Accuracy when Measuring (Sensing)

Input Characteristics (Sensing Capabilities)

Specifications for the input/sensing capabilities on the AL-4010 can be found in the table below.

Sensing Voltage Range	Sensing Voltage Accuracy	Sensing Current High Range	Sensing Current High Accuracy	Sensing Current Low Range	Sensing Current Low Accuracy
±8V	±1mV	±2.2A	±2mA	<10mA	±10µA

Resolution

The AL-4010 is equipped with Analog to Digital Converters (ADCs) with 18-bits of resolution i.e., the resolution is 18-bits, and the board has an internal operating range of ±10V, and with that information we can calculate the code width using this formula:

Code Width = Range/(2^bits)

So, for the AL-4010 we will get:

Code Width = 20/2^18 = 0.000019073486328125V = 19.073486328125µV

This is not the actual accuracy or code width of the AL-4010 itself since other components in the signal paths will impact overall performance when measuring the simulated cell values, but instead provides information about the performance of the onboard ADC.

Sensitivity

Sensitivity is the degree to which a change in the input signal is reflected in the data i.e., it defines the change in voltage required for the AL-4010 to register a change in value. Sensitivity is mostly influenced by the internal noise in the signal path + external noise in the overall signal path.

A device with high internal noise in its signal path will respond poorly to minor changes on the input signal, whereas a device with low internal noise in its signal path will be able to detect minor changes on the input signal.

The sensitivity can be larger than the code width and the benefit is that it picks up less noise and can better represent a signal as a constant voltage.

Example:

We are measuring a cell voltage that varies more than the code width, but it varies less than the sensitivity levels, resulting in the measurement is represented by the same digital value. Once the voltage exceeds the sensitivity level it will be represented by another digital value.

AL-4010 has been designed to have low noise in its signal paths so it can respond nicely to minor changes on the input signal.

Accuracy

The specifications for measuring/sensing the simulated cell values state ±1mV and include internal and external noise in the signal path.

Accuracy is the capability of the AL-4010 to indicate the measured signal in a truthful manner, or how close the measured signal is to the actual known value, and the accuracy can never be better than the code width.

The measurement accuracy on the AL-4010 will be impacted by things such as the temperature, offset errors, thermal noise in components, and other noise uncertainties.

Precision

An Analog to Digital Converter (ADC) is precise when it can return consistent measurements, so the values returned will not vary much, and the variance is low. So, it defines the stability of the AL-4010 and its capability to return the same measurement value repeatedly for the same input signal.

This does not necessarily mean that the ADC is accurate, it might still have an offset with respect to a known value, but the returned values are stable.

Accuracy when measuring stacked simulated cell voltages

The specifications for measuring/sensing the simulated cell values state ±1mV, and this specification is valid for an individual cell measurement even in a setup where the simulated cells are stacked (connected in series) to simulate a battery pack.

The total accuracy for the entire simulated battery pack when summing all the measured voltages will however not be ±1mV, since the total accuracy of a stacked setup will scale linearly based on the number of cells in the setup.

Example:

An engineer wants to validate a smaller battery management system that consists of a battery management unit (BMU) and a single cell monitoring unit (CMU) that can monitor 16 cells and instead of connecting it to a real battery he wants to simulate the battery and will therefore need three (3) AL-4010’s to achieve this (3 x 6 or 18 cells in total, so two cells will not be used).

Total Accuracy (worst case) = Number of Cells x Accuracy per Cell

Total Accuracy (worst case) = 16 x ±1mV = ±16mV

In real life the total accuracy will be better, one cell at a specific moment in time might have an accuracy of 0.3mV and another cell have an accuracy of -0.3mV, so they would even out each other.

Example:

The table and the plot found below shows a possible scenario where 16 cells are measured through the sensing capabilities on the AL-4010, and we can see that all measurements per cell are within the specifications (±1mV), here represented by the red lines. The mean value is represented by the green line and the expected cell measurement is represented by the blue line.

This is an example of where the system is both accurate (close to the actual signal) and precise (stable), and where the mean value (green line) is close to the actual signal (blue line), and all measurements per cell are within specifications.

Parameter	Value
Expected Stack Voltage	48.0
Actual Stack Voltage	48.00469614369332
Total Accuracy	-0.004696143693323052
Mean	3.0002935089808327
Variance	2.2315825754543953e-07
Standard Deviation	0.0004723962928997639

Down below you see the standard deviation plot using the same data as above.

Example:

The table and the plot found below shows a possible scenario where 16 cells are measured through the sensing capabilities, and we can see that all measurements per cell are within the specifications (±1mV), represented by the red lines. The mean value is represented by the green line and the expected cell measurement is represented by the blue line.

In this example we added an offset to illustrate a system that is precise (stable) but not accurate (not close to the expected signal due to the offset) and it is clearly visible since the mean value (green line) is far from the expected signal (blue line) but all measurements per cell are still within specifications. In the table we can see that we have a large error due to the offset, but that the variance and standard deviation values are low indicating a stable system.

Parameter	Value
Expected Stack Voltage	48.0
Actual Stack Voltage	47.84469614369332
Total Accuracy	0.15530385630668064
Mean	2.990293508980833
Variance	2.2315825754543953e-07
Standard Deviation	0.0004723962928997639