3 – 4
Circuitry
Battery lifetime is temperature dependent, especially at the higher
end of the useful temperature range. Users concerned with maximiz-
ing the battery lifetime who are operating several SIM928 modules
in the same SIM900 mainframe can reduce maximum temperatures
by spreading out modules with higher power dissipation like the
SIM928 evenly over the mainframe slots. In any case the e
ff
ects on
actual lifetime should be small.
3.1.2.3
Battery voltage monitoring
While the battery is connected to the charger circuit, its cathode is
referenced to
−
15 V. A battery voltage monitor made of R216, R217,
U203 and QN204 level shifts the battery anode voltage to comply
with the microcontrollers internal ADC input range of 0 V to
+
5 V.
The circuit is essentially a precision current mirror built with a 2.5 V
voltage reference. The mirrored current creates a voltage drop across
sense resistor R119 which is referenced to the microcontroller ADC’s
voltage reference (generated by U105). The circuit has a battery
voltage range of approx. 20 V–35 V and is factory calibrated to have
millivolt o
ff
set voltage and drift. The microcontroller’s 10-bit internal
ADC achieves a resolution of approx. 15 mV
/
LSB which is further
increased by averaging. The e
ff
ective voltage resolution per NiMH
cell is better than 100
µ
V, allowing it to follow battery charging very
precisely.
3.1.2.4
Switches and voltage regulators
The batteries are connected to the charger and the isolated voltage
regulator through relays K201–K204. The microcontroller algorithm
makes sure that only combinations of switches can be activated which
isolate the charger from the isolated output stage. Coupling between
input and output is therefore purely capacitive, albeit the capacitance
of approx. 40 pF changes by a few picofarads, depending on the
actual charger configration.
Since the batteries can be connected to di
ff
erent absolute potentials,
these changes in potentials and capacitance can generate small cur-
rent spikes during battery switching events. Similar but smaller
spikes can occur when the charge
/
discharge circuits are activated.
The module has a set of opticaly isolated Mosfet switches U204
/
205
with 1 M
Ω
series resistors R218–R221 which pre-charge the battery’s
stray capacitance to minimize these current spikes. However, user’s
who are very sensitive to charge transfers on the order of a few
nanocoulombs need to take precations against these unavoidable ar-
tifacts by either shunting these charges against ground with a capac-
itor or by stretching them out with series resistors. Alternatively, the
SIM928
Isolated Voltage Source
