i want step down 8vdc to 3.6vdc to recharge cell phone battery

[fayyaz0346]

can any one helpe. i am useing 3.6v cell phone battery in a 2 bright light leds touch.problem is when i use normal cell phone charger its out put voltage is about 8v dc which is harm full to battery i want a circut diagram for cell phone battery charger which can cut off after battery is full.

 

[aquamon]

3.6V zener + resistor...

or

 

[fayyaz0346]

tried zener+resistor but zener diod gets very hot. im useing 120 ohm resistor .

 

[aquamon]

tried zener+resistor but zener diod gets very hot. im useing 120 ohm resistor .

Why would you use a 120 ohm resistor?

Here, try multiplying that by 10 as a start.

What value is the zener?

As you seem to be lacking in some electronix knowledge, just try using a resistor in series to act as a **broken link removed**.

 

[fayyaz0346]

Thanks for your help. my prob is solved accroding you advice yes you are correct i am new to electronics its my hobby.thanks once again for this cool link it will help me.

 

[amando96]

Just a warning, but since it's a cell phone battery, it's likely to be li-ion, as far as i know it doesn't just take current flowing through them to charge them, won't need a chip like **broken link removed** to manage the power properly?

 

[photonic]

Recharging Li-Ion/Poly cells safely

Hi all.

Having had at least one cell nearly explode due to overcharging, I would like to point out that a simple voltage regulator WILL NOT work.

Li-Ion cells (and their polymer cousins) NEED a proper charger, the precise voltage varies depending on type (CoO, spinel, etc) but is normally around 4.05V to 4.1V and no higher.
In addition if the cell is below 3.1V is needs a 1/20C (i.e. for 500mAh cell 25mA constant current) to 3.2V then current limited constant voltage Vmax until the drawn current drops below 1/10 capacity (i.e. 50mA for the above cell)

the cell then needs to be disconnected from the charger as even ten minutes of charging can damage it by plating out lithium metal onto the interior of the cell causing potentially dangerous instability even after several discharge/recharge cycles.

if you happen to see a "blown" (i.e. bulging) lithium cell then in all likelihood it has been either overcharged or overdischarged causing the electrolyte to destabilise and making the cell unsafe to use.

additionally if a cell is discharged below 2.75V then copper shunts form and cause internal leakage resulting in a dangerous temperature rise during any attempt at charging. You absolutely cannot "zap" them back to life so don't even try.
this is a different mechanism to NiCd whiskers and zapping does not work because the copper just ends up as debris in the electrolyte and contaminates it resulting in capacity loss and in worst cases a catastrophic internal short at or near full charge (read- BOOM).

LiFePO4 cells (easy to detect as their open circuit voltage is typically 3.25 to 3.35V) need a different charging algorithm, the "maximum" charge voltage here is 3.65 to 3.7V.
Under overcharge or overdischarge they just lose capacity and the internal resistance goes up so they still need a dedicated charger.
Interestingly with some of the newer generation this is less of a problem, as long as you don't charge them for days on end they somewhat self regulate and the current falls off to less than a mA once charged.

-A
memo to self:- $2 a unit for phosphate cells = counterfeits.

 

[cowana]

What exactly is this for? We might be able to suggest a better solution.

 

[aquamon]

Err, maybe you need to start your own thread on:
i want step down 8vdc to 3.6vdc to recharge cell phone battery
or
Recharging Li-Ion/Poly cells safely

 

[bountyhunter]

can any one helpe. i am useing 3.6v cell phone battery in a 2 bright light leds touch.problem is when i use normal cell phone charger its out put voltage is about 8v dc which is harm full to battery i want a circut diagram for cell phone battery charger which can cut off after battery is full.

You need to know what kind of battery chemistry and the battery charge information from the manufacturer.

As said above, some Li cells can be dangerous if charged improperly.

 

[audioguru]

The complicated charging circuit is inside the phone. The charger is not a charger, it is a power supply for the charger circuit in the phone.

Cell phones have used Lithium batteries for many years. They have a typical voltage of 3.6V or 3.7V but are charged with a limited current, not a voltage but the voltage must also be limited to 4.20V to avoid a fire. The battery charging current must be monitored and when it drops to a few percent of the battery's rated current then the charger must turn off. A lithium battery is never trickle-charged.

The lithium battery also needs a low voltage shutoff circuit so that it does not catch on fire.

 

[bountyhunter]

Li batteries use a charging method called CC-CV which stands for constant current/constant voltage. The charger limits it's current to a fixed amount until the battery voltage reaches the target value (typically 4.1 - 4.2V for Li) at which time the voltage control loop in the charger tapers off the charging current to hold the cell voltage at the correct value.

 

[unclejed613]

what previous posters have said, i will reiterate and expand on it a bit.... overheated or overcharged Li-Ion batteries are dangerous, in fact, they can explode. or catch fire, or both. some Li-ion batteries are in a pressurized rigid container and can fragement..... for more info, look here:
HowStuffWorks "What is causing lithium-ion laptop batteries to catch fire?"