What is good internal resistance of lipo battery?

Sep 12, 2019   Pageview:34

Lithium polymer batteries which are also known as "LiPo" batteries are the most common type of batteries used today in most of the consumer electronic devices. These batteries have gained popularity over the last few years in the radio control industry and now it is the most preferred option for those who want batteries of high performance and long run times or durability. LiPo batteries provide many advantages, but each user has to understand the few drawbacks of lithium batteries. Generally, there is nothing to worry about with LiPo batteries as long as you know what you’re doing and how to maintain the batteries.


What is internal resistance·

LiPo batteries (and in fact, all power supplies) are not ideal sources of voltage. According to Ohm's Law, when a circuit with a load resistance RL is connected to a voltage source with voltage (V), the amount of current (I) that flows through the circuit is V divided by RL.


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But then, in our world, this doesn’t always make sense. There are quite some inefficiency such as heat-induced loss or resistance of the wire. Therefore, Internal resistance is a common idea in batteries used to describe one (or more) of these inefficiencies. The truth is that there is a resistance in the battery which acts against the flow of current from leaving the battery.


It is quite important to note that there are no actual "internal resistors" in batteries. Batteries store and release their charges through chemical processes that differ in many ways from the other typical voltage sources.


Why is internal resistance important·

Users need to be able to accurately compare the quality and performance of batteries, especially before buying them. Almost all batteries have two "C ratings" printed on their labels: one is for sustained loads and the other is for short bursts. C-rating by definition is "The maximum amount of electricity/current that the battery can safely provide". Unfortunately, in this definition (the term "safe") requires interpretation and does not make much sense in practice.


For this reason, the RC community decided that the internal resistance of a battery was a good measure of battery comparison. However, after studying the data, some people still disagree that internal resistance is a good metric for comparison.


How to calculate the internal resistance of a Lipo battery·

Since we are trying to calculate the resistance of a resistor that’s not actually a single real resistor, it may not be possible for it to be measured directly. So we instead need to take some few other measurements and apply circuit fundamentals in other to calculate the value. To determine the internal resistance, we will use two circuit laws, using voltage sag and current.


The first law we would use is Ohm's Law. This law states that the drop of voltage across a resistor is equal to the current multiplied by the resistance.


V = I * R


The second law is Kirchoff's voltage law. This law states that the sum of all the voltage drops in a closed circuit has to be zero. In other words, the LiPo open-circuit voltage (VOC) must be equal to the voltage at the load resistance (VL) plus the voltage which is at the modeled "internal resistor" (VI).


V_OC = V_I + V_L


We may now use Ohms law to the VI with I * RI.


V_OC = (I * R_I) + V_L


And finally, the resolution for RI provides the following equation for the internal resistance:


R_I = (V_OC - V_L) / I.


It is important to note there are some other versions to this equation. For example, it is possible to use Ohm's Law to replace VL with I * RL. But then, in my tests, I used small resistance to achieve a fairly high energy consumption. I didn’t use a high precision ohmmeter (NB: the average multimeters do not measure resistances more accurate than 1 ohm). So, if you rely on this metric, some errors are likely to occur. Current and voltage are easier to measure accurately.


To calculate the internal resistance of a battery, it is necessary to measure the voltage of your battery without connecting a load (VOC), the current (I) and then the voltage (VR) with the load connected. The method of collecting this data is extremely important. There are some resources on websites and YouTube which can help deal with the measurement of internal resistance, and quite a number of them make simple mistakes.


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But the method below is error-free:

· Connect a fixed load to your battery with an ammeter and at the same time, the Voltmeter should be connected as well.

· Connect the circuit and leave it connected for a fixed time (I used up about 20 seconds of the battery life, but other batteries may have a different requirement or value –this depends on the battery being measured).

· Observe the current consumption (I) and the load voltage (VL), do this as quickly as soon as possible, then disconnect the load.

· Wait for a fixed and specific duration (I used 3 minutes between my readings).

· Then measure the voltage on the battery terminals on your VOC.


A small note on how I chose the duration of my steps 2 and 4. The time delay has to do with the capacitance. I essentially wanted to make sure I waited enough to stabilize the system and leave the non-linear area of · · the curve where capacitance still affects. Capacitance explains the need for consistency with time. For example, when you sample a voltage at 1 second and 5 seconds, you would get different values.


The AC method

There is another method of measuring the internal resistance, in which an AC frequency is applied to the battery and observed. This method requires special equipment and may not be a very good method to get internal resistance if you are interested in steady-state and high load scenarios. It should also be noted that some LiPo chargers can both measure and report internal resistance.


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