Internal resistance is sometimes known as the “gatekeeper” of a battery. Batteries are a source of voltage, but they all have a certain amount of internal resistance that restricts the voltage delivery and determines the battery’s runtime. The internal resistance of a battery is calculated based on various parameters like the size of the battery, state of charge, age of the cells, temperature, internal chemistry, and current discharge and is measured in milliohms. Understanding the internal resistance of a battery will enable you to know its state of health and power capabilities and estimate its performance.
There are two main purposes for measuring the internal resistance of a battery.
The first reason for measuring internal resistance is to ensure quality control throughout production. It is possible to determine the quality of a battery by measuring its internal resistance.
For the production of BSL forklift lithium batteries, the incoming materials from the batteries are sent to the BSL automotive-grade module production factory after factory inspection. Because factors such as vibration during transportation and even the passage of time can cause defects, workers at the BSL factory conduct acceptance inspections on each of the incoming cells before assembling them into modules and battery packs. It is very important that all cells in a given battery pack have equal internal resistance. If one or more cells have high internal resistance or have degraded, they will become a bottleneck and limit the capacity of the battery pack. Simultaneous use of tests that monitor cell internal resistance ensures consistent battery quality within specification. It plays an important role in maintaining battery quality during production!
In order to improve the quality of the battery pack, it is important to select cells with equal internal resistance
The second reason for measuring internal resistance is for battery maintenance. The internal resistance of a battery gradually increases as it is used. The power from a battery comes from the chemical reaction between the electrolytes and the electrodes. However, over a long period of time, the chemical reaction will slow down due to rust and corrosion inside the electrodes. As the chemical reaction slows down, the internal resistance of the battery increases.
When it comes to forklift batteries and Airport GSE, it is critical that the batteries provide sufficient power when needed.
As already indicated, the lower the battery’s internal resistance, the more capable it is to deliver more power. This is especially useful when the battery supplies its power to heavy loads like Airport GSE and Electric Forklift. On the other hand, batteries with high resistance typically heat up, resulting in the voltage supplied dropping under load leading to a premature shutdown.
In this blog, we’ll go over why internal resistance has an effect on a lift truck battery’s performance and discuss ways to check the internal resistance and mitigate some of that effect.
The forklift is one of the standard power tools in industrial applications that dramatically benefit from batteries with low internal resistance. Traditionally, forklifts are used to lift and transport heavy loads over short distances like in warehouses. This means they usually need significant amounts of power to raise the loads consistently. Thus, most forklifts are fitted with batteries with exceptionally low internal resistance and the ability to sustain high voltage demands over long periods. This allows them to lift heavy loads and carry them over the short distances.
Lithium-ion batteries are commonly used in forklifts due to their very low internal resistance, which remains relatively stable from full charge to full discharge. This type of battery can continuously provide a high current and offers the forklift the grunt to lift the load and carry it to the required position. Lithium-ion batteries consistently provide high current power until the charge is depleted. Thus, they allow for the continuous running of forklifts in industrial applications.
On the other hand, lead-acid batteries have a relatively low internal resistance, but they cannot provide the high current required by forklifts for extended periods. Their voltage drops significantly after a couple of seconds, and they need to rest before they can give another burst.
What causes higher internal resistance in a battery? It varies depending on of the battery’s composition.
Nickel-based batteries:
For nickel-based batteries, crystalline formation contributes to a higher level of internal resistance and can be reversed with deep cycling.
Crystalline formation, sometimes known as the “Memory effect”, occurs when a nickel-based battery is not charged properly.
If they are repeatedly recharged after being only partially discharged, the battery appears to “remember” the lower level of capacity and its overall capacity is reduced.
To prevent this effect, users should periodically discharge the battery down to one volt per cell, about every few months or so.
Lead Acid batteries:
For lead acid batteries, sulfation and grid battery corrosion can cause an increase in internal resistance, as well as a lower temperature.
Grid corrosion happens when the lead of the positive grid converts into lead dioxide and collapses the lead plates as a result.
Grid corrosion is one of the most common failures in a lead acid battery’s service life and can be reduced but can’t be avoided altogether.
Sulfation can be avoided with proper charging practices, which means avoiding partial charges and always allowing it to fully discharge, fully recharge and go through its required cool down period after charging.
Lithium-ion batteries:
For lithium-ion batteries, aging and continued use will lead to a build-up of solid electrolyte interphase, or SEI, on the anode. This build-up grows thicker over time and obstructs interaction with the graphite, which increases its internal resistance.
Many lithium-ion manufacturers have developed their own way to offset this effect using special additives to the electrolyte. As lithium-ion battery technology has evolved, their efficiency has greatly improved.
When electrical current is flowing through a circuit, the voltage output is always lower than the input - that difference is known as the voltage drop.
Voltage drop is the product of current and resistance - so by understanding the amount of voltage drop, you can determine how energy efficient your battery.
The lower the internal resistance, the better the battery performance is. The higher internal resistance in a battery, the more the battery will heat up creating a voltage drop.
One of the main reasons you might want to test for a battery’s level of internal resistance is to understand its condition, as a higher-than-normal reading will indicate that the battery is near the end of its working life.
This reading is most useful when compared to a reading taken from the battery when first put into service.
There are different ways to check the internal resistance of a battery, and it is by looking at the Direct Current (DC) and Alternating Current (AC):
Direct Current - measures the voltage drop at a given current, looks only at pure resistance
Alternating Current - takes reactance into account and provides the impedance, which may be different from the DC reading but both are correct
A battery’s internal resistance level varies depending on all the factors listed above but also on its state of charge.
Nickel-based batteries:
The internal resistance in these types of batteries decreases throughout discharge until about halfway through the discharge cycle, then it steadily rises up again.
Lead Acid batteries:
The internal resistance rises throughout the discharge, which is caused by a depletion of the electrolyte as it becomes more watery, which decreases its specific gravity.
Lead acid batteries have a voltage drop from internal resistance from continuous high current discharge.
Lithium-ion batteries:
The resistance of lithium-ion is fairly flat from empty to full charge, allowing for its performance to remain stable.
Lithium-ion batteries have one of the lowest internal resistances available, and their high level of Coulombic Efficiency, or CE rating, is a testament to this.
Internal Resistance and Lift Truck Battery Performance
Understanding the concept of internal resistance is useful for anyone working with and making decisions about forklift batteries on a regular basis.
Batteries are becoming more than just an after thought power source; they are a technology investment that can help increase productivity, output, and overall organizational efficiency.
Lithium-ion batteries have low internal resistance which is why they outperform other types. Understanding the role that internal resistance plays will put you in a better position to make informed decision about your forklift battery.
