Precise battery measurements are essential to achieve reliable results. With 4-wire measurement, it is possible to eliminate cable resistance in the measurement setup and effectively avoid measurement errors.
The precise measurement of batteries is an indispensable basis for the development, testing and optimization of modern energy storage systems. Whether in research, quality control or product development – high measurement quality is the key to achieving reliable and reproducible results. There are some essential principles that must be observed in order to minimize measurement errors and maximize the significance of the results.
What is the 4-wire measurement?
In electrical engineering, only the 2-wire measurement is often known. A battery is connected to a lamp via two cables – a simple and everyday example. As long as the current is low, the lamp lights up as expected, as the voltage losses along the cables are almost negligible. But if the current increases, the situation changes: the resistances of the cables and connections lead to voltage drops, so that less voltage arrives at the lamp than was originally supplied by the battery. This may be less critical for the lamp, but when measuring battery characteristics, where precision is essential, such losses can render the results completely useless.
This is where the 4-wire measurement method comes in: It separates the current circuit from the voltage measurement. While the measuring current flows via a separate pair of cables, the voltage is measured directly at the battery points. This separation eliminates the effects of cable and contact resistances and ensures precise measured values.
Structure:
- Two power lines (I+ and I-):
- A defined measuring current I is passed through the battery via these cables.
- The current flows through the test terminals and the battery.
- Voltage drops from the cables, but this does not matter
- Two voltage lines (V+ and V-):
- These cables measure the voltage directly at the battery.
- As almost no current flows through these cables, the cable resistances have no influence on the voltage measurement.
Procedure:
- The measuring current I is generated by a measuring device and flows through the battery.
- The voltage V is measured directly at the battery terminals without the lead resistances distorting the measured value. The voltage drop across the cables is 0 V.
When is the 4-wire measurement important?
The 4-wire measurement is particularly important when charging and discharging with high currents, as the cable resistances make the measurements completely unusable for 2-wire measurements. The 4-wire measurement method is also essential for long tests in which faults accumulate.
Further measures for good measurement quality
The use of the 4-point measurement is the most important point to avoid the grossest error. The aim should always be to run the voltage and current-carrying cables together as late as possible. This is often not always perfectly possible because the geometry of the room conflicts with. In this case, the common conductors should be made as thick as possible so that the resistance across this comductor is as low as possible.
In practice, it is also often the case that the measuring device is located in an electromagnetically disturbed environment. This means that other test devices nearby can interfere with the measurement. This can then be noticeable through a strong scattering of the measurement results. This can make measurements completely unusable, particularly for resistance measurements and especially for electrical impedance measurements (EIS).
There are several measures that can be taken to minimize the influence of electromagnetic interference:
- Twisting the measuring cables: Twisting the cables cancels out the effect of parasitic electric fields, eliminating or reducing the interference. The tighter the twisting, the better the reduction in interference. I+ and I- and V+ and V- are always twisted.
- No metal surfaces in the vicinity: If the input and output conductors are close to metal surfaces (e.g. metal table, housing of test devices, etc.), eddy currents occur which affect the voltage cables and cause incorrect measurements. The solution is to increase the distance between cables and metal surfaces.
- Cables as short as possible: Even if the cable resistance does not play a role, short cables are advantageous. The shorter the length, the less opportunity there is for electromagnetic interference to affect the cables.
