Specifications

the battery (charging cycles). Given the fixed nature of the battery to begin with,

making the battery last as long as possible is a consideration.

Since the cell being used is a two- cell configuration (in order to reach the

required minimum battery voltage), these numbers are doubled. The required

charge voltages go to about 8.4 Vdc. The charging circuit must be able to supply

such a voltage. In addition, the current should be proportional to the capacity of

the battery. Since this battery is 850 mAh, the current should optimally be around

0.85 A for charging.

However, additional logic is required in this case in order to constantly read the

input from the current or voltage sensor, detect an end-of-charge condition, and

terminate charging. This arrangement utilizes a microcontroller and a voltage

sensor to achieve the necessary level of automation to make such a circuit work.

When an end-of-charge condition is detected, the microcontroller will use the

power transistor to “turn off” the V

charge

, the charging voltage. Note that an

analog-to-digital converter is not detailed here (in order to be able to read

useable values from the thermal probe). It is assumed to be part of the

microcontroller.

There exists an integrated IC that combines all the intelligent charging logic into a

single package. The BQ24005 chip by Texas Instruments is a package specially

designed to act as a nearly complete charging solution for two cell lithium ion or

lithium ion polymer batteries. This matches the application here.

Implementing this package into the board circuit will require several capacitors,

resistors, and LEDs. An example implementation is seen in Figure 2.12.3-1.

Several aspects of this chip can be configured to match the desired application.

Since an 8.4Vdc charge voltage is desired, pin VSEL will be connected to high.