Why does your battery voltage drop when you accelerate? Why do old batteries get hot? The answer lies in Internal Resistance. In this guide, we explore the physics of IR, how to measure it correctly using AC vs DC methods, and why matching IR is critical for parallel group balancing.
The Hidden Enemy Inside the Can
You charge your battery to 4.20V. It looks full. You connect your motor, hit the throttle, and... the low voltage alarm screams, and the power cuts out. You measure the battery again: 4.15V. What just happened?
You just met Internal Resistance (IR).
Internal resistance is arguably the most important metric for assessing the health (State of Health - SOH) of a lithium cell, yet it is invisible to a standard multimeter. It represents the friction that electrons experience as they move through the battery's chemistry and mechanical connections.
1. The Physics: Where does IR come from?
A battery is not a perfect voltage source. It can be modeled as a perfect voltage source in series with a resistor. This resistance comes from three main areas:
- Ionic Resistance: The speed at which Lithium ions can move through the electrolyte and separator. Cold temperatures turn the electrolyte into "sludge," increasing this resistance massively.
- Electronic Resistance: The resistance of the materials themselves—the aluminum and copper foils, the active material paste, and the spot welds on the tabs.
- Polarization Resistance: The chemical reaction rate at the anode and cathode interfaces.
2. The Consequences of High IR
Resistance does two terrible things in a battery system:
A. Voltage Sag (Ohm's Law)
When you pull current (Amps) through a resistor, voltage drops across it.
Formula: $V_{drop} = Current (I) imes Resistance (R)$.
Example:
You have a 13S e-bike battery (48V) with a total internal resistance of 0.3 Ohms.
You pull 30 Amps up a hill.
Voltage Drop = $30A imes 0.3Omega = 9.0 Volts$.
Your 54.6V (Full) battery effectively becomes 45.6V instantly.
If your battery was at 50% charge (48V), it would drop to 39V, likely triggering the controller's cutoff.
B. Heat Generation (Joule Heating)
That missing voltage isn't gone; it turns into heat.
Formula: $Power (Heat) = I^2 imes R$.
Using the example above: $30^2 imes 0.3 = 270 Watts$.
You are effectively running a 270-watt heater inside your battery case. This heat degrades the SEI layer, decomposes the electrolyte, and can lead to thermal runaway.
3. Measuring IR: AC vs. DC
If you look at a datasheet, you will often see two different IR values. It is vital to know the difference.
AC Internal Resistance (Impedance) - $AC_{IR}$
This is measured by injecting a 1000Hz (1kHz) AC signal into the battery. It mostly measures the "Electronic Resistance" (tabs, grid, welds).
Tool: YR1035+ Meter.
Use: Great for quality control (QC) and checking if a cell is genuine or fake. It is fast and doesn't drain the cell.
DC Internal Resistance - $DC_{IR}$
This is measured by applying a heavy load (e.g., 10A) for a short time and measuring the voltage drop. This accounts for the "Ionic Resistance" and polarization.
Tool: Electronic Load or advanced charger.
Use: This is the "Real World" resistance your motor will feel. It is always higher than AC IR.
4. Matching Cells for Parallel Groups
When building a battery pack, specifically a parallel group (e.g., 4P), it is vital that the cells have identical IR.
The "Lazy Worker" Effect:
Imagine a 2-cell parallel group.
Cell A has 20mΩ IR.
Cell B has 100mΩ IR.
Electricity follows the path of least resistance. When you hit the throttle, Cell A will provide the vast majority of the current. Cell A will get hot and age faster.
Even worse, when you stop, the voltage of Cell B (which didn't work as hard) will be higher than Cell A. Cell B will then charge Cell A, causing "cross currents" inside the pack even when the bike is turned off.
5. The Aging Curve: When to Retire?
As a battery cycles, the SEI layer thickens, and the electrolyte dries out. Both cause IR to rise.
The Rule of Thumb: When a cell's Internal Resistance doubles from its original datasheet value, it is considered End of Life (EOL) for high-power applications.
- New Samsung 25R: ~13mΩ.
- Used 25R (Good): ~18mΩ.
- Old 25R (Retire): > 30mΩ.
A high IR cell might still have capacity (Ah), but it is useless for high loads. However, these cells are perfect for "Second Life" projects like Solar Power Banks where the current draw is low.
Conclusion
You cannot see Internal Resistance, but it dictates the performance of your entire system. Investing in a proper IR meter separates the novice battery builder from the professional. By sorting and binning your cells based on IR, you ensure that every cell shares the load equally, maximizing the power and lifespan of your creation.
