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Is It Better To Have High Or Low Resistance In 18650 Batteries?

Is It Better To Have High Or Low Resistance In 18650 Batteries?

You may be intrigued by the above question. Isn’t it obvious that low resistance batteries are always the best? After all, internal resistance wastes battery energy. Intuitively, you might argue that having a zero-resistance battery would be ideal! Well, not really, since zero resistance increases the risk of short circuits.

The role of resistance in electrical circuits is similar to that of friction in our daily lives. For example, can you imagine a world where there is no friction? In such a world, even standing or walking would be problematic, as everything would be sliding around. In the same way that friction regulates mechanical movement, electric resistance regulates the flow of electrical current. In any battery application, matching the power supply to the power demand is important and resistance plays a role.  But at what level is resistance unacceptable? Let us explore this topic with focus on the popular 18650 battery cell.

What Is Battery Internal Resistance?

The internal resistance of a battery is the resistance offered to the flow of internal electric current between its electrodes. It depends on several factors such as battery type, chemistry, age, temperature, state of charge, and application.

Imagine that electric current flowing in a battery system is like Water flowing out through a tap fixed to a storage tank. Then, the voltage difference across the battery electrodes is like the level or pressure difference that pushes Water out of the tank. The internal resistance of a battery can be compared to the resistance of the tap as shown in Figure 1.Figure 1 Hydraulic Analogy Internal Resistance Reduces Current Flow


The rate at which a Lithium ion battery can supply current depends upon the speed at which Lithium ions move between Anode and Cathode. Internal resistance slows down their rate of travel. The internal resistance of Lithium ion batteries can be divided into three components, namely:

  • Diffusion Resistance
  • Charge Transfer Resistance
  • Electrolyte resistance

Figure 2 Components Of Lithium Ion Battery INternal Resistance

These are illustrated in Figure 2 and described below:

Diffusion resistance

This is related to the ionic conductivity within the electrodes. Lithium ions moving from one electrode to another, detach themselves from the electrode structures and dissolve into the electrolyte. Within the electrode structures, these migrating ions are restricted by local atom arrangements and electrostatic interactions , resulting in diffusion resistance.

Charge Transfer Resistance

This is the resistance faced by Lithium ions when crossing the Solid Electrode Interphase (SEI). The SEI is initially formed during cell production on the Anode, to prevent reactions between electrode and electrolyte. While it allows Lithium ion to migrate, it blocks electrons. The SEI layer thickness will build up over the service life of the battery. For the purpose of quantitative analysis of SEI resistance, it is usually modelled as combination of charge transfer resistance and a double layer capacitance. This is illustrated Figure 3.

Figure 3 Charge Transfer Resistance At The SEI

Electrolyte Resistance

The speed at which Lithium ions move through the electrolyte depends on the chemical and physical properties of the electrolyte. The resistance of the electrolyte and separator is also termed bulk resistance or Ohmic resistance. It is extremely sensitive to low temperatures, which is one of the reasons why at sub-zero temperatures, Lithium ion batteries experience severe capacity loss.

How To Measure The Internal Resistance Of 18650 Battery?

Each component of electrical resistance has unique characteristics, necessitating different measurement techniques. You should also be aware that internal resistance is not a constant value, but dependent on the cell’s temperature, state of charge (SoC), chemistry, construction and age. So, the conditions under which measurements are performed must be explicitly defined. Common resistance measurement methods include direct current internal resistance (DC-IR), alternating current internal resistance (AC-IR) and electrochemical impedance spectroscopy. These are explained below:

DC-IR Resistance Measurement

In this method, a direct current demand in the form of  direct current demand in the form of timed pulse  is applied across the battery terminals and the voltage response is measured. The resistance is than calculated by dividing voltage drop by the current demand. A typical current pulse lasts for about 30 seconds. Instantaneous and time delay voltage drops are measured. For example, as shown in Figure 4, the instantaneous voltage drop is measured at 0.1 seconds and corresponds to the ohmic resistance, ‘Ro’. The voltage drop after 1 second is attributed to charge transfer resistance, ‘RCT’. Finally, the extended decay of voltage upto 20 seconds is a measure of the diffusion resistance, RD.

