Specifications

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

10.5

11.5

12.5

13.5

100

Battert Termina Voltage

Percentage of Battery Charge Remaining

Battery Terminal Voltage vs. Percentage of Battery Charge

Remaining