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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 |
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 |
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.