Relaxation delta Voltage

We see Relaxation delta Voltage (RdV) as a safety diagnostic that can be used in a number of areas.

LG Energy Solutions describe it as a technique used to find micro internal short circuits in cells [1].

LG Energy solutions Relaxation delta Voltage (RdV)

Showing a cell with a Hard Short that is resulting in an Open Circuit Voltage drop of 30mV/day. Hence being applied to a cell at rest. By default this means the charge or discharge demand has been removed and the cells left open-circuit.

internal cell short through separator

An example of a cell internal short-circuit through the separator.

This could have been due to metal particles in the active materials or created during the cell assembly process being included in the cell.

The shorting of the cell can take some time to occur as the cell swells over it’s lifetime and the pressure between the layers eventually results in a bridge between the anode and cathode.

Images coutesy of Andrew Lamb.

Relaxation results from the diffusion of lithium ions into the active materials, resulting in impedance changes and the equilibration of the cell voltage to its open-circuit voltage (OCV) as over-potentials dissipate [2].

Alexa Fernando et al [2] also show that relaxation behavior and open-circuit voltage settling times are influenced by:

  • depth of discharge
  • rate
  • current flow direction
  • cell chemistry

Primarily this research was looking at using the OCV as a lookup for SoC estimation.

This post has been built based on the support and sponsorship from: Thermo Fisher Scientific, Eatron TechnologiesAbout:EnergyAVANT Future MobilityQuarto Technical Services and TAE Power Solutions.

The OCV of the cell also drops due to self discharge, this is very small in lithium-ion cells.

relaxation of cell OCV

Bob Zollo [3] suggests a Rule of Thumb for determining when relaxation changes to self discharge:

A rule of thumb to determine how long to wait would be to compare the expected change in OCV due to self-discharge to the change in OCV due to relaxation. If you expect 3 mV of change in OCV due to self-discharge, then you should wait until the change in OCV due to relaxation is down to 30 µV, or 1% of the expected OCV change due to self-discharge value. This could take a day or more. 

An Li et al [4] look at the characterisation of this relaxation and in particular a method to do so economically.

relaxation delta voltage

If you can characterise and model the relaxation it means that you can then use it within a measurement system to find faults within the pack. Essentially you will be looking for devaiations from the normal behaviour that could be due to soft short circuits inside or outside the cell.

In a battery pack with lots of cells in parallel you will only be able to isolate to the parallel group.

References

  1. The Beginning of Commercial Vehicle Innovation, LG Energy Solution’s Automotive Batteries for Commercial Vehicles
  2. Alexa Fernando, Matthias Kuipers, Georg Angenendt, Kai-Philipp Kairies, Matthieu Dubarry, Benchmark dataset for the study of the relaxation of commercial NMC-811 and LFP cells, Cell Reports Physical Science, Volume 5, Issue 1, 2024
  3. Measure Self-Discharge Using OCV on Lithium-Ion Cells, ElectronicDesign
  4. Li, A.; Pelissier, S.; Venet, P.; Gyan, P. Fast Characterization Method for Modeling Battery Relaxation VoltageBatteries 2016

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