Specifications

70
Figure 2.12.3-2: Configuration of the APG configuration (as opposed to the
thermistor configuration) of the BQ24005. Reproduced with permission of Texas
Instruments.
2.12.4 Charging Circuit Comparison
Several different charging circuits were compared, with a focus on reducing cost,
size, and improving quality. While the simpler circuits offer less complexity as
well as possibly less cost, a single IC solution from ISL6291 is less complex than
the other options when viewed from a top down assessment. As such, the
optimum choice is the BQ24005.
While the BQ24005 works best when connected to a thermistor, the battery
selected does not include this feature. As such, it could be connected, but
disassembling the pack and reassembling could introduce failures which may
cause the charge to prematurely terminate charging, or charge past the proper
thresholds and cause dangerous conditions within the battery, leading to a fire or
explosion. As such, it is prudent to simply use the other indicators without
complicating status feedback from the battery. If there are any failures, it should
be visible within the circuit LEDs, if so connected.
As previously mentioned, this will be implemented as a built-in solution to the
portable unit. While this will require the user to go to a service center to
replacement a defective or worn battery, it is optimal for the reasons previously
mentioned. In addition, the existing applications of a fixed lithium ion batteries
show a sufficient success rate to show this strategy as acceptable. A summary of
the comparisons can be found in Table 2.12.4-1.