Understanding Internal Resistance in Batteries: Why It Matters for RC

When it comes to powering your remote-controlled (RC) vehicles, whether it's a car, drone, or plane, the battery is at the heart of the operation. But there’s a crucial aspect of battery performance that often goes unnoticed—internal resistance. This hidden factor can significantly impact your RC experience, from how long your battery lasts to how well your vehicle performs during high-demand activities. Understanding internal resistance is essential for hobbyists seeking to optimize their setups.

What is Internal Resistance?

At its core, internal resistance is the opposition to the flow of current within the battery itself. Every battery, whether it's a LiPo, Li-ion, NiMH, or lead-acid type, has some degree of internal resistance. This resistance is caused by the materials inside the battery, the design of the battery, and even the age of the battery. It’s a natural characteristic of the chemical processes that take place inside the battery as it discharges.

When a battery discharges, the internal resistance causes a voltage drop across the battery's internal components, which reduces the amount of energy available for your RC vehicle. The higher the internal resistance, the greater the energy lost as heat, leading to less efficient performance.

How Internal Resistance Affects RC Hobbyists

As an RC hobbyist, you're likely pushing your vehicles to their limits with high-speed runs, jumps, or even aerobatics. These high-power demands put a heavy load on your battery, making internal resistance a critical factor in performance. Here’s why:

Reduced Power Output
When internal resistance is high, it causes a voltage drop within the battery as the current increases. This means less voltage is available for the motors and other components in your RC vehicle, leading to reduced power output. In practice, this could mean slower speeds or less torque, especially during moments when your vehicle needs extra power, such as during acceleration or steep climbs.
Increased Heat Generation
As internal resistance increases, more energy is wasted as heat. This not only reduces the efficiency of your battery but can also cause overheating, which may lead to premature battery wear or even thermal runaway in extreme cases (particularly with lithium-based batteries like LiPo). Overheated batteries can become unstable and may even pose a safety risk.
Shorter Run Times
With higher internal resistance, your battery will struggle to deliver consistent power over time. This can lead to quicker voltage sag under load and shorter run times. Even though the battery might show a full charge when you start, the actual usable energy is diminished because of the internal resistance.
Faster Voltage Drop Under Load
RC vehicles often experience sharp current draws during intense usage, like a quick acceleration or a large jump. If the internal resistance is high, these surges in current can cause a rapid drop in voltage, leading to poor performance (e.g., slower speeds or stuttering control). This can also trigger low-voltage cutoffs, causing your vehicle to shut down prematurely.

Factors That Affect Internal Resistance

Internal resistance isn't a static number—it can change depending on several factors:

Battery Chemistry: Different types of batteries exhibit varying levels of internal resistance. LiPo (Lithium Polymer) batteries, for example, tend to have lower internal resistance compared to older chemistries like NiMH. LiFePO4 (Lithium Iron Phosphate) batteries are known for being more robust and having relatively low resistance, but they also typically offer lower power density compared to LiPo.
Age and Usage: Over time, the internal resistance of a battery increases. This is partly due to the wear and tear on the battery's internal components, including the electrodes and electrolyte. Frequent deep discharges or high-temperature exposure can accelerate this aging process.
Temperature: Internal resistance can fluctuate with temperature. Cold weather increases resistance, while heat can lower it but lead to overheating and damage in the long run. This is one reason why it's important to store and use batteries at recommended temperatures.
Manufacturing Quality: Not all batteries are created equal. Higher-quality cells tend to have lower internal resistance, while cheaper or lower-quality cells may suffer from higher resistance right out of the box.

Measuring Internal Resistance

If you're serious about optimizing your RC vehicle's performance, it’s helpful to know how to measure a battery’s internal resistance. Many battery testers and chargers come with built-in resistance measurement tools, often displayed in milliohms (mΩ). While most batteries will have internal resistance in the range of a few milliohms, anything significantly higher might indicate a potential issue with the battery.

For RC hobbyists, checking internal resistance regularly can be a good way to assess the health of your batteries and predict when they might be nearing the end of their life. If you're using a high-performance setup, knowing how your battery responds to high current loads will help you avoid unexpected performance drops.

How to Minimize the Impact of Internal Resistance

While you can’t eliminate internal resistance, there are ways to minimize its impact and extend the life of your RC batteries:

Choose High-Quality Batteries
When selecting batteries, opt for reputable brands and high-quality cells. Batteries from well-known manufacturers like Turnigy, Venom, or Gens Ace typically offer lower internal resistance and better overall performance.
Proper Storage and Care
Store your batteries at the recommended voltage level (typically around 3.8V per cell for LiPos) and in a cool, dry place. Avoid overcharging or deep discharging, as both can increase internal resistance over time.
Monitor Battery Temperature
Always keep an eye on your battery temperature during use. Overheating can increase internal resistance, so try to avoid running your RC vehicle for too long at high power. You can also use external cooling systems, like heatsinks or active cooling fans, to help regulate the temperature of your batteries during intense use.
Balance Charging
Use a quality balance charger to ensure your cells are charged evenly. Imbalanced charging can increase internal resistance and lead to inconsistent battery performance.
Regular Testing
Invest in a battery tester that can measure internal resistance, and perform regular checks to monitor the health of your batteries. This can help you spot aging batteries before they start affecting performance.

Conclusion

Internal resistance is a critical yet often overlooked factor when it comes to battery performance in the RC hobby. A battery with high internal resistance can lead to reduced power output, increased heat generation, shorter run times, and overall diminished performance. By understanding what internal resistance is and how it impacts your RC setup, you can make informed decisions about battery selection, usage, and maintenance, ensuring that your RC vehicles perform at their best every time you hit the track, field, or sky.

Happy RC’ing!