Are the batteries arranged in series like your drawing shows? If so your discharged voltage will be around 15V and your charged voltage will be about 16.8V (1.25V/cell discharged, 1.4V/cell charged). LM317 reference is 1.25V across the 68 ohm resistor for charge current of (1.25/68) 18mA charge current. Dropout voltage on the LM317 is for that rate is never more than 2V (page 6 from the datasheet tyblu linked to, very conservative estimate).So minimum input voltage needs to be (conservatively) 2V + 1.25V + 16.8v = 20.05V

How did you choose the value for the resistor? A safe charge rate (if you're watching it to turn it off) is C/10 which should be (100-mAh/10) 100mA charge current. That gives a resistor value of (1.25V/.1A) 12.5 ohms with power rating of (.1^2*12.5) .125W. Are these 12 cells in series or parallel?

Are the batteries arranged in series like your drawing shows?

If so your voltages will be

• ~15V (1.25V/cell discharged - reference Roomba chargers)
• ~16.8V (1.4V/cell charged)

LM317 reference is 1.25V across the 68 ohm resistor for charge current of (1.25/68) 18mA charge current. Dropout voltage on the LM317 is for that rate is never more than 2V (page 6 from the datasheet tyblu linked to, very conservative estimate).So minimum input voltage needs to be (conservatively) 2V + 1.25V + 16.8v = 20.05V

How did you choose the value for the resistor? A safe charge rate (if you're watching it to turn it off) is C/10 which should be (100-mAh/10) 100mA charge current. That gives a resistor value of (1.25V/.1A) 12.5 ohms with power rating of (.1^2*12.5) .125W. Are these 12 cells in series or parallel?

Are the batteries arranged in series like your drawing shows? If so your discharged voltage will be around 15V and your charged voltage will be about 16.8V (1.25V/cell discharged, 1.4V/cell charged). Safe (or, safest) charge rate is C/10 which is 1000mAh/10 =100mA so the voltage drop across the 68 ohm resistor is (V=IR, .168 = 6.8) 6.8V. Dropout voltage on the LM317 is for that rate is never more than 2V (page 6 from the datasheet tyblu linked to).So minimum input voltage needs to be (conservatively) 2V + 6.8V + 16.8v = 25.6V

Are the batteries arranged in series like your drawing shows? If so your discharged voltage will be around 15V and your charged voltage will be about 16.8V (1.25V/cell discharged, 1.4V/cell charged). LM317 reference is 1.25V across the 68 ohm resistor for charge current of (1.25/68) 18mA charge current. Dropout voltage on the LM317 is for that rate is never more than 2V (page 6 from the datasheet tyblu linked to, very conservative estimate).So minimum input voltage needs to be (conservatively) 2V + 1.25V + 16.8v = 20.05V

How did you choose the value for the resistor? A safe charge rate (if you're watching it to turn it off) is C/10 which should be (100-mAh/10) 100mA charge current. That gives a resistor value of (1.25V/.1A) 12.5 ohms with power rating of (.1^2*12.5) .125W. Are these 12 cells in series or parallel?