Specifications
34
In order to sense the voltage at the battery, a simple voltage divider circuit can be
employed to drop the voltage swing from the 10.5 – 13.5VDC range to the 0 – 3VDC
range. This will enable the voltage to be read at the analog inputs of the
microprocessor, as most microprocessors read in analog voltages between 0 and
5VDC. The voltages can then be scaled back up mathematically in the programming of
the power calculation. This method of measuring the voltage at the battery can also be
used to calculate the charged state of the battery. Since voltage and percent charged
have a linear relationship, all we need to know to calculate the percentage of battery
charge remaining is what the maximum, minimum, and instantaneous voltages of the
battery are. This graph of the battery terminal voltage vs. percentage of battery charge
remaining for the Optima yellow top D51 battery can be seen below. For example, if the
battery voltage is measured to be 11.5V, there is approximately 40% of the batteries
charge still remaining.
Figure 25 - Battery Terminal Voltage vs. Percentage of Battery Charge Remaining
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To sense the current, there are a few different possible methods. The first possible
method is to use a Hall Effect sensor. A Hall Effect sensor uses the magnetic field
created by the current within the wire to calculate the current running through the wire.
This value is then output by the sensor in the form of a DC voltage. The current can
then be mathematically calculated from this voltage within the program of the
microcontroller. Different Hall Effect sensors have different relationships between their
output voltages and the current they sense in the wire. The second possible method is
to put a shunt resistor in series with the load and use a current sense amplifier to
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10.5
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11.5
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12.5
13
13.5
0
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100
Battert Termina Voltage
Percentage of Battery Charge Remaining
Battery Terminal Voltage vs. Percentage of Battery Charge
Remaining