Is the max charged voltage of the battery 4.1V or 4.2V?
What is the recommended charging current for the battery?
What are the spec's for the temperature sensor in the battery?

All you are doing is repeating what the cell phone designer has done to make the charger. Except you probably won't be able to find out what is the recommended voltage and current and details about the temperature sensor for the battery.

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Uncle $crooge

 

the max. voltage for a li-ion cell battery is 4.2V

on our charger, i've set the charging voltage to 3.9V - 4V with 400mAH (clarification: to limit the output current of a power supply, a series resistor on the ground should be placed? 4V / 10ohms = 400mA)

we don't have temperature sensor for the charger circuit.

if we have 3.9V as a charger voltage, does the battery voltage reaches at 4.2V? even though the charging voltage is lower than 4.2V?

 

The battery won't be anywhere near fully charged when the charging voltage is only 3.9V without a load. The voltage will be lower with the battery connected because your 10 ohms resistor isn't a current regulator.

The 10 ohm resistor limits the current to 400mA for a dead short. When the battery voltage reaches 3.0V then the 10 ohm resistor limits the current to only 100mA. When the battery voltage reaches 3.5V then the 10 ohms resistor limits the current to only 50mA. You need an active current regulating circuit to provide 400mA until the battery voltage reaches 4.2V.

You must use temperature sensing circuit to avoid a fire and manufacturers of Li-Ion batteries also recommend timers as another backup.

Read all about one manufacturer's Li-Ion battery charging here:
http://www.panasonic.com/industrial/...ion/index.html

EDIT: Panasonic shows that their Li-Ion cells are only half charged when the voltage reaches only 3.9V.

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Uncle $crooge

 

Look here too: http://shdesigns.org/lionchg.html

But you cant just hook this up to your cell phone, because it allready has a charger built in.

This will give you some idea what is needed for charging lithium batteries but, it has no extra safety features.

sam

 

I forgot to say that when the battery voltage reaches 4.2V then it is 90% fully charged. It needs another 40 minutes at 4.2V for its charging current to automatically drop to 100mA when it will be 100% fully charged.

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Uncle $crooge

 

how did you compute this? at NO LOAD the limited current is 400mAH...

when there's a load with 3.7V, the limited is equal to 370maH? or
400maH - 370mAH = 130maH?

 

At no load then there isn't any current.
When the supply is 4.0V then a dead short creates a current in the 10 ohms resistor of 400mA.
When the battery voltage rises to 3.0V then the resistor has 1.0V across it and Ohm's Law calculates the current to be only 100mA.
When the battery voltage rises to 4.0V then there won't be any current but the battery will not be fully charged.

You need to have an active current regulator to keep the current steady while the battery is charging. A simple resistor won't do it.

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Uncle $crooge