> ## Documentation Index
> Fetch the complete documentation index at: https://docs.ionworks.com/llms.txt
> Use this file to discover all available pages before exploring further.

# Internal Resistance

> How internal resistance causes voltage drop, power loss, and heat generation, and how it changes with SoC and aging.

**Internal resistance** is a key property that determines how much of a battery's stored energy can actually be delivered as useful power. It causes voltage drops under load, limits power output, and generates heat during operation—making it critical for both performance and safety.

## What Is Internal Resistance?

Internal resistance refers to the opposition a battery presents to the flow of current. Just like any electrical component, a battery isn't a perfect energy source—it has inherent resistance to:

* Movement of ions in the electrolyte
* Movement of electrons through the electrodes and current collectors

This resistance causes:

* Energy losses
* Voltage drops under load
* Heat generation during operation

<Note>
  Internal resistance isn't a fixed value; it varies based on state of charge,
  temperature, battery chemistry, and aging. As a battery degrades over time,
  its internal resistance typically increases, leading to greater energy losses
  and reduced efficiency.
</Note>

## Measurement Methods

### Direct Current (DC) Method

This involves applying a current pulse to the battery and measuring the resulting voltage change. The resistance is calculated from Ohm's Law:

$R_{DC} = \frac{\Delta V}{\Delta I}$

The measured value depends on when the voltage is sampled:

* **Immediately after pulse** (within milliseconds): Captures primarily ohmic resistance from electrolyte, current collectors, and contacts
* **After several seconds**: Includes charge-transfer resistance and some diffusion effects

<Note>
  DC resistance measurements are simple and widely used for quality control and BMS algorithms, but the result depends on pulse duration, current magnitude, temperature, and state of charge. Always specify test conditions when reporting DC resistance values.
</Note>

### Alternating Current (AC) Method

A more sophisticated method, **Electrochemical Impedance Spectroscopy (EIS)**, applies an AC signal at different frequencies and measures the impedance response of the battery.

| Advantage                   | Description                                                             |
| --------------------------- | ----------------------------------------------------------------------- |
| Separation of contributions | Distinguishes charge transfer resistance from diffusion-related effects |
| Deeper insights             | Provides information about battery aging and performance                |

EIS requires specialized equipment but provides more comprehensive information.

## Impact on Battery Performance

Internal resistance plays a major role in battery performance through a series of interconnected effects:

<Steps>
  <Step title="Voltage Drop">
    Higher resistance leads to greater voltage drops under load, reducing the
    power a battery can deliver.
  </Step>

  <Step title="Efficiency Loss">
    More energy is wasted as heat instead of powering the device.
  </Step>

  <Step title="Temperature Rise">
    The generated heat raises the battery's temperature.
  </Step>

  <Step title="Accelerated Degradation">
    Elevated temperature can accelerate degradation and, in extreme cases, lead
    to thermal runaway.
  </Step>
</Steps>

For these reasons, internal resistance is one of the key metrics monitored by Battery Management Systems (BMS) to optimize performance and maintain safe operation.

## Connection to Thermal Behavior

One of the most significant consequences of internal resistance is heat generation, making it impossible to assume a constant battery temperature. Predicting temperature dynamics is the main goal of [thermal models](/guide/batteries-101/thermal-modelling).

## Related Topics

* [State of Charge](/guide/batteries-101/state-of-charge)—how resistance affects SoC estimation
* [State of Health](/guide/batteries-101/state-of-health)—using resistance increase to track degradation
* [Thermal Modelling](/guide/batteries-101/thermal-modelling)—how resistance causes heat generation
* [Degradation Overview](/guide/batteries-101/degradation)—mechanisms that increase resistance over time
* [Battery Management Systems](/guide/batteries-101/battery-management-systems)—how the BMS monitors resistance