Selection of a resistor for bleeding LiIon cells

[element]

Hi,

I am designing a Li-Ion pack and was wondering....would I have to use those 5W resistors for bleeding the cells as they are quite big in size for a PCB...I am using 5W resistors because maximum voltage of a single cell can be 4.2V and I need atleast 550mA bleeding speed. 2W resistors won't do it...Is there any other alternative or resistors that are small in size yet are capable of dissipating this much amount of heat??...The resistor values available should also be atleast 6Ohms

 

[Audioguru]

Why are you "bleeding" a Lithium rechargeable battery cell??
If it is discharged below about 3.2V then it is ruined.
Maybe you are discharging it down to about 3.7V for storage?

For the winter, I discharged my Li-Po batteries down to 3.7V per cell for storage using a tail light bulb for a car.

 

[element]

This is to balance them while they are charging....instead of cutting off the string, I want to bleed the cell which reaches 4.2V to the level of the cell with the 2nd highest voltage and then open the switch...In this way, all of the cells would be charged to 4.2V at the end of the charge cycle.

 

[ArticCynda]

If it is discharged below about 3.2V then it is ruined.

This depends on the cell chemistry and the manufacturer. LiCoO2 can be discharged to 3.0 V (deep discharge). Optimal storage charge for most chemistry types is 60% at lower temperatures (i.e. a fridge).

 

[Audioguru]

The Li-Po batteries for my radio controlled electric model airplanes charge to 4.2V per cell. While powering the airplane, if the voltage drops to 3.15V then the motor pulses as a warning that the voltage is too low and if the voltage drops to 3.05V or less then the motor stops but the guidance servos still function. The manufacturer says that if the low voltage protection occurs often then the life of the battery is shortened. The manufacturer says to store the batteries at 3.7V per cell.

 

[ArticCynda]

if the voltage drops to 3.05V or less then the motor stops but the guidance servos still function.

At that voltage you should immediately disconnect any loads. Rotating servos will lead to current spikes, causing the voltage to drop below 3.0V. At this voltage, the lithium ions will be reduced to metallic lithium, a process which is irreversible. This leads to permanently lost capacity.

 

[Audioguru]

The servos on my lightweight radio controlled model airplanes use low current.