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December 8, 2016

Battery impedance and resistance

Why does battery internal resistance increase over time?

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Battery impedance versus battery resistance

Resistance is the opposition to current flow. Impedance includes resistance and any added opposition to alternating current flow due to factors such as inductance, capacitance and rectification. In most battery applications impedance = resistance, but higher frequency impedance measurements have some utility in pulsed applications and in battery testing. In addition, the AC impedance measurement is quicker, and includes the DC measurement so sorting batteries is practical.

General factors influencing battery resistance

Factors that influence battery resistance are
  • Conductor resistance, both in the metal component of electrode plates and conveyer and interconnection wires, plates and foils
  • Electrolyte resistance.
  • Ionic mobility
  • Separator efficiency
  • Reaction rates at the electrodes
  • Concentration polarization due to the transport of reactance and the removal and conductivity of reaction products
  • Activation polarization due to bottlenecks in the charge-transfer step of the the electrode reaction
  • Temperature effects of the reaction and transport rates

Conductor resistance

As a battery ages, corrosion of the metal current carriers, particularly of the plates or foils substrates that the active materials are supported on can decrease their cross-section, and therefore increase their resistance

Electrolyte resistance and ionic mobility

Electrolyte resistance is dependent on the number of charge carriers and the mobility of the charge carriers through it. As the battery ages, components of the electrolyte can be consumed in corrosion of metal components in the battery, and in other secondary chemical reactions, thus reducing their concentration and decreasing the number of charge carriers. Build-up of reaction products can also increase the viscosity and polarize the electrolyte so that the ion mobility is reduced.

A specific example is the lead-acid chemistry. As the battery is discharged the electrolyte concentration is reduced, becoming pure water when the battery is fully discharged. Because of this change in electrolyte concentration the battery resistance increases during discharge.

Loss of electrolyte is also a frequent cause of increased electrolyte resistance. This can happen through water migration through plastic or rubber seals, overcharging and venting.

Separator efficiency

Separators are non-conductive sheets that prevent electrodes from coming into electrical contact, but still must be porous to allow ions to flow through them. As a battery ages reaction products and corrosion products can clog the pores, thus decreasing the ion flow and increasing battery resistance.

Reaction rates at the electrodes

During battery aging the electrodes can change in porosity, crystal structure, and chemical composition which increases the battery resistance.

In nickel metal hydride batteries the negative electrode consists of a layer of metal particles bound to a nickel foil. The metal particles absorb hydrogen during charging and desorb during discharging. The electrolyte slowly corrodes the metal particles, increasing the dead layer at the surface of the grain, which makes it harder for the hydrogen ions to get to and from the metal, increasing the resistance.

In lead acid batteries large, non-conductive, less soluble crystals of lead sulfate grow when the battery is left uncharged or partly charged, which increases the resistance of the battery.

In lithium ion batteries the ion receptor channels in both the positive and negative electrodes can collapse or get clogged with lithium metal or corrosion products

Concentration polarization

Liquid electrolyte batteries rely on diffusion to get fresh reactants to the electrode surfaces. Diffusion is typically a slow process, so the reactants are typically depleted near the electrode surface. The difference in concentration existing between the electrode surface and the bulk electrolyte creates a potential difference which looks like an increase in resistance.

Activation polarization

Activation polarization comes from the speed of the chemical reaction at the electrode surface. The factors that influence this are the thermodynamics of the reaction and the surface area. Porous electrodes are often used to increase the surface area, but as the battery ages the pores can get plugged by reaction products, spalling of the electrode material, or corrosion products.

Temperature effects

A fundamental law of nature is that chemical processes slow down with lower temperature. This is because diffusion slows down, and the average kinetic energy of the molecules is reduced. So as the temperature goes down you can expect higher impedance of the chemicals. The resistance of the metals in the battery is lowered as the temperature goes down, but this is very small effect compared with the chemical effects.

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