SHDesigns: Embedded Systems Design, Consulting and Developer Resources Page hits:

Modifying the design for different cell sizes:

R1 sets the current limit. Panasonic says to use 0.7C max current. 1 ohm works for 900mah or so. You can adjust R1 for a pack of any size. As long as the current is smaller than 0.7C, then there will be no damage to the battery. To adjust R1 for a different pack size:


 Design Parameters  Value  Example for 600 mAh
 Max Current (amps)  0.7 * C  0.600 * 0.7 = 420ma (0.420 amps)
 R1  0.65/Max Current  0.65 / 0.420 = 1.55 ohms
 R1 Power Dissipation  Max Current * 0.65  0.420 * 0.65 = 0.273 watts

You can not easily buy a 1.55 ohm resistor. So either use one that is close or make one by paralleling resistors. If you connect N resistors in parallel, the resistance divides by N and the power multiplies by N. So to try to get 1.55 we can use three 4.7 ohm, 1/4 watt resistors in parallel:

R=4.7/3 = 1.567 ohms - Close enough
P= 0.25 *3 = 0.75watts
Try to make a R1 value that is as close as possible or larger.

Here's some calculated values for R1:
 Pack Size  Calculated R1  Parallel R to use  Result  Max Current
 800mah  1.16 ohm  4.7 ohm * 4  1.175 ohm  553ma
 1200mah  0.774 ohm  2.2 ohm *3  0.773 ohm  886 ma
 600ma  1.548  4.7 ohm *3  1.567  415 ma

You can parallel different resistors to get the desired current. Here's a table of current per resistor:
Resistor Value  Current  Power Dissipated
 1.0 ohms  650 ma  0.423w, use 1/2w
1.5 ohms  433 ma 0.282w, use 1/2w
 2.2 ohms  295 ma 0.192w, use 1/4w
 2.7 ohms  241 ma  0.157w, use 1/4w
 3.3 ohms  197 ma  0.128w, use 1/4w
 4.7 ohms  138 ma  0.090w, use 1/4w
 6.8 ohms  96 ma  0.062w, use 1/4w
 10 ohms  65 ma  0.042w, use 1/4w

So if you wanted about 500ma max you could use a 2.2 and 3.3 ohm resistor in parallel for 295+197=492ma, close enough. 1/4 watt resistors are all that are needed for paralleled operation if the value is greater than 1.69 ohms.

Note: the LM317 is limited to 1.5 amps. A LM350 can be used for up to 3 amps.

Modify the number of cells:

R2 sets the voltage range. To change the number of cells, change R2 and adjust voltage as shown below:

# Cells  R2 Voltage Min. Supply
Voltage
 1  1K ohm  4.2  6v
 2  2.2K ohm  8.4  12v
 3  3.9K ohm  12.6  16v
 4  5.6K ohm  16.8  20v
 5  7.5K ohm  21  24

Increasing charge speed:

Panasonic specifies stopping charge at 4.2v/cell. But they say 4.3v/cell is the maximum allowed. For 2-cells that is 8.4 and 8.6v respectively. As the cell approaches 4.2v/cell the current drops significantly. I set mine for 4.25v/cell (8.5v.) This is under the maximum and reduced the charge time a bit. The laptop I took the cells from actually charged to 8.6v. So, 8.5 is a good value for better charge time. But don't leave the batts on this charger for more than a few hours. The improved charger design can be used. This will turn off the LED when the batteries are charged fully.

Change the LED turn off current:

R6 sets the current that the LED turns off. This is about 10ma. Use 22 ohm for 25ma, 27ohm for 20ma, 33 ohm for 15ma. Currents are approximate as the tolerances in the transistor and diodes are not exact and may vary a bit with temperature.