I would like to know if my assumptions are right. I'm quite new to electronics, but after some work in my electronics lab, I could find out that a battery draws less and less current while it' s charging. One of my old battery chargers didn't work any more so I opened it up. Now to my question : Does this circuit have only a visual notification that the battery is charged ? And how does it work ? Like when it's fully charged, the LED goes out because the battery doesn 't draw any current ?

Battery Charger :

 

The leds are in series with a limit resistor , and this is in parallel with a diode, so as the battery is charged the voltage drop across the resistor has a Large voltage across it and lights the led. When the battery charges up the voltage across the resistor gets less and less until it is insufficient to light the led.

These are usually found in Nimh or nicad chargers.

 

The LEDs have a threshold voltage of about 2 volts so they have that built in characteristic which can be used as a decision voltage.

The principle is scaleable. For instance, I lucked on to a pile of surplus transformers for way cheap and made some car battery chargers. I found that when the charging current gets down to 3 amps, the battery starts bubbling. Bingo! Make a circuit which turns the charger off (or switches to a float voltage) when less than 3 amps flow and I have a fairly reliable shut off function. So yes, your observations are correct.

 

I could find out that a battery draws less and less current while it' s charging.

This depends on battery chemistry and charger design.

Old nicads and nimh could be charged up to 5C, IIRC, to about 98% of capacity. But you had to monitor both the cell voltage (for a characteristic dip) and temperature (for a characteristic increase).

Modern lithium cells use two stage charging: constant I followed by constant V.

 

Thank you all for the answers ! It's been a while ago since I've posted this question, and I learned much more about electronics by reading books ( for example Art of Electronics ) and by practice in real life.