[SOLVED] charging li-ion battery safely

i want to charge my opened li-ion battery. it is rated with 3.7v and it is working properly. but my old mobile(phone) is broken. i also have the different model mobile but, both battery model is same size and same ratings. but i have opened the battery so that i cant charge battery by plugging in with the mobile. when i connect the multimeter with the two + and - terminals of the mobile, i measures 0.05 v :-? and when i connect multimeter with the output of charger, it is 7.8 v:?: ,while rating of charger is 5v dc . i also found that the mobile and the multimeter is working properly.

i go for checking third smartphone device, the pin outs giving the reading of 4.2v. which is perfect for charging li-ion battery

what is wrong with my old mobile ?

if i connect two terminals of battery directly to the 4.2 v supply,will it disconnect automatically after full charging?

A lithium battery will blow up or catch on fire if you do not correctly charge it by limit the charging current, limiting the maximum voltage and disconnecting the charger when it is detected to be fully charged.
Your "charger" is not a charger, instead it is the power supply for the charger circuit inside the phone. The charger circuit for most phones has an input of 5V.
If you connect a lithium rechargeable battery cell to a current-limited 4.2V power supply then it will keep overcharging until it blows up or catches on fire.
Most charger circuits also detect when a Lithium battery voltage is too low and will refuse to charge it or will try charging with a very low current and if the voltage does not rise soon then refuse to charge it.

but what will happen if i give li-ion battery 4.2v from voltage limited power source?
which kind of power source is charger? current controlled or voltage controlled power source or both?

You have already been warned about the dangers of playing around with Li-Ion batteries. They are very dangerous if you dont know what you are doing. The charging circuit controls both voltage and current.

I think primary concern is the current while charging li ion battery. While charging the battery, my understanding is current can be controlled ony by controlling voltage across the battery. Charging current can only be increased if terminal voltage is increased.

samEEEf said:

I think primary concern is the current while charging li ion battery. While charging the battery, my understanding is current can be controlled ony by controlling voltage across the battery. Charging current can only be increased if terminal voltage is increased.

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Absolutely not! Please post a video of your battery exploding. Also please learn about how to charge a Lithium battery. Go to www.batteryuniversity.com .
The first stage of charging a Lithium battery is feeding it a constant current that is recommended by the battery manufacturer. If you feed it a voltage then the current will be massive and KA-BOOM!
The second stage is to limit the voltage to 4.20V per cell.
The third stage is to wait until the charging current drops to 3% to 5% of its mAh rating then disconnect the charging because a trickle charge is not allowed.

samEEEf said:

I think primary concern is the current while charging li ion battery. While charging the battery, my understanding is current can be controlled ony by controlling voltage across the battery. Charging current can only be increased if terminal voltage is increased.

Click to expand...

dear, sameeef;

when i was new to the electronics, i also feel that ohm's law. like if i want to increase the current, i should apply more voltage. but, it is for the passive elements like resistor, inductor, etc they are following the ohm's rules. for this, the passive element's resistance is constant.

but for power source, the the resistance is not constant. so power source can deliver with any combination of voltage and current. like it can deliver 10000000A current at 1v and 9999999v at 0.111111A current.
this all because of internal resistance of power source.


because of the internal resistance, the batteries are not able to deliver this type of power.

Hi,

This is a nice introduction to battery charging for:

"...charger circuits for use with Nickel-Cadmium (Ni-Cd), Nickel Metal-Hydride (Ni-MH), and Lithium-Ion (Li-Ion) batteries."

Audioguru said:

Absolutely not! Please post a video of your battery exploding. Also please learn about how to charge a Lithium battery. Go to www.batteryuniversity.com .
The first stage of charging a Lithium battery is feeding it a constant current that is recommended by the battery manufacturer. If you feed it a voltage then the current will be massive and KA-BOOM!
The second stage is to limit the voltage to 4.20V per cell.
The third stage is to wait until the charging current drops to 3% to 5% of its mAh rating then disconnect the charging because a trickle charge is not allowed.

Click to expand...

Thanks for clarifying li ion battery. Is it ok for Lead Acid battery?

A lead-acid battery is charged in my car and I have never touched it. I have never used an old lead-acid battery for electronics. If the charging current is too high then the electrolyte boils away.
The Battery University tells you a problem that occurs when the charging current is too low and another problem when the charging current is too high. They say charging current, not charging voltage.