Self Discharge of Cells

self discharge of cells can be an issue when the car is left over a holiday

Self discharge of cells is dependent on the chemistry, temperature and age of the cell. These reactions occur in any electrochemical systems and are very difficult to avoid completely. This is because some self discharge reactions involve chemical reactions which will always take place at a rate proportional to temperature.

However, this does mean the lower the temperature, the lower the rate of self discharge. Hence long term storage of lithium ion cells is best at around 10°C.

Chemistry

ChemistrySelf Discharge Rate [% / month]
Lead Acid4 – 6
Nickel Metal Hydride15 – 20
Nickel Cadmium30
Lithium Ion2 – 3
This is percentage of capacity of total Ah lost / month

In new NMC cells high self discharge rates can occur in the first month ~2.5% and then this rate decreases to <0.5%/month.

Causes

  • Internal electron leakage
    • electrolyte partial conductivity
    • other internal micro shorts
  • External electron leakage
    • poor isolating properties of the battery seals or gasket
    • external finite resistance between the leads (this could even be due to corrosion and the build up of an ionic pathway)
  • Electrode/electrolyte reactions
    • anode corrosion or cathode reduction by the electrolyte or impurities
    • Partial dissolution of the electrodes active material.
    • Electrode passivation by decomposition products (insoluble species or adsorbed gases).
  • Electrode mechanical disintegration or isolation from current collectors.
  • Internal pressure built up and electrolyte leakage through seals

Dependencies

  • Chemistry – see table
  • Cell temperature
    • higher temperature => higher self discharge rate [3]
  • State of Charge
    • higher SoC => higher self discharge rate
  • Age / Abuse
    • ageing introduces micro-cracking, damage to SEI and
    • discharging to 0V and then recharging increases the self discharge rate
    • Over-voltage charging increases the self discharge rate

References

  1. Boman Su, Xinyou Ke, Chris Yuan, Electrochemical Modeling of Calendar Capacity Loss of Nickel-ManganeseCobalt (NMC)-Graphite Lithium Ion Batteries, Department of Mechanical and Aerospace Engineering, Case Western Reserve University, Cleveland, Ohio 44106
  2. Eduardo Redondo-Iglesias, Pascal Venet, Serge Pelissier. Measuring Reversible and Irreversible Capacity Losses on Lithium-ion Batteries. VPPC, Oct 2016, Hangzhou, China. ff10.1109/VPPC.2016.7791723ff. ffhal-01393614v2f
  3. Self discharge of batteries, corrosion-doctors.org
  4. Guo, J.; Li, Y.; Pedersen, K.; Stroe, D.-I. Lithium-Ion Battery Operation, Degradation, and Aging Mechanism in Electric Vehicles: An Overview. Energies 202114, 5220. https://doi.org/10.3390/en14175220

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