Background

During charging and discharging of batteries, the capacity will be influenced by the overvoltage caused by internal resistance. As a critical parameter of battery, internal resistance is worth research for analyzing battery degradation. The internal resistance of a battery contains:

• Ohm internal resistance (R_Ω) –The resistance from tabs, electrolyte, separator and other components.
• Charges transmission internal resistance (R_ct) – The resistance of ions passing tabs and electrolyte. This represents the difficulty of tabs reaction. Normally we can increase the conductivity to reduce this resistance.
• Polarization Resistance (R_mt) is the internal resistance caused by density uneven of lithium ions between cathode and anode. Polarization Resistance will be higher in situations like charging in low temperature or high rated charge.

Normally we measure the ACIR or DCIR. ACIR is the internal resistance measured in 1k Hz AC current. This internal resistance is also known as Ohm resistance. The shortage of the data is that it cannot directly show the performance of a battery. DCIR is measured by a forced constant current in a short time, in which the voltage continuously changes. If the instantaneous current is I, and the change of voltage in that short term is ΔＵ, according to Ohm law Ｒ＝ΔＵ／Ｉ We can get the DCIR. DCIR is not about only Ohm internal resistance, but also charge transfer resistance and polarization resistance.

Analysis on standards of China and other countries

It’s always a difficulty on the research of DCIR of a lithium-ion battery. It’s mainly because the internal resistance of a lithium-ion battery is very small, usually just some mΩ. Meanwhile as an active component, it is hard to measure the internal resistance directly. Besides, the internal resistance is influenced by the status of environment, like temperature and charges status. Below are standards that have mentioned about how to test DCIR.

• International standard:

IEC 61960-3: 2017: Secondary cells and batteries containing alkaline or other non-acid electrolytes – Secondary lithium cells and batteries for portable applications – Part 3: Prismatic and cylindrical lithium secondary cells and batteries made from them.

IEC 62620:2014: Secondary cells and batteries containing alkaline or other non-acid electrolytes – Secondary lithium cells and batteries for use in industrial applications.

• Japan: JIS C 8715-1:2018: Secondary lithium cells and batteries for use in industrial applications – Part 1: Tests and requirements of performance
• China does not have relevant standard about DCIR testing.

Varieties

The testing methods are similar among IEC 61960-3:2017, IEC 62620:2014 and JIS C 8715-1:2018. The main distinctions are as followed:

1. Testing temperatures are different. IEC 62620:2014 and JIS C 8715-1:2018regulates a 5℃ higher of ambient temperature than IEC 61960-3:2017. Lower temperature will make it higher viscosity of electrolyte, which will cause lower movement of ions. Thus the chemical reaction will slower, and Ohm resistance and polarization resistance will become larger, which will cause a trend of DCIR increase.
2. SoC is different. The SoC required in IEC 62620:2014 and JIS C 8715-1:2018 is 50％±10％, while IEC 61960-3:2017 is 100%. The status of charge is very influential to DCIR. Normally DCIR testing result will get lower with increase of SoC. This is related to the procedure of reaction. In a low SoC， the charge transfer resistance R_ct will be higher; and R_ct will decrease with increase of SoC, so as DCIR.
3. The discharging period is different. IEC 62620:2014 and JIS C 8715-1:2018 requires a longer discharge period than IEC 61960-3:2017. The long pulse period will cause a lower increasing trend of DCIR, and present a deviation from linearity. The reason is that the increasing of pulse time will cause a higher R_ct and become dominant.
4. The discharge currents are different. However the discharge current does not necessarily directly relate to DCIR. The relation is determined by the design.
5. Though JIS C 8715-1:2018 refers to IEC 62620:2014, they have different definitions on high rated batteries. IEC 62620:2014 defines that high rated batteries can discharge no less than 7.0C of current. While JIS C 8715-1:2018 defines high rated batteries are those can discharge with 3.5C.

Analysis on Testing

Below is the voltage-time function chart of DCIR testing measure. The curve shows the resistance of cells, so that we can evaluate the performance.

• As shown in the picture, the red arrows represent R_Ω. The value is related to iR-drop. iR-drop means the sudden change of voltage after the current change. Normally when a cell is electrified, there’s a drop of voltage. Therefore we can know that the R_Ω of the cell is 0.49mΩ.
• The green arrow represents R_ct. R_ct and R_mt need some time to activate. Normally it happens after drop of Ohm Voltage. The value of R_ct can be measured 1ms after current change. The value is 0.046mΩ. Normally R_ct will decrease with the raise of SoC.
• The blue arrow represents the change of R_mt. The voltage keeps decreasing because of lithium-ion uneven spread. The value of R_mt is 0.19mΩ 

Conclusion

DCIR test can show the performance of batteries. It’s also a critical parameter for R&D. However there are some issues to be considered in order to keep the accuracy of measurement.

• The way of connection between batteries and charge and discharge equipment should be considered. The connection resistance should be as low as possible (suggest not larger than 0.02mΩ).
• The connection of voltage and current collection wires is also important. It would be better to connect in the same side of tabs. It should be noticed that do not connect the collection wires to the charging wires of equipment.
• The accuracy of charge and discharge equipment and the response time should also be taken into consideration. The response time is suggested not longer than 10ms. The shorter of response time, the more accurate of result.