Specifications

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Figure 2.12.3-1: A typical implementation of the Texas Instruments BQ24005.
Reproduced with permission of Texas Instruments.
While lithium ion batteries do not require any sort of special “maintenance” type
of charge modes, they do have to be charged in certain ways depending on their
states of discharge. A deeply discharged battery cannot be charged as a battery
with a moderate amount of charge left. The BQ2400x is capable of sensing this
condition and will enter a precondition mode, where it can compare the battery
voltage to an internal, fixed threshold. The battery is charged at a low current
until the battery can be brought up to a safer voltage before ramping up the
current to normal charge current.
This current can be limited by adjusting the Rsense resistor. There are additional
methods of detecting any unsafe conditions with the battery, including reading a
thermistor value as noted in the initial block diagram. However, no thermistor will
be used in this design. Alternatively, system input voltage may be monitored by
utilizing the AGP/THERM pin. This configuration is shown in Figure 2.12.3-2.
The values of R1 and R2 can be calculated using a formula. This will allow the
circuit to recognize when there is an external power source connected (i.e., when
it is connected to the charger power).