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

From: Ben Smith (QM Systems) (ben.smith_at_qm-systems.com)
Date: Wed Jan 23 2002 - 04:20:18 EST


> -----Original Message-----
> From: newtontalk-bounce_at_newtontalk.net
> [mailto:newtontalk-bounce_at_newtontalk.net]On Behalf Of
> Gruendel, Frank
> 3837 S-WF-RD6
> Sent: Tuesday, January 22, 2002 4:58 PM
> To: 'newtontalk_at_newtontalk.net'
> Subject: [NTLK] Misc. replies regarding battery tray thread
>
> 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.

The object you are referring to is not a thermistor, it is a fuse -
plain and simple!!!

>
> 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.

Again wrong, what you are calling a resistor is in fact the
thermistor, it enables the Newton to monitor the battery pack
temperature and reduce the charge current if the pack starts to get
too hot. (however it is not entirely foolproof, a faulty cell can
still overheat and burst.)

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

NO

>
> 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.
>
>
> > - 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.

No, you are confusing the thermistor with the fuse.

>
> > 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

The instructions on this page seem ok, the thermistor (NOT resistor)
could be a bit better placed, or some of the gel type heatsink
compound could be used to improve the thermal contact (not the type of
heatsink paste that you use with CPU's but the type that sets like
white silicone sealant).

A thermal fuse connected between the 2 end batteries would be nice,
these can be purchased quite easily, one rated at 70-80c and 5A would
suit. it could be laid between the batteries in a similar way to the
thermistor in an original battery pack (but on the lower pair of
cells).
However if you use a thermal fuse, remember that you have to use a
good heat shunt on the leads when you solder it or else the heat from
the soldering iron will blow it. (in commercial production they spot
weld them as this produces far less heat).

Ben.

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