"NiCad" battery charger

LM 317 is a good IC Voltage regulator for small power supplies. FIG. 1 shows another application of LM317 as "Constat Current source" for "NiCd" battery charging.

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FIG. 1

Example: we need a NiCd battery charger for a 700 mAh battery case.
How is it the "R" value?

Answer: NiCad batteries must be charged with 10 percent of their capacity...
that means 700 x 10% = 70 mA or 0.07 A... =>

R = 1.25 / 0.07 => R = 17.85 OHM


The commercial resistors have standard values 10, 12, 15, 18, 22, 27, 33 .... so, the closer value for 17.85 is a 18 Ohm resistor .

Imprortant notes:
1) input voltage must be 5-6 Volts higher... i.e. for a 7.2 V battery case the input voltage must be 12.2 V or higher.

2) For currents above 50 mA, LM317 needs a Hotsink but keep in mind:
the central conductor has an internal - connection with metal- body of IC .
DON'T put the IC-body direct to a GND-metal box for hotsink without isolation! In this case we have short-circuit between pin2 of LM317 and ground (--)

Finally, during charging "NiCd" batteries have a thermal-loss of about 40-50 %. That means a higher total charging period... i.e. for a 700 mAh battery case it is not enough a 10 hours period (10 x 70 mA =700 mAh) but a 14-15 hours period (total battery capacity "plus" thermal loss).


The following table has various Battery capacities and "R" values

Battery capacity
"R" value
Time period
400 mAH
33 Ohms
15 Hours
500 mAH
22 Ohms
14 Hours
700 mAH
18 Ohms
15 Hours
1000 mAH
12 Ohms
14 Hours


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