Hi,

I recently bought this charger: http://www.rcbazaar.com/products/992-wolfpack-b3ac-2s3s-compact-charger.aspx
I tried to charge a 4200mAh 3S LiPo pack with this charger. When the charging starts, a sharp high-pitched noise can be heard from the charger. After about a minute or so, this noise completely fades away. Is this normal?

I also tried a 15 minute test run. The battery pack does not get warm and the charger runs slightly warm.

I too have a similar problem with a similar simple charger but not from RCB.
When I charge  a 3S 2200mAh battery I get a high pitched noise. It is just because the maximum current that the charger could supply is 400mA but the battery is capable of draining more ,this increases the load in the primary switching circuitry which causes the noise.As the charging nears completion the charge current consumed by the battery reduces to minimum i.e. within 400mA and hence the noise fades away.

Senior members correct me if I am wrong.

The battery has no mechanism to draw higher current than what is provided. It'll accept only what is fed into it.

More details at
 http://www.physicsforums.com/showthread.php?t=650961
 
 http://www.edaboard.com/thread213912.html

it could well be the cooling fan noise

The charger doesnt have a fan. Just opened up my charger, there is only a transformer and inductor that could possibly generate a noise among the rest of the components.

http://en.wikipedia.org/wiki/Coil_noise

And if its the inductor, then I assume there is nothing much to worry about if the charger is functioning normally. And these chargers do get a bit warm at the beginning of charging. Have been using mine for 2 years, doesnt make a sound tho.FYI The capacity of your battery is too large, it would take 4-5hrs to charge it @ Charging current rate 850mA.

Yes. It does seems to come from the inductor.