Figure 4 Li Ion Cell DC-IR Measurement

AC-IR Resistance Measurement

The AC method utilizes a constant-current AC signal to the battery. This AC signal generally has a fixed frequency of 1 kHz, although some products allow the frequency to be varied. The small voltage generated by the varying current is used to calculate the impedance values. Please note that the term impedance is used to describe resistance in AC circuits. This is due to the reactance component, comprising inductive and capacitance effects, which oppose current flow. There is a phase difference between reactance and the ohmic resistance, and they are added vectorially to calculate impedance.

The measured reactance and ohmic resistance values for a range of frequencies are then plotted on a Nyquist diagram. Figure 5 shows a plot for a frequency range of 1 kHz to 1 Hz . 

Figure 5 Nyquist Plot For An AC-IR Test

From the horizontal axis, it is possible to segregate the battery’s total internal resistance into components such as diffusion resistance, charge transfer resistance and electrolyte resistance.

Electrochemical Impedance Spectroscopy (EIS)

EIS is primarily a research tool to measure battery dynamic impedance. The technique uses a potentiostat /galvanostat instrument to apply a small signal AC or voltage stimulus over a wide range of test frequencies, ranging from a few millihertz to megahertz. The resultant voltage or current response is digitally processed, and impedance values measured at various frequencies. Results are shown on a Nyquist plot. The EIS setup is very expensive, complex and time-consuming and not suitable for production line operations.

Is It Better to Have High or Low Resistance in 18650 batteries?

To answer this question, we need to look at the market segments where the 18650 dominates. Being energy dense, compact, and an early market entrant, the 18650 finds use in a wide variety of applications. These span gadgets and toys, mobile phones, computers, power storage, drones, electric bikes and electric vehicles. Each application imposes different patterns of power demand on the battery. For example, 18650 cells in flashlights or laptops need to deliver small but constant currents, but when used in power tools, must deliver large, short-duration currents. In the case of mobile phones, the current demand would be in the form of pulses as depicted in Figure 6.

Figure 6 Current Pulse Discharge In a Mobile Phone

You also need to remember that the internal resistance of 18650 cell varies with operating temperature, age and state of charge (SoC). Devices such as laptops will not experience extreme temperature. However electric vehicles and large power storage devices will be exposed to a range of climatic conditions.  Figure 7  shows  the variation of internal resistance with temperature values and SoC of a 18650 Battery made by Tianjin Lishen Company.

Figure 7 Internal Resistance of 18650 cell Vs Temperature & SoC

You can see from Figure 7 that the internal resistance values of the tested model range from 60 to 140 milliohms. These are typical resistance values and will vary for various models of 18650 from different manufacturers. Based on this resistance data, what are the best-fit applications for 18650 batteries?  Figure 8 shows battery and pack resistance mapped against common applications. You will observe that the majority of applications use batteries with internal resistance in the range of 10 milliohms to 100 milliohms. This largely coincides with the market segments where 18650 batteries have found success. In the case of EV batteries lower cell resistance is preferred.  So even though Tesla adopted 18650 batteries for their early sedans, it was not an ideal choice. They have now moved on to larger formats and lower-resistance batteries.

Figure 8 Battery Application Map Vs Internal Resistance

We can conclude, therefore, that a resistance range of 10 milliohms to 100 milliohms is satisfactory for the 18650 batteries to retain their markets in the consumer electronics and gadgets space. Higher resistance is not desirable.

What are the disadvantages of 18650 battery with high internal resistance?

