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Shunt resistor calibration

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Last Updated
11th of October, 2018

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It is very important to know how much current is flowing in and out of the battery bank in a renewable energy system. When is charging the current flowing into the batteries should never be more than 10% of the battery capacity - e.g. a 100Ah battery should not be charged with more than a 10 Amp current or it may be damaged and/or overheat. It is also very useful to know how much current is being generated by a wind turbine or solar panel, because that information helps to calculate how much power is being generated.

Shunt Resistor and Ohm's Law:-

A shunt (a current shunt resistor or an ammeter shunt) is a high precision resistor which can be used to measure the current flowing through a circuit. Using Ohm's Law we know that the voltage dropped across a resistor divided by the resistance of that resistor is equal to the current, therefore if we measure the voltage across a shunt resistor in a circuit, we can easily calculate the current.

With Ohm's Law we can make calculations to quantify the power, voltage, current, and resistance in renewable energy systems with ease.

ohms-law.gif

Above is a handy chart showing all of the inter-relationships between power, current, voltage, and resistance in which:-

>_V is measured in Volts.
>_R is measured in Ohms.
>_I is measured in Amps.
>_P is measured in Watts

Shunt Resistor Calibration:-

For example, pictured above is a typical 100 Amp shunt resistor. This can be used to measure currents of up to 100 Amps - although to prevent overheating it should really only be used to measure continuous currents of no more than 60-70 Amps. * If a shunt resistor overheats it can permanently change the resistance of the shunt.

This shunt is calibrated such that the voltage drop across it is 100mV when the current flowing through it is 100 Amps. Therefore we can calculate the resistance of this shunt to be voltage divided by current = 0.1 / 100 = 0.001 Ohms (typically to within ± 0.25% accuracy). Therefore if a voltage drop of 28mV is measured (using a standard multimeter or 0-100mV range voltmeter), we know that the current flowing is 0.028/0.001 = 28 Amps.

The power wasted by the shunt resistor is given by multiplying the voltage by the current = 0.028 * 28 = 0.78 Watts in this example.


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