Hi folks I wonder if anyone could help me out with a circuit ?
:roll:
I have designed a simple slow NICAD charging circuit using the LM317 to charge a 5 cell NICAD battery pack at 9volts 700mA. For a Kirby caving lamp.

Instead of using a moving coil ammeter to monitor charge I would like to use a variable colour LED to monitor charging. With it changing from Red to Green as the battery charges.
I have looked around the web but to no avail yet. Does anyone have any ideas, advice or links ?

My electronics is a bit rusty.......

Thanks in advance,

__________________
Gary Collier, UK

 

From your post I am not sure what current you are charging the Nicads at.
I would assume you have the LM317 set up as a constant current charger.
If that is the case it is fairly easy to add a LED to monitor that the batteries are charging, provided your DC voltage source will add about
2 to 3 volts. By using a comparator you can detect when the batteries are
fully charged, as nicads have a individual cell voltage of very near to 1.5 volts per cell at room temperature when fully charged.

Several years ago I build a number of nicad chargers that had 3 LED's,
1 indicated applied power, 1 indicated that the batteries were charging,
and the last one illuminated when the battery pack was charged. It is a pretty straight forward circuit I would be glad to share with you if you
are interested.

__________________
The great thing about electronics is unlimited ways to do the job. The only limit is one\'s imagination. I generally think my way is best.
Show me a different way. I have an open mind.

 

It seems like this method could be tricky...

I thought a property of batteries NiCd's in particular was that when near full charge, the voltage changes very little. I mean the difference between say 95% charged and 99% is not that much. Now when they are almost fully discharged, that's a different matter. The voltage will drop drastically below around say the 2% level...

A comparator does not need to sense much change in voltage at all to trip but its reference input would need to be calibrated so-to-speak at very near the 100% level and how could one do that?

I've read that one of the more reliable ways of sensing 100% charge is to measure temperature of the cell. Given a constant charging current, when the cell reaches full charge, power from the cc generator must be dissipated as heat and the cell temp will begin to rise significantly, if one can sense this, then they can place the charger in a trickle state and of course, give an indication of full charge via LED or whatever..

I myself have never designed a charger so I can't speak from experience.
I'm interested to know how you accomplished 100% charge sensing without overcharge by measuring cell volts..

 

I think I know of a circuit that may be very similar to the one you used. Is it based on a comparator circuit and 4 PNP transistors ?

http://homepages.paradise.net.nz/bhabbott/charger.html

The cells are contained in a caving lamp battery pack so sensing the temperature would not be easy to do really. I will provide a link to the page showing the lamp and also one for the charger.

The lamp is the 2nd kit down known as the Kirby Kidney lamp:
http://www.cavinglamps.com/lamps/lamp_page.html

The charger is the 2nd kit down also. I think the kit uses the LM317 with just a moving coil as an indicator:
http://www.cavinglamps.com/chargers/chargers_page.html

Maybe I should consider using the PNP transistor circuit rather than the LM317 ? Maybe I could try and incorporate a comparator into the LM317 circuit..... hmmm So many options......

__________________
Gary Collier, UK

 

[quote="garycollier"]I think I know of a circuit that may be very similar to the one you used. Is it based on a comparator circuit and 4 PNP transistors ?

http://homepages.paradise.net.nz/bhabbott/charger.html

/quote]

The design shown from this URL illustrates my point about the sensitivity of peak charge detection. The peak detect circuit used is clever.

In your design, I think you will want to provide the same kind of functionality as this one does.

 

Nicad batteries have some interesting properties. The voltage on the cell
terminals
during the charge process can reach almost 1.5 volts. If the cell voltage is at
cutoff (1.0 volts) when the charger is applied the voltage will increase as the cell
charges. Let's assume the charge current is 0.1C and the temperature is 20
degrees C. When the cell is fully charged two other things happen, on the cell
temperature reaches a peak and the internal cell pressure also reaches a peak. The
interesting thing is that the cell voltage peaks a little before the other two peaks,
in fact the cell voltage peaks before full charge and the dips a little at full charge
and this dip occurs at the peak of the temperature and pressure.

I worked for over 10 years at a electronic distributor that in the value added
department, rebuilt nicad battery packs. I worked in that department for a while
and learned from the supervisor that if one is to charge a nicad battery at
.095C (capacity of the cell in Ma X.095) the cell will never over charge at room
temperature. I also learned that quality packs had an internal thermostat the
had a set of normally closed contacts. When the internal temperature reached
the temperature of the thermostat the contacts opened and current would neither
flow into the pack or out of the pack.
The attacked schematic is for a charger that was specific for a 7.2V 170MA pack, but can be modified for other batteries

The supplied schematic is a design for a 7.2V 170Ma nicad battery. By changing the values of R8(75
ohms)and R2(22ohms)it will work with other Ma capacity batteries. By changing the value of R3(560k)
the circuit will also work for other voltages. The circuit as shown will have very close to 9V at the
output terminals when the 7.2 volt battery is fully charged. This is assuming a 1.5 volt per cell for a
6 cell (7.2 Volt) nicad battery. At this condition the voltage on the high side of the 100k pot. is 0.8V.
Voltage drops for other components are typically:
R8 1.25V
R1 0.375V
D1 0.5V
LM317 2.0V
For the circuit to operate correctly the input voltage should be a minimum of 5.0 Volts higher than
the full charge voltage of the battery.
All resistors excepty R1 and R8 are 1/4 watt. The wattage of R1 and R8 depends on the charge current.
The LM317LZ will handle charge currents of to 100Ma, for higher charge currents us a LM317T.

The green LED is on when power is applied, the red LED is on when the battery is charging, and the
yellow LED comes on when the battery is charged, or if the charger is connected to power and no
battery is connected.

My recommendation is that you choose a charge current less than capacity/10. If you will give me the
charge current I will recalculate the values for R1,R8 and R3.

Attached Images

NICAD_CHARGER.JPG (29.0 KB, 931 views)

__________________
The great thing about electronics is unlimited ways to do the job. The only limit is one\'s imagination. I generally think my way is best.
Show me a different way. I have an open mind.