Complications in voltage sensing of Lithium ion batteries

The concept of Kelvin sensing is explained below and why it is useful in battery voltage sensing.

A current carrying conductor drops voltage across it, and in battery packs this current can be really high at times, which means that the voltage dropped will also be higher. The voltage sensing circuitry is usually far away from where the actual cells are, how then can we accurately sense battery voltage without causing a drop in the sense lines? To better articulate the problem, here’s a diagram attached below

Shown above is a pack with 4x 18650 cells and the red, and black voltage sense lines connect to a voltmeter (V) far away.

If the same lines, are used for carrying the current to the load, and also for sensing this could lead to a potential problem in sensing the true voltage of the cells by the voltmeter V. This is because the current i will cause a voltage drop in the wires equal to i*R where R is the resistance of the wires.

An effort can be made to reduce the resistance of the wires by reducing their length, or by increasing its thickness but this might not always work out. Length of the wires is often governed by the mechanical structure of the battery pack and the product as a whole. The length would be the distance between the actual cell assembly and the battery management system where the voltage is being sensed. Increasing the thickness of the wire means that we have to add more copper which is going to bring up the cost of the pack. How then do you solve this issue?

Let’s first understand how multimeters sense resistance. When you put the meter in resistance measurement mode, it does the following –

The multimeter sends out a known current Is to the device R that you want to measure the resistance of. It then measures the voltage drop across the resistance and calculates the resistance as Vm / Is.

This is called two wire resistance measurement and works most of the times as long as the resistance R is much higher than the resistance of the contacts and probes. What happens if the resistance R that you want to measure is really small?

As shown in the diagram on the left, if Rlead which is the resistance of the multimeter probes and contact points is greater than the resistance R that you want to measure, it will lead to issues. This is because to measure a small resistance R, the multimeter would need to send out a higher current Is. Why? Because the voltage measuring device Vm cannot measure voltages that are too low. So increasing the current Is flowing through R, increases the voltage drop across it so that Vm can measure it more accurately.

There is a drawback of doing this – the voltage that Vm reads will be equal to –

Vm = Is * (R + 2Rlead)

If R << Rlead, then we cannot determine R properly.

This is fixed using Kelvin sensing also known as four wire sensing. In this case, the current carrying wires are separated from the voltage sensing wires. The voltage sense wires are tapped as close as possible from the contact points of the resistance R.

The current flowing through the sense lines is very low, and thus Rlead on the sense lines does not affect the voltage measurement Vm.

A similar technique can be used in battery management systems to increase the accuracy of cell voltage measurement.

When the high current carrying wires are separated from the sensing wires, the sense wires can be really long without affecting the voltage measurement.

We believe that the best way to learn is through hands-on experiments. Our courses are built on this, which is why we design specialized hardware kits to accompany the course. You don’t need an expensive multimeter which has an inbuilt 4 wire measurement mode. It can be achieved using a digital power supply, a simple multimeter and a small resistance that you want to accurately measure.

The MakerMax EV Battery masterclass is an opportunity to understand such concepts in an engaging and detailed way. We have recently sent you an email about the exclusive subscriber only registration which will be opening soon and we look forward to your participation in the course. MakerMax is working with some of the top EV companies, professionals and students worldwide to assist them in accelerating to a sustainable future for all of us.