Specifications

8.4. BATTERY SELECTION AND CHARGING
Figure 8.2: Circuit diagram of switching DC-DC converter
for the GSM module (3.8V).
8.4 Battery Selection and Charging
8.4.1 Hardware
Many secondary power sources were considered for the backup battery. The
decision to use a sealed lead acid (SLA) battery is based on economic factors.
As mentioned in Section 5.4, the RFID modules require a high voltage.
Although LION and Ni-MH batteries have higher energy-per-mass and energy-
per-volume figures, it is significantly more costly to construct a 12V pack from
these cells than to purchase a single SLA battery. Since space and mass are
not a primary concerns, but cost is, we opt for the older SLA technology.
SLA batteries have a typical lifespan of five years, but this can decrease to as
low as two years depending on the levels of charge and discharge, operating
temperatures and rates of charging and dis charging [23]. Long-life batteries
are also available which have service lifespans of over 10 years under ideal
conditions.
The approximate remaining capacity of the battery can be accurately
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