Internal resistance is your enemy!

Internal resistance is your enemy!

Internal resistance (IR) is a characteristic of a battery cell that is often overlooked. This is actually one of the most important features.

Why is this important?

What is the impact of having cells with different internal resistance?

In this article we will cover some theory and translate the results into practical impacts.

 

The theory

The internal resistance (IR) of a battery is a complex system made up of different factors. To simplify, let’s represent a battery as a pure voltage source in series with a resistance.

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Several factors influence internal resistance (IR):

– the capacity of the cell (C)

– state of charge (SoC)

– temperature

– the state of health of the cell (SoH)

– cell technology

– the current passing through the cell.

 

The typical internal resistance for a 100 Ah cell is less than 0.6 milliohm and less than 0.4 milliohm for cells above 300 Ah (Winston data). There are significant variations depending on cell technology and manufacturing process.

 

Most BMS use algorithms to estimate internal resistance. Although the measurement they give is different from the manufacturer’s specification, these estimates are consistent over time and can be used to quantify battery aging.

 

Impacts of internal resistance

High internal resistance of cells has two major impacts:

 

More energy is dissipated when charging or discharging

Indeed, the greater the resistance, the more the current passing through it will generate heating and therefore dissipation of energy in the form of heat. This energy is considered lost because it cannot be used for the purpose for which it was initially intended (charging the battery or powering equipment).

 

Effective charge and discharge voltages are reduced

We can schematize this type of system in the form of a watercourse whose direction of flow (load/discharge) we can control. On one side of this stream is what is external to the battery and on the other side is what is internal to the battery.

 

 

Between these two environments we can represent internal resistance as an obstacle opposing the proper flow of water in one direction or the other. The more important this obstacle is, the more we will observe a difference in flow (voltage drop) between upstream and downstream of the obstacle.

  • Load:  The stronger the internal resistance, the more energy it dissipates, the greater the corresponding voltage drop (Vir).

 

 

The voltage measured across the battery (V) is higher than the actual (effective) charging voltage of the cell.

  • Discharge:  The stronger the internal resistance, the more energy it dissipates, the greater the corresponding voltage drop (Vir).

 

 

The voltage measured across the battery (V) is lower than the actual (effective) charging voltage of the cell.

 

What does that look like in practice?

  • To avoid too much heat, the continuous charge/discharge current should be lower for cells with higher internal resistance.
  • Under the same charge/discharge current, the cell with higher internal resistance will heat up, age faster (which will increase its internal resistance even more), and enter a death spiral.

 

What precautions should be taken to avoid this type of problem?

When purchasing lithium battery cells, you should check and compare the internal resistance as well as any other characteristics specified by the manufacturers.

You can also check the manufacturer’s recommended maximum continuous current as it will be lower for cells with high internal resistance (assuming the manufacturer is reputable and provides actual measured data).

 

Pack composed of cells with different internal resistances

If cells in a pack have different internal resistance, current will have uneven distribution between cells in parallel and voltage will have unequal distribution between cells in series. This will result in:

  • Cells do not charge and discharge at the same rate, creating an imbalance in the cells.  (See article “Why balance your lithium cells?”)
  • The capacity of the battery pack is reduced.
  • The aging of certain cells will be more rapid.

 

The internal resistance of a cell is not a directly controlled parameter in the manufacturing process but is the result of a number of raw materials and manufacturing parameters. We therefore expect to see significant internal resistance variances between cells (some research articles mention variances of 20% or more).

 

It is also important to carefully interconnect the cells to reduce each connection resistance. The interconnection resistance has the same effects as the internal resistance of the cell.

 

“A 20% difference in internal cell resistance between two cells operating in parallel can lead to approximately a 40% reduction in lifespan compared to two cells with very similar internal resistance”

 

“Internal resistance matching for parallel-connected lithium-ion cells and impact on battery pack cycle life”

 

Journal of Power Sources 252: 8-13 – Avril 2014

A single cell with an internal resistance significantly different from other cells can significantly reduce the lifespan of a battery pack.

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