NiMH battery pack

[John Riley]

I have a number of 4 cell, 4.8V NiMH mobile phone batteries that I
want to string together to make a larger battery pack.
When I pull one apart, I find that the centre of the three terminals
on the case is the positive, the other two are straight negative, and
negative with a very small component in the lead about 1mm in diameter
and a few mm long. The voltage reading between pos and straight neg is
5.19V and between pos and this other neg with the unknown component is
5.13V. Can anyone tell me what this strange (to me) component is?
Perhaps a diode, for charging purposes? Thanks for any help, regards,
John

 

[Puckdropper]

I have a number of 4 cell, 4.8V NiMH mobile phone batteries that I
want to string together to make a larger battery pack.
When I pull one apart, I find that the centre of the three terminals
on the case is the positive, the other two are straight negative, and
negative with a very small component in the lead about 1mm in diameter
and a few mm long. The voltage reading between pos and straight neg is
5.19V and between pos and this other neg with the unknown component is
5.13V. Can anyone tell me what this strange (to me) component is?
Perhaps a diode, for charging purposes? Thanks for any help, regards,
John

Some batteries have fuses in them. They look quite a bit like a diode,
but with no band. (Usually you'll see an amperage rating on them.)

Puckdropper

 

[John Riley]

This seems too small to be a fuse, and why the connection to the other
terminal wirhout it? Seems to me it has to be something that needs to
be different between charging and draining the battery. And this fuse
would need destruction of the casing to change it. It is all glued
together. This tiny little component seems to have some sort of
marking like a diode with a triangle pointing in one direction. It is
so small I can't figure it out with my strongest glasses and a
magnifying glass out in the sunlight.

 

[Phil Allison]

"John Riley"


** DO NOT TOP POST !!!!!!!

This seems too small to be a fuse, and why the connection to the other
terminal wirhout it? Seems to me it has to be something that needs to be
different between charging and draining the battery.

** Very likely it is a thermistor.

A special resistor that changes value with temperature, possibly quite
drastically.

The phone uses it to sense if the battery is too hot and stop charging.



....... Phil

 

[John Riley]

A special resistor that changes value with temperature, possibly
quite drastically.

The phone uses it to sense if the battery is too hot and stop
charging.

Thanks Phil. That seems to make sense. I will heat it up and see if
the ohms rise accordingly.
This will make little difference to my setup as I will just be
shorting the two negative terminals together, as the bottleneck in my
larger battery will be the fine wires used to hook the cells together.
If one of the negs acquires a bit more resistance, then that's fine. I
will be charging and discharging manually with temperature and current
monitoring both ways. ps, sorry about the top posting, but I've been
lulled into a sense of stupidity by Yahoo Groups

 

[John Riley]

Thanks Phil. That seems to make sense. I will heat it up and see if
the ohms rise accordingly.
This will make little difference to my setup as I will just be
shorting the two negative terminals together, as the bottleneck in
my larger battery will be the fine wires used to hook the cells
together. If one of the negs acquires a bit more resistance, then
that's fine. I will be charging and discharging manually with
temperature and current monitoring both ways. ps, sorry about the
top posting, but I've been lulled into a sense of stupidity by Yahoo
Groups

Before I heated the thing, I tried to measure its resistance.
I couldn't get a reading. Open circuit?

I then put it in series (both ways) with a battery and a globe
that glowed. Nothing, and even with a meter in series as well,
zero milliamps.

But, measuring the voltage of the battery with this thing in
series showed a slightly smaller voltage (~30 mV) than the
direct voltage measurement.

Could this be a capacitor? What would be its purpose?

 

[Rich Grise]

Before I heated the thing, I tried to measure its resistance. I couldn't
get a reading. Open circuit?

Did you try it with the meter leads the other way around?

I then put it in series (both ways) with a battery and a globe that
glowed. Nothing, and even with a meter in series as well, zero
milliamps.

But, measuring the voltage of the battery with this thing in series
showed a slightly smaller voltage (~30 mV) than the direct voltage
measurement.

Could this be a capacitor? What would be its purpose?

No, it couldn't be a capacitor.

It sounds like a diode, that's used as a temperature sensor by the
smart charger. You won't read much voltage from it, and probably
won't see much change in its forward drop, but the smart charger
had a circuit that _does_ see the change, and uses it to decide
when to stop charging the cells.

Good Luck!
Rich

 

[John Riley]

Did you try it with the meter leads the other way around?


No, it couldn't be a capacitor.

It sounds like a diode, that's used as a temperature sensor by the
smart charger. You won't read much voltage from it, and probably
won't see much change in its forward drop, but the smart charger
had a circuit that _does_ see the change, and uses it to decide
when to stop charging the cells.

Good Luck!
Rich

Thanks for your thoughts, Rich, but I measured the current and
resistance both ways, and it acts exactly like an open circuit.
What other component shows "infinite" DC resistance, but you can
measure voltage through it? My extremely limited experience suggests a
capacitor, which as you have suggested, makes no sense. It seems
strange to me to put one component of a smart charging circuit right
inside the pack against a cell, if it isn't there to monitor
temperature. I was chuffed when it was suggested to be a thermistor.
But this thing would allow no charge to flow. It could however monitor
voltage.
Aha, I wonder if its voltage reading varies with temp, and so it could
tell the smart charger when to stop charging by its signal, rather
than directly reducing charging current itself. I'll give that a try.

 

[John Riley]

Hooley Dooley, I just put my meter in series with it and a
battery, and as before, zero amps. I heated it and lo and
behold, it started transmitting several amps.

I tried the voltage and at room temp, it measured 4.99V and
at hot, it went up to a steady 5.05 V which is the full
voltage of the battery.

Sooo, what sort of component, (group of components) has
infinite resistance at room temp, and around 2 ohms when
heated? Does this reconcile with the voltage readings?

 

[[email protected]]

John Riley,

I'm sorta throwing this out and hope that it may be the right answer
for you.

I have a Newton which also uses a rechargeable pack which consists of
(4) AA NiMH Cells. As part of it's construction it too has a thermistor
installed. The themistor is present to monitor the temperature. This
thermistor in the Newton Rechargeable Pack has a particular value which
only activates at a predetermine temperature range. Since I'm not
electronically inclined I can not describe all the mechanics, but in
the Newton when the Rechargeable Battery Pack reaches this set
temperature limit the Newton will stop charging the pack to prevent
over-charge or possible explosion of the pack (which might be just a
POP! and the cells crack open.)

But since we don't know where you got this battery pack(s) from and
what type of possible protective features or charging setup was
implemented there's no real way to determing the use of this unknown
item unless you contact the manufacturer whom might explain it's use OR
knew first hand the item in question.

Sonny

 

[Eeyore]

John Riley,

I'm sorta throwing this out and hope that it may be the right answer
for you.

I have a Newton which also uses a rechargeable pack which consists of
(4) AA NiMH Cells. As part of it's construction it too has a thermistor
installed. The themistor is present to monitor the temperature. This
thermistor in the Newton Rechargeable Pack has a particular value which
only activates at a predetermine temperature range. Since I'm not
electronically inclined I can not describe all the mechanics, but in
the Newton when the Rechargeable Battery Pack reaches this set
temperature limit the Newton will stop charging the pack to prevent
over-charge or possible explosion of the pack (which might be just a
POP! and the cells crack open.)

No. The thermistor is indeed there to monitor battery temp but this additionally
gives vital inforamtion about its state of recharge.

Graham