Portable battery-powered applications sometimes also
permit operation from the power grid. If so, the application
must choose the appropriate available power source—the grid
or the battery. If the battery is rechargeable, the system also
requires a charging circuit.
Instead of employing a dedicated charge controller for the
backup-power controller, designers can choose the more economical
option of using an inexpensive eight-pin microcontroller,
available from many manufacturers, as a charge controller
for a variety of charging algorithms and battery chemistries.
An added advantage of this approach is that if the intended
application uses a microcontroller, the same device can implement
the charging algorithm, provided that the application has
a few I/O lines and code space available.
For our design, we chose an Atmel AVR Tiny13, which
works over a wide supply voltage range from 1.8 to 5.5 V (Fig.
1). The controller monitors a 9-V nickel-cadmium (NiCd)
rechargeable battery. If its voltage drops below 9.6 V, the controller
detects—through transistor Q3—whether or not grid
power is available. If available, the controller starts a batterycharging
cycle. The charging circuit employs pulse-width
modulation (PWM), with the PWM duty cycle inversely
proportional to the battery voltage. So, as the battery gets
charged, the PWM duty cycle is proportionally reduced.
The battery charges through transistor Q2 and transistor Q1.
A logic 1 at Q2’s base turns on the transistor, which then turns
on Q1 and starts the charging current. When the battery voltage
reaches 9.8 V, the charging cycle is terminated.
While the battery is charging, indicator LED D1 is turned on.
If the grid power is available, LED D2 also turns on. The microcontroller
is operated at the lowest clock frequency possible
on the Tiny13—128 kHz—in order to minimize the power
consumption.
The charging algorithm can be modified for lithium or nickelmetal-
hydride (NiMH) batteries. When you use these types
of batteries, which require constant-current charging, Q1 can
be replaced with a suitable current source. If required, battery
temperature can also be monitored. The Tiny13 has additional
analog-to-digital inputs available. A suitable temperature sensor,
such as a thermistor, that’s placed in thermal contact with
the battery can monitor the temperature. Thus, the controller
can modify the charging process appropriately.
The control program for the Tiny13 was implemented in C
using the AVRGCC compiler under a Windows operating system.
The AVRGCC is available for Linux as well. Figure 2 illustrates
the flow diagram for the control program. For the code listing, click here.
