# "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

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