For spec class racing the self proclaimed holy grail on any Lithium Polymer Battery is the lowest possible Internal Resistance. From our own perspective the most important objective from your LiPO power source is voltage hold - meaning what is the amount of potential energy in the LiPO after use, the higher the number the better followed by amperage - what amperage can the power source deliver to your system.

 

To check if your LiPO delivers the voltage and amperage that you need is easier than we might think. Look at your lap times - did you do consistent lap times through the race, was your fastest laps towards the second half of the race of even right towards the end? If so you have all the power you will need. We have found in many cases the Internal Resistance in any system is usually higher than almost any brand of new LiPO batteries, meaning that your ability to accept power and amps in the speed controller and motor is limited more by IR than the power source.

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To help customers and racers we have put together the following information in an effort to help you get the most performance from your power source.

 

It is worth noting that in most cases the Internal Resistance shown on almost every brand of RC LiPO charger is inaccurate - in the iCharger DX8 manual it even makes statement to the same, meaning, it does not show the correct internal resistance number for a number of factors. In essence the charger passes current to the battery, causing lithium ions to move through the electrolyte from the cathode to the anode and then the charger manufacturer assumes the internal resistance calculations - these assumptions are what leads to inaccurate readings - that said, assuming that the LiPO battery, charge cables and power source remain constant the data can provide the user with directional information, changing any of these mainstays will lead to further inaccuracies.

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While it might seem counterintuitive to the local RC LiPO expert stock class racer at your local track, the internal resistance of a battery doesn't actually decrease when the charge amperage is increased. In fact, the opposite is usually true: higher charge currents will most likely lead to increased internal resistance due to factors like Joule heating and electrochemical reactions within the battery.

 

However, there's a misconception that might be leading to this these people to believe otherwise. When measuring the internal resistance of a battery, techniques like the pulse discharge method are often used. 

 

1. These methods involve applying a brief high-current pulse to the battery and measuring the resulting voltage drop. 

 

2. Under these conditions, the battery's internal resistance might appear lower due to the way the measurement is taken.   

 

Here's why:

 

Chargers typically use Pulse Discharge Method. This method relies on the assumption that the battery's internal resistance remains constant during the short pulse, your charger will only check the IR every 30 or 60 seconds. However, this isn't always accurate, especially for batteries with significant internal resistance. The high current pulse can temporarily heat the battery, which can lead to a slight decrease reading on the charger in internal resistance. This can make the measured resistance appear lower than the actual value under normal operating conditions.

 

Battery Chemistry and Design: The internal resistance of a battery is influenced by its chemistry, design, and state of charge (SOC). Llthium-ion (the chemistry we use in our Lithium Polymer batteries) can exhibit lower internal resistance at higher charge rates due to improved ion transport within the electrolyte solution. However, this effect is typically limited and can be offset by other factors, such as increased Joule heating.

 

While the internal resistance of a battery might appear lower under certain measurement conditions, it's important to remember that this doesn't necessarily reflect the true behaviour of the LiPO battery under normal operating conditions in our RC Cars. Higher charge currents can typically increase internal resistance, leading to reduced battery performance and lifespan.

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In more simple terms - whilst charging and only whilst charging you will get data on your screen that shows lower numbers however in reality during discharge this will lead to increased internal resistance which is where we really want to see the benefits from lower IR.

 

Internal resistance of a LiPO battery actually decreases as the temperature of the cells increases due to the following factors:

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1. Increased Ionic Mobility:

• Electrolyte: The electrolyte within a battery is a solution containing ions that carry the electric charge. As the temperature rises, these ions gain more kinetic energy, making them move faster. This increased mobility allows them to conduct electricity more efficiently, reducing the resistance within the electrolyte.  

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2. Reduced Viscosity:

• Electrolyte Flow: The viscosity of the electrolyte decreases with increasing temperature. This means the electrolyte can flow more easily, reducing the resistance to the movement of ions.

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3. Improved Electrode Reactions:

• Chemical Reactions: The chemical reactions occurring at the electrodes of a battery, where ions are exchanged, are temperature-dependent. Higher temperatures can accelerate these reactions, leading to a smoother flow of charge and lower internal resistance.  
   

Important Note:

While a moderate increase in temperature generally lowers internal resistance, it's crucial to maintain the battery within its optimal temperature range. Excessively high temperatures can lead to accelerated degradation and potential safety hazards.   

 

Therefore, while a moderate temperature increase can improve battery performance, it's essential to balance this with the need to avoid extreme conditions that could damage the battery. 

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We recommend to avoid deep discharges - more damage is done to the lipo through deep discharge which elevates temperatures well above a safe range. This means careful consideration around the capacity requirements for your racing.

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If you would like further reading on the above we recommend reading the published case study from Yun Bao at the Department of Applied Physics, Donghua University. 

 

As well a review of the Patent for pulse discharge battery testing as published online (google patents).