With that last clarification i think i understand what you are saying mr. Lefty. Please correct me if i am wrong:

You are basically saying that as the battery charges up, its voltage increases. A constant current charger will try to maintain a constant charging current and therefore will also increase its charging voltage more or less in the same manner as the battery itself increases its own voltage (since it charges up). When the battery is at full charge, its voltage (aka EMF) will start dropping. The charger will keep the charging current constant and will therefore have to decrease its battery charging voltage in a similar manner, following the voltage drop (-deltaV) of the battery. By measuring the drop in voltage of the constant current charger we can identify a drop in the voltage and therefore stop the charging process since the battery is full.

Am i correct?

[@ Mr.Lefty]

@ Mr.Lefty
I know for a fact that in order to charge a battery that has an open circuit voltage of lets say 6 volts, at any current, the voltage you supply to the battery (aka power supply voltage) has to be higher than the 6 volts of the battery. If you connect a voltmeter across a charging battery then you would essentially measure the voltage supplied to the battery by the charger in order for it to charge and not the actual EMF of the battery.

@ Mr.Retroplayer
Say i have a 9 volts supplied to the circuit you are suggesting. The voltage drop across the resistor is 1 volt and the current through it (and the battery etc) is 1 ampere. Then as you suggest the voltage across the battery would be 8 volts, and as Mr.Lefty said earlier, this could be simply done using a voltmeter across the battery, since i am not interested in the current going to the battery (there is no need for the shunt resistor). The voltage across the battery is the voltage provided by the charger to the battery and not the EMF of the battery. I want a way to measure the EMF of the battery, if there is one, other than disconnecting the battery and measuring the open circuit voltage.

[how]

What i am asking is how do the chargers to this. ie how can it be done with a microcontroller. I would like some technical details. I mean, the "loop" that the microcontroller would have to repeat in order to figure out when this -deltaV has been reached and some circuitry that would be required.

@Mr.Retroplayer:
You kind of confused me there. If i understood correctly, you are basically saying that by knowing the voltage you provide to the battery through the series resistor and the voltage drop across the resistor in series with the battery you can calculate the EMF of the battery ?? (ie EMF of the battery = voltage supplied to the resistor and battery - Voltage across the series resistor)
>Wouldn't this equation give me the voltage that is used to charge the battery?
>Would it give me the actual EMF of the battery in order for me to log the data down and be abple to determine the -deltaV?
> Wouldn't i have to stop the charging process, measure the open circuit voltage of the battery and the continue with the charging process in order to measure the EMF of the battery?
>By the way, i know that for lipos, its mostly the cell voltage that changes (drops) while the battery is drained (increase in internal resistance has a smaller effect). Does this not apply for NiCads and NiMhs? Does internal resistance vary greatly while the EMF of the battery remains more or less constant?

@Mr.Lefty:
How do you continuously monitor the battery terminal voltage while charging? What to you mean the change in direction of the voltage?

Again thanks for the help people.

As you know, one way to detect whether a NiCad or a NiMh battery is fully charged is when the battery voltage starts dropping (-deltaV). My question is: How do chargers detect such a drop in voltage while the battery is charging? The only way to measure the battery voltage is to stop the charging process and measure the voltage right? How would a microcontroller be programmed to detect such a voltage drop? (ie charge for 2 mins, stop charge, measure voltage, store value, charge for 2 mins, stop charge, measure voltage, store value, compare the present value to the value obtained 2 mins ago)

Can someone please educate me on this matter?

Thanks in advance

Thanks for the info Lefty

I just googled it and the search came up with results of a crazy scientist and the flux capacitor and time travel etc. It's a TV show right? I must have watched one or two episodes of the show a long time ago as i remembered his face but not enough shows for me to know the title of the show or to have witnessed the extraordinary/amazing power of the FLUX CAPACITOR :O

And whats a flux capacitor? :O

In the past i opened some cordless drills and i noticed that their motors would have a metallic ring around the motor's metal casing. Why is that? What is it used for?

(The ring i am talking about can be clearly seen in this picture http://images.auctionworks.com/hi/59/59048/johnsonmotordrillwswithc.jpg)

Or an even smaller value to reduce the power dissipated and increase circuit efficiency

Thanks for the help Mr.Lefty and Mr.Mike
By the way these sensors are extremely expensive. I guess i ll stick to the good old resistor for measuring current :d 

What is the minimum amount of current that these sensors can sense? The current i would like to measure would be between 5mA to 10A or even more.

Is there another better/more sophisticated way to measure current other than to pass the current through a resistor and measure the voltage drop across it?

I would like to know how do electronics (ie smartphones,mobilephones,laptops etc) know exactly what % of juice is left in the battery and how long that juice is going to last.

Thank you in advance

Thanx for the help people i appreciate it a lot. I just discovered what that tiny thing is. I forgot to mention it only has two leads.

It is basically a battery thermal switch a.k.a. thermostat. When the battery gets too warm, the contacts inside separate and the charging process stops since the circuit is open. Once the temperature climbs back down to safe levels, the contacts touch and the charging procedure resumes. As i said previously it was found in the heart of a Ni-Cad drill battery not Lipo.



 

I opened a bad old cordless NiCad drill battery last week. The battery has three terminals. The tow terminals on the sides are labeled + and - and as expected are connected onto the ends of the series cells of the batteries, but there is another smaller connection which doesn't state anything. Its not used by the drill but by the charger and when opening the battery, a small,thin, rectangular object reading  ""Klixon 7-S 4MM45B2-21V Z3J" was on it. I haven't been able to find any datasheets for it.

Could someone tell me what that rectangular thing is and what it is used for? I found a similar one i another NiCad drill battery.

Thanx in advance... Have a great day