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Itamar FrankenthalJanuary 18, 20227 min read

How Load Current Affects a Lithium-Ion Battery's Capacity and Runtime

How Load Current Affects a Lithium-Ion Battery's Capacity and Runtime

Capacity, the total amount of electricity generated through a battery's electrochemical reactions, is a function of the battery type, load current, temperature, and age of the cell. For example, the capacity of lithium-ion (Li-ion) batteries can be reduced by as much as 25% when used under higherg loads than 20% of its rated capacity (C rating) or when operating temperature very cold. Therefore, when determining the actual capacity at your specific use conditions, you will need to conduct additional engineering.

The battery's internal impedance reduces capacity and runtime at high load currents. However, basic battery calculators don't account for the reduction in capacity. There are four methods to account for load current in capacity and runtime calculations accurately. The best one is to generate empirical cycling data at the desired current or use an advanced battery calculator that accounts for the cell's unique impedance profile. It is helpful to review some fundamentals of cell capacity to understand the impact of neglecting this impedance.

Cell Capacity 101

What is a Cell's Rated Capacity? A Lithium-Ion battery's published rated capacity is the amount of electricity that an be delivered by the cell when the load current is one-fifth of the C Rate (the rated capacity). When the current varies from C/5, the capacity changes due to chemical reaction rates. These variations include a chemical effect called concentration polarization, a variation in a component's concentration near a membrane surface due to selective transport through the membrane.

What Is the Capacity of a Cell When the Load Current Increases? If the load current is greater than one-fifth of rated capacity, capacity and runtime decrease in an inverse relationship. The cell's capacity will be less than the rated capacity.

What Is the Capacity of a Cell When the Load Current Decreases? If the load current is lower than the rated capacity, capacity and runtime increase inversely. The cell's capacity will be greater than the rated capacity.

With this foundation in mind, here are the four ways to compute capacity and runtime for a cell.

Four Methods to Calculate a Cell's Capacity and Runtime

Method 1 – Standard Battery Calculators

Standard online battery calculators employ the formula:

Picture1

The calculator is helpful because it provides a first-order approximation for the runtime. You can experiment with our standard calculator here. However, this formula is only an approximation because it uses the same rated capacity regardless of the load current. The actual capacity of a cell varies with the load current as described above. 

Method 2 – Use a Cell Data Sheet Discharge Curve

A more accurate calculation approach uses a cell data sheet (e.g., MoliCell ICP103450DA 2200 mAh Cell) for a specific cell. You can use the discharge curve to approximate the capacity at the desired load current and then apply the runtime formula with capacity defined at the load current condition.

When available, discharge curves provide a more precise calculation for runtime because they are based on empirical data. However, even when vendors provide datasheets, they are not always helpful. For example, the LG INR 21700 M50LT and Samsung NR21700-50E are popular lithium-ion 21700 cells with ~4900 mAh capacity. However, their datasheets only contain the cell's standard discharge rate capacity, one-fifth of the rated capacity (C/5). Furthermore, discharge curves for the Moli INR-21700-M50A, only show a few different load profiles. The capacity must interpolate within the data set for any load profile not displayed, which approximates the real value. In addition, discharge curves only show the capacity of a fresh battery and do not consider how the capacity changes over time.

Method 3 – Use an Advanced Lithium-Ion Battery Calculator

Advanced battery runtime calculators account for internal impedance by utilizing empirical cell cycling data to provide a more accurate runtime calculation than discharge curves can. Each cell has a unique chemical profile that is captured through cell cycling and uploaded to a database. The advanced calculator implements the cell's impedance profile to calculate the capacity at the load current. You can try our Advanced Lithium-Ion Battery Calculator to see results from this type of analysis.

Advanced battery calculators are a great way to get a quick answer and provide the ability to run scenarios comparing different cells simultaneously to assess the differences. A drawback of this calculation is that advanced battery calculators require an extensive database of cell cycling data. The cell you are frequently considering may not be part of the dataset. As a result, engineers most effectively use advanced battery calculators to understand the relative performance of different batteries under varying load conditions.

Method 4 - Use Empirical Cell Cycling Data

The most precise way to determine runtime is to cycle multiple cells on a battery analyzer under the cell's operating conditions. Analyzers can run with:

  • Varying currents
  • Constant current loads
  • Constant power loads
  • Pulse currents
  • Multiple cycles

The resulting data provides the voltage curve over time and the variance from cells within a lot.

Cycling data will provide the most accurate results. Subjecting a battery to hundreds of cycles can also determine how capacity desegregates with use. Using a thermal chamber, engineers can perform cycling studies at varying temperatures. The most significant limitations to collecting a dense matrix of cycle data are time and cost. Cell cycling data can take weeks or months to generate, especially under low currents. This reality means that the cycle testing consumes costly resources over a long duration.

Securing time in the test lab is always challenging. To offset the bottleneck, Rose offers contract lab testing services to help you generate this critical cycle data. In addition, we provide the first cycle for free! To find out more, contact us for lab testing services.

 

Capacity and Runtime Example Calculation: MoliCell's ICP103450DA 2200 mAh Cell

The MoliCell ICP103450DA is a very popular prismatic cell and has a rated capacity of 2200 mAh. However, from the below discharge rate characteristics, its capacity is only at 0.44A, representing one-fifth of the rated capacity. Therefore, at a 0.44A current load, the cell will have a capacity of 2200 mAh and a runtime of 5 hours (2200 mAh divided by 440 mA).

If we increase the load current to 4A, what will happen to the runtime? We will apply the four methods described in this article to determine the outcomes.

  • Method 1: Standard Battery Calculator - Using the standard battery calculator formula, the rated capacity of 2200 mAh is divided by 4000 mA, yielding a runtime of 0.55 hours (33 min). From the standard calculation definition, 0.55 hours is only an approximation of runtime.
  • Method 2: Using Use a Cell Data Sheet Discharge Curve (figure 1 below)

chart 1 

The above chart and this datasheet show that a 4A load decreases the battery capacity to about 1980 mAh. The runtime is calculated the same way, dividing 1980 mAh by 4000 mAh for about 0.4950 hours (29.7 min). The capacity decreased because the high current increased the internal impedance.

Method 1 variance: 11%

  • Method 3: Using an Advance Battery Calculator

Advance battery calculators use empirical data. Under lab conditions, the capacity may be closer to 1982 mAh. The runtime is 1982 mAh divided by 4000 mAh or about 0.4955 hours (29.7 min). The capacity decreased because the high current increases the internal impedance.

Method 2 produces a similar result to method 3 because both approaches use empirical data

  • Method 4: Using Empirical Cell Cycling Data

Under lab conditions, the capacity may be closer to 1982 mAh. Therefore, the runtime will be similar, with 1982 mAh divided by 4000 mAh to produce a runtime of about 0.4955 hours (29.7 min). However, the capacity decreased due to the high current increases the internal impedance.

Method 2, 3 and 4 will produce similar results because they use empirical data

 

Conclusion

A simple battery calculator is a great way to get a first-pass approximate calculation for battery runtime. However, as the load current varies from one-fifth of the rated capacity, the analysis becomes less accurate. The best way to calculate runtime is to use empirical data unique to the cell because each cell has a different non-linear impedance profile. Engineers can obtain empirical data using a datasheet, Advanced Battery Calculators, or cell cycling, which produce similar results.

 

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