Specifications

The battery itself must be recharged. While various configurations were

discussed earlier, the built-in battery configuration turns out to be the optimal

solution. To this point, a charging circuit then becomes necessary, or else the

battery would become a single-use battery, necessitating a visit to a service

center every time the battery was exhausted. Generally, batteries must be

charged at a voltage higher than their output voltages. With lithium ion chemistry

batteries, the nominal charging voltage is 4.1-4.2Vdc. Since this is a two-cell

lithium ion battery generating 7.4Vdc, the charging voltage is 8.1-8.2Vdc. The IC

detailed earlier would automatically take care of this. However, an external

source is still needed.

The external source will be a commonly available AC-DC power supply. It could

also be recharged using a simple adapter which plugs into the 12V supply in a

vehicle. The specs of this unit will be a 12V supply, as these are commonly

available. The current capability should be at least 2A. The charging IC

recommends the input voltage to be no higher than 10Vdc. However, to achieve

this, an additional regulator would need to be connected between the external

input and the input to the charging circuit. This is undesirable as current limits

would then have to be taken into consideration. While it is possible to drive the

regulator at its upper limits, it is not advisable for longevity.

However, another option of driving the charging circuit at 12Vdc is available.

While not within the recommended operating conditions specified by the

BQ24005, it is specific as the maximum safe limit for the input voltages. It is still

possible to maintain this level with an acceptable life to the IC. However, it may

be necessary to utilize heatsinking due to the heat generated by this IC even

under regular operating conditions.

The two status LEDs D1 and D2 will provide feedback about the charge status of

the battery. No additional power protection is provided in the portable unit’s

power supply as not only would it occupy additional space, but there is also a

design assumption that the external power supply provided will have some basic

power protection as a part of its circuitry. These LEDs are capable of providing a

fairly thorough overview of the various states the charging circuit could

encounter, including fault conditions. These states are summarized in Table

3.3.1-1.