead pack
the way I describe on my site, re-refurbishing it will be a matter of
15 - 30 minutes.

> From: Someone whose name I forgot to copy

> The essential thing is that you need to add a thermistor and the
> appropriate connections to your battery tray, otherwise the Newt will not
> charge your NiMH cells in the battery tray.

Let's get one thing straight: You need at least one element to make the
Newton charge, you need a second element if you want this thing to be
electrically identical.

Not being a native English speaker, this is just an assumption, please
correct me if I'm wrong:

a) A thermistor behaves like a switch. The simplified version of it's
behavor
   (just to save us the necessity to explain what a hysteresis is) is that
   it cuts a connection when the temperature exceeds a specific value and
   it re-establishes the connection when the temperature drops below it.

c) The resistor behaves like a temperature dependant potentiometer. The
resistance
   changes with the temperature, the amount of change tells the Newton
   the change in temperature.

The following usage of the words "resistor" and "thermistor" is based on
the
assumption that this is correct.

You need the resistor, otherwise the Newton wouldn't charge. Its specs
should
match the original one's specs as closely as possible (which you could
guarantee
if you used a resistor from an old pack).

You do not need the thermistor. The Newton will charge just fine without
it.
As long as nothing goes wrong, you are safe. But only as long as that.

> From: Achow2_at_aol.com

>> What would be the resistor values?

> - Resistor: ??

At room temperature the resistance is about 7.5 kiloOhms. I have never
tested
it with other temperatures, but you must make sure it behaves like the
original one.

> - Thermistor: 15K based on battery hack article

I might be mistaken, but if with "15K" you actually mean 15 kiloOhms, I
would
like to disagree with all due respect. The thermistor is connected serially
between two cells. Any current through it would not be available at the
battery's terminals. You have 4.8 volts for four cells, the Newton will
work
until the voltage drops to about 3.6 volts. In other word, the maximum
voltage drop allowed across the thermistor would be 1.2 volts. Assuming
that
your Newton needs an average current of 30 mA (can be more or less
depending
on the hardware installed and on what you are currently doing with it), a
40 Ohms resistor would result in a voltage drop of 1.2 volts. 15 kiloOhms
would never let the Newton even start. Actually, the thermistor is as close
to 0 Ohms as one can build it.

> From: "Daniel Padilla" <daniel.padilla_at_wanadoo.es>

> I set up a (quite simplistic) web page with photos and some intructions
on
> how to convert the standard battery tray to be able to use rechargeable
cells.
> I've been using this for two years now without a problem. I think it's a
> fast and clean way to achieve this.
>
> http://dev.newtontalk.net/~dpadilla/battery.htm

Again, "clean" is a function of one's own definition. I agree to "fast".
You left out the thermistor which is fine unless something
goes wrong. Also, the resistor you use isn't the original one which may or
may
not be a problem depending on its specification. It also appears that the
resistor
is located pretty much at the tray's bottom although it should be located
as
close to the cells as possible. After all, its job is to sense their
temperature,
and being several millimeters away from their surface in an otherwise open
box
won't allow it to do this job with the required accuracy. This is why in
the
Newton and eMate rechargeable packs it is fixed with good expensive tape to
the cell's surface. This is also the reason why the temperature sensor in
the
MP1x0 devices (you can see it in the battery compartmen) is shaped the way
it is shaped.
I apologize for this sounding arrogant (it really isn't meant that way),
but...
I'm sure this setup works just fine. However, considering the fact that
2x00

machines don't exactly grow on trees anymore, it doesn't quite match my
personal
quality standards ;-))

Frank

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