The main disadvantages of high internal resistance are:

  • Excessive Heat production: Termed ‘Joule Heating’, this arises from energy lost when current flows through a resistance. Uncontrolled heating can cause thermal runaway and fire in Lithium ion batteries.
  • Power loss: The effective power delivered by the battery is reduced due to internal resistance.
  • Battery life: Due to faster capacity loss, the frequency of recharge cycles will increase. Figure 9shows the effect of battery resistance on runtime of a mobile phone battery. A 100 milliohm battery provides twice the runtime of a battery with 400 milliohm resistance.

Figure 9 Effect Of Battery Resistance On Runtime

What Affects the internal Resistance of a Battery

The following are the main factors affecting the internal resistance of Lithium ion batteries:

  • Battery chemistry: Electrode and Electrolyte composition affect Lithium Ion availability and movement.
  • Battery Age: The SEI layer builds up with age, increasing the resistance.
  • State of Charge (SoC): Resistance increases as SoC decreases.
  • Temperature: Resistance increases as Temperature decreases.
  • Battery construction: Batteries with the same chemistry and size, but produced by different manufacturers, show different internal resistance. This is due to differences in the cell construction method.

What Is The Internal Resistance Of Different Types Of Lithium Batteries?

Internal resistance as we learnt earlier, is a variable quantity dependent on many parameters. Manufacturers generally do not publish resistance curves. However, some research data is available comparing resistance of different batterie. One such European study published in 2022 measured the impedance and ohmic resistance of Lithium ion cells from different manufacturers. The measurements were done using EIS over a range of frequencies and states of charge (SOC).  The measured ohmic resistance at 50% SOC for 10 different models of 18650 format Lithium ion NMC cells, were as shown in the following table:

 

Manufacturer Model Capacity mAh Nominal Voltage (V) Ohmic Resistance At 50% SOC (milliohms)
Keeppower P1834J 3400 3.7 100.07
LG Chem HG2 3000 3.6 17.76
LG Chem M26 2600 3.65 40.66
LG Chem MJ1 3500 3.635 43.93
Murata V3 2250 3.7 33.86
Murata VTC5A 2250 3.7 73.56
Murata VTC6 3120 3.7 12.74
Nitecore NL1835HP 3500 3.6 53.67
Samsung 30Q 3000 3.6 13.24
Sanyo ZT 2700 3.7 41.10

 

Why Is Lithium Batteries’ Internal Resistance Lower Than Dry Cell Batteries?

The main reason for this is the underlying chemistry by which electrical current is produced. In the case of Lithium ion batteries, the electrolyte is designed to be chemically stable and conductive to Lithium ions though out its cycle life. There is no build-up of reaction products. On the other hand, dry cells, which are mostly Zinc-Carbon type, are based on irreversible chemistry. The Zinc Cathode in these cells reacts irreversibly with the aqueous electrolyte to release electrons. Both Zinc and electrolyte are consumed and a layer of reaction product builds up at the Cathode. In various types of Dry cells, the electrolyte may be Ammonium Chloride or Zinc Chloride or an Alkali such as Potassium or Sodium Hydroxide. Figure10 illustrates how Zinc reaction products builds up at the Cathode, increasing the internal resistance of a dry cell.

Figure10 Build up of Reaction Products at Zinc Cathode

Under What Circumstances Will The Internal Resistance Increase?

Once you have selected your Lithium ion battery type and model then you will notice increase in resistance due to the following reasons:

  • Age: As batteries become older, they will display increased internal resistance
  • Low temperature: The electrolyte resistance increases at low temperature. That is why electric vehicle batteries must be heated-up in cold climates.

What is the Effect of Internal Resistance On Batteries?

The main effect of internal resistance is on the performance of the battery. This shows up as rapid voltage drop and the battery will not be able to deliver power as per its rated capacity. The other main problem is heating up. This increases the cost of cooling systems when large battery packs employing thousands of batteries are used, as in the case of electric vehicles. Hot batteries increase the risk of thermal runaway.

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