lithium battery charger voltage question

[curtqn]

I use a simple plan from a rc magazine to build a li-po battery charger.
A "10K-Ohm 15 turns trimmer" is used to adjust the voltage.
The problem is the voltage only goes up around 12 volts.
Any guru in electronics can give me some ideas how I can make the voltage go to 12.6 volt for 3 cells.

Please take a look at the picture:

http://www.geocities.com/curtqn1/lithibattery.html

[Andy W]

What is your input voltage? What is the voltage on the "In" pin of the regulator?
..a

[Bruce Abbott]

According to my calculations the highest voltage you could get with those values is 11.9V. Replace the 2200 Ohm resistor with a 2700 Ohm resistor, and you should be able to get about 13.6V maximum (assuming the supply voltage is high enough. You will need about 15V for the regulator to operate properly).

[curtqn]

Thank for your reply.
My input voltage is 13.8 volt from a power supply.
If I use 2700Ohm resistor instead of 2200Ohm will it cause any damage to the battery?
I charge at 1C. It take over 1 hour to charge 2 cells but when I go to 3 cells it take the whole night to charge. I wonder this charger does work with 3 cells

[KillerWatt]

To use these type chargers (which seem to require a lot more input voltage than what's needed out, but not quite double) on 3S LiPo packs, a power supply of about 18 volts DC works real good......Something like a "Dell" brand laptop switching supply gets the job done and the DC voltage from it is real clean and free of noise.........kw

[Bruce Abbott]

If I use 2700Ohm resistor instead of 2200Ohm will it cause any damage to the battery?
Not if you adjust the open circuit output to 12.6V :).

13.8V is not enough supply voltage. The regulator has a dropout voltage of about 1.6V at 100mA and 2.0V at 1A. Depending on the charge current you will need between 14.2 to 14.6V minimum, preferably more since the light bulb also absorbs some voltage.

[Andy W]

Thank for your reply.
My input voltage is 13.8 volt from a power supply.

That's what I figured. You have to supply at least 15V here.. I think most folks use 18V..
..a

[Juko]

Who sells voltage regulators for 5-6 cell battery packs?
Need it to be small enough to fit inside a 2 meter sailplane fuselage.

[zagisrule!]

Like other suggest, the internal voltage drop of the regulator will prevent you from obtaining your desired amount with that supply. 15V would be adequate, but 16 would be a safer bet and allow for some headroom so to say.

I am working on a SMPS to boost 12VDC up to around 25VDC at several amps for my newest lithium, ni-cd, and ni-mh charger. I'll post the schematics when I am done, I already have stock of the transformers we need, which were custom made and are very hard to find.




-Matt