How to Measure the Internal Resistance of a LiPo Battery

If you're into high-performance hobbies like RC cars or FPV drones, you’ve probably heard people talking about a battery’s “IR,” but what does that actually mean? The internal resistance of a LiPo battery is one of the most important—and most overlooked—indicators of its health. Think of it as the battery's internal bottleneck. It’s a measure of how much the battery itself resists letting its own power flow out. A low IR number is good; a high IR number is bad. Understanding this simple number can tell you a lot about your battery’s condition and how it's going to perform.

Why Should You Actually Bother Measuring Internal Resistance?

So, why should you care about this number? It might seem like a technical detail for engineers, but measuring the internal resistance of a LiPo battery is like taking its pulse. It gives you a clear and honest look at its health, how it will handle being pushed hard, and how much life it really has left in it.

A battery with a low internal resistance (IR) can dump a lot of power quickly without breaking a sweat. In the real world, this translates to more "punch" off the line for your RC car or less voltage sag during a high-throttle climb with your drone. On the flip side, a battery with high IR will struggle to deliver that power. This shows up as a noticeable drop in performance, a battery that gets unusually hot during use, and shorter runtimes. Tracking the IR over the life of a pack is the best way to know when it’s getting tired and needs to be retired for safety reasons.

What Gear Do You Need to Measure It?

The great news is you don’t need a bunch of expensive lab equipment to measure IR. Most modern hobby-grade chargers have this function built right in, making it super easy to do.

Here’s what you’ll need:

• A Smart Charger or Battery Analyzer with an IR Function: This is the key piece of gear. When you’re shopping for a charger, look for one that specifically lists "Internal Resistance Measurement" or "IR Check" as a feature. Many popular brands include this on their mid- to high-end models.
• The Right Connectors: You'll need your main power leads (like an XT60 or Deans connector) and, just as importantly, the balance lead. The most accurate IR readings are taken through the balance plug, as it allows the charger to measure each cell inside the battery individually.

How Do You Actually Measure the Internal Resistance? A Step-by-Step Guide

Measuring the IR is a straightforward process, but if you want to get useful, consistent results that you can track over time, you should try to do it the same way every time.

Step 1: Get the Battery Ready

For the most reliable and consistent readings, it's best to measure your batteries under the same conditions each time. A great habit to get into is to measure them when they are fully charged (4.2V per cell) and have cooled down to room temperature. A hot battery right off the charger or a partially discharged battery will give you a different reading, so consistency is key.

Step 2: Hook It Up to the Charger

Securely plug both the main power lead and the balance lead of your LiPo battery into the correct ports on your charger. The charger needs both of these connections to be able to perform the test and give you a reading for each individual cell.

Step 3: Start the IR Test

Navigate through your charger’s menu system to find the internal resistance function. This might be labeled as "Battery Resistance," "IR," or something similar. Once you find it, select that option and let the charger do its thing. The process is usually very quick and only takes a few seconds.

Step 4: Read and Record the Results

The charger will then display the IR value for each cell in your battery pack, as well as an overall value for the whole pack. The numbers will be shown in milliohms (mΩ). It’s a fantastic idea to write these numbers down, maybe on a small sticker you put on the battery or in a logbook. This way, you can easily see how the IR changes as the battery gets older.

What Can Affect Your Internal Resistance Readings?

If you measure the same battery on two different days and get slightly different numbers, don't panic. The IR reading isn’t a fixed number; it can be influenced by several factors. This is why doing your measurements under the same conditions every time is so important for getting data you can actually compare.

• State of Charge: A battery’s IR will be slightly different when it's fully charged versus when it's at storage voltage. This is why it's best to always measure at the same charge level, preferably when it's full.
• Temperature: This is a big one. A warm battery will always show a lower IR than a cold battery. Always let your batteries cool down to room temperature before measuring to get a true, comparable reading.
• Battery Age and Usage: This is what we're trying to track! As a battery gets older and has more charge/discharge cycles on it, its IR will naturally and slowly increase.
• Battery Size and Capacity: This is also very important. A big, high-capacity battery designed for high-current applications will naturally have a much lower internal resistance than a small, low-capacity one. You can't directly compare the IR of a 5000mAh pack to a 1500mAh pack and expect them to be the same.

What's a "Good" Internal Resistance for a LiPo?

This is the golden question everyone asks, and the honest answer is, "it depends." However, there are some really good general guidelines you can follow to judge the health of your packs. The numbers below refer to the internal resistance per individual cell.

Brand New, High-Quality Pack

A fresh, top-tier LiPo should have an IR below 10mΩ per cell, with many high-performance packs coming in as low as 2-5mΩ per cell when they are brand new. The cells should also be very closely matched, ideally within 1-2mΩ of each other.

A Healthy, Used Pack

As you use a battery, the IR will slowly start to climb. A pack with an IR between 10-20mΩ per cell is still in great shape and should provide solid performance.

An Aging or Worn-Out Pack

Once the IR starts to creep above 20mΩ per cell, it's a sign that the battery is getting tired. It will likely show a noticeable drop in performance and may get warmer during use. While it might still be usable for less demanding, low-power applications, its life as a high-performance pack is nearing the end.

A Potentially Dangerous Pack

If a cell's IR shoots up dramatically, or if one cell is much higher than the others (for example, one cell is at 40mΩ while the others are at 15mΩ), that is a huge red flag that the cell is failing. The battery should be retired immediately for safety reasons.

Remember, a battery with a higher capacity should generally have a lower resistance. A large 6000mAh pack with 15mΩ of resistance is in much worse shape than a small 1300mAh pack with the same reading. Context matters!

How Can You Keep Your Internal Resistance Low?

You can't stop a battery from getting old, but you can definitely slow down the aging process and keep the internal resistance of your LiPo battery as low as possible for as long as possible by taking good care of it.

• Don't Over-Discharge: This is the fastest way to kill a LiPo. Never run your battery down below 3.0V per cell. When your low-voltage alarm goes off, it's time to stop.
• Don't Let Them Sit Fully Charged: Leaving a LiPo fully charged for days on end is very stressful for it. If you aren't going to be using your batteries for more than a day or two, put them at a storage charge (around 3.8V per cell).
• Manage the Heat: Never charge a battery when it’s hot from use. Let it cool down first. And whatever you do, never leave your batteries in a hot car.

Conclusion: Give your battery a quick checkup

Checking the internal resistance of a LiPo battery is like giving it a regular health check-up. It’s a quick, simple way to monitor its condition, know what kind of performance to expect from it, and most importantly, identify when it’s time to safely retire an old pack. By getting in the habit of checking your batteries regularly, you can ensure you get the best performance and the longest possible lifespan out of every single one. It’s a simple step that every serious hobbyist should make part of their routine.