Specifications

8.4. BATTERY SELECTION AND CHARGING

connected to PortA.RA4 on the GSM Logger’s Microprocessor. This is an

open collector output with FET output. It is thus able to turn T9 oﬀ by

pulling the base low when the battery is fully charged. Thus, should the

battery be completely depleted, it would immediately begin charging until it

is suﬃciently charged for the microprocessor to power-up and regain charging

control. Should the power be connected when the battery is already fully

charged, the microprocessor would detect this and stop charging to prevent

a harmful over-charging condition.

Provision has also been made for the microprocessor to sense the presence

or failure of the supply by sampling the voltage us ing an ADC channel.

8.4.2 Software

An additional FSM is added to the PIC 18F452 Microcontroller on the uplink

logger for battery charging and power management. Figure 8.6 illustrates

the operation of the charging algorithm. Under normal operating conditions

(25

◦

C with mains power supplied), the battery charges to 13.8V and then

stops. When the voltage drops to 12.8V, charging resumes to the 13.8V

threshold. This cycle continues indeﬁnitely or until mains power fails. If

a power failure is detected, the state changes to Power battery. Normal

operation continues until a low power condition is detected. At this point, the

GSM module is started and all collected data transmitted. When the battery

is depleted, all activity is stopped. Only the Real Time Clock continues to

operate. This is done to conserve power. If the device is allowed to continue

to ope rate in this condition, the battery will be damaged. SLA batteries

should not be allowed to deeply discharge. However, this situation should

not arise often; battery backup is in the order of many hours and it is highly

unlikely that the unit will be without mains power for this length of time.

When the power is restored, a report is delivered to notify the operator before

normal operation is resumed. If power is restored at any point before a low

battery condition, no report is sent and operation will continue transparently.