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Next we need to build a battery charger.
Battery charger
To safely charge the battery, we need to know how much volt-
age we must drop between the DC power supply and the bat-
tery and what current we can safely run through the battery for
extended periods of time.
The camera owner selected a PowerEx 9.6 volt NiMh battery
with a capacity rating of 230 mAH. This means that the battery
will supply 9.6 volts when fully charged and will deliver 230 mA
(0.23 Amps) for an hour. Thus, a fully charged battery will run
the camera for almost two hours (0.23 / 0.12 = 1.92).
These sorts of batteries will accept a charge current of 0.1 C
forever without damage. Okay, what’s C? Not the speed of
light, as in the theory of relativity. C stands for the Capacity of
the battery. So, a 230-mAH battery will stand 23 mA (0.023
Amps) or less of charge current on an ongoing basis. So, we
design the circuit to supply this current to the battery.
There is a diode between the DC power supply and the resistor
connected to the battery. This is a check valve to keep the bat-
tery voltage from feeding back into the power supply. As I said
DCC Impulses Column - 5
7: Battery charger circuit diagram.
7
MRH-May 2013
