You have a 2-year-old battery. It looks fine. It charges to 4.20V per cell perfectly. It balances perfectly.
But when you put it in your car, it feels... slow. It lacks punch. And after just 5 minutes of driving, your Low Voltage Cutoff (LVC) alarm starts screaming, even though the battery is still 80% full.
Is your ESC broken? Is your motor dying?
The "Voltage vs. Resistance" Analogy
To understand IR, think of a garden hose.
- Voltage is the water pressure (how much water is in the tank).
- Capacity (mAh) is the size of the tank.
- Internal Resistance is the width of the hose.
A brand new battery
...is a fire hose. Water (current) flows out massively and easily.
A high-IR battery
...is a drinking straw. You can have the same pressure (16.8V) behind it, but when you open the valve, only a tiny trickle comes out.
When your motor demands 100 Amps to do a backflip, a high-IR battery simply cannot deliver it. The flow is choked, and the energy that can't get out turns into Heat.
How to Measure IR
Most modern 4-button chargers (SkyRC, ISDT, Hota, Gens Ace) have a function to measure IR. It is often a separate menu option or a screen you can toggle to during charging.
The Golden Rule of Measuring: Temperature Matters!
Internal Resistance fluctuates wildly with temperature. A cold battery has high resistance. A warm battery has low resistance.
To get an accurate reading: Measure your batteries at room temperature (about 72°F / 22°C) after they have been sitting for an hour. Never compare a hot battery off the charger to a cold one from the garage.
The Health Chart: What do the numbers mean?
Resistance is measured in milliohms (mΩ). The lower the number, the better.
Note: These numbers apply to standard sized 5000mAh+ car packs. Smaller batteries (like 1300mAh drone packs) naturally have higher resistance.
| Reading (per cell) | Status | Usage |
|---|---|---|
| 0 - 5 mΩ | Brand New / Race Grade | High-speed runs, Racing, 6S bashing. Maximum punch. |
| 5 - 10 mΩ | Healthy / Average | Standard bashing. This is where most reputable batteries sit after 20 cycles. |
| 10 - 20 mΩ | Tired / Aging | Practice laps, trail trucks, or low-speed bashing. You will feel less "pop" off jumps. |
| 20+ mΩ | RETIRE / DANGEROUS | Do not use in high-amp cars. High risk of puffing or overheating. Recycle soon. |
The "Delta" Warning: The Imbalance Killer
Sometimes, the total number isn't the problem-the difference is. Imagine a 4S battery with these readings:
This battery is dangerous. Cell 3 is a bottleneck. During a run, Cell 3 will heat up much faster than the others. It will drop voltage faster, triggering LVC while the other cells are still full.
If you continue to force this battery, Cell 3 will eventually puff and fail, possibly taking the whole pack (and your car) with it.
Rule of Thumb
If one cell is more than 50% higher than the average of the others, the pack is dying.
Why does Resistance Increase?
It's simple chemistry. Inside the battery, lithium ions move through a liquid electrolyte between the anode and cathode.
Decomposition
Over time, the electrolyte breaks down and creates a solid buildup (SEI layer) on the electrodes.
Clogging
This buildup is like plaque in arteries. It makes it harder for ions to move.
Result: The harder they have to push to get through the "plaque," the more friction is created. Friction = Heat.
When to Retire (The Checklist)
You should retire a battery when:
-
IR is consistently over 20mΩ per cell (for a standard 5000mAh pack).
-
The battery gets HOT (too hot to hold) after a normal run. This heat is the internal resistance converting your energy into waste.
-
It Puffs: High IR generates gas. If the pack is swelling, the chemistry is irreversibly damaged.
Conclusion
Don't fly blind. Voltage tells you quantity, but Resistance tells you quality.
Pro Tip
Check your IR once a month. Write it on the battery with a sharpie (e.g., "Jan: 4mΩ").
When you see that number creep up to 15 or 20, you'll know exactly why your car feels slow, and you can retire the pack before it fails catastrophically.