Introduction to Internal Resistance and Measurement Methods of Batteries

    Every battery has an internal resistance. Different types of batteries have different internal resistances. Similar types of batteries have different internal resistance due to inconsistent chemical characteristics. The internal resistance of a battery is very small, so we define it in units of micro ohms or milliohms. Internal resistance is an important technical indicator for measuring battery function. Under abnormal conditions, batteries with low internal resistance have strong discharge ability under high current, while batteries with high internal resistance have weak discharge ability.

    Take a simple example: an old-fashioned digital camera that uses a No. 5 battery (such as the CANON 210, which consumes a lot of power), powered by a No. 5 alkaline battery, may take up to ten consecutive photos; But with the power supply of the No. 5 dry battery, it can only automatically shut down after taking as many photos as possible, but the dry battery is not completely dead; Replacing with a rechargeable nickel hydrogen battery of size 5 may result in more photos being taken. After actual measurement, we may know that the internal resistance of nickel hydrogen batteries is less than that of alkaline batteries and less than that of dry batteries. This example illustrates that in the utilization of high current discharge, it is necessary to choose batteries with lower internal resistance. In the schematic diagram of the discharge circuit, we may consider separating the battery from the internal resistance and dividing it into a completely non internal resistance battery connected in series with a very low resistance resistor. If the external load is light at this time, the voltage allocated to this small resistor will be small. Conversely, if the external load is heavy, the voltage allocated to this small resistor will be relatively large, and some local power will be consumed on this internal resistance (which may be converted into heat or some complex reverse electrochemical reactions). The internal resistance of a rechargeable battery is relatively small when it leaves the factory, but after long-term use, due to the depletion of the electrolyte inside the battery and the decrease in the activity of the chemical substances inside the battery, this internal resistance will gradually increase until the internal resistance is large enough that the battery's internal charge is released abnormally, at which point the battery will; Died unexpectedly; That's it. The vast majority of locally aged batteries are caused by excessive internal resistance, which results in no useful value and can only be scrapped.

1、 Internal resistance is not a fixed value. The trouble is that the internal resistance value of the battery varies when it is in different battery levels; The internal resistance value of a battery varies depending on its lifespan. From a technical perspective, we can separately consider the resistance of a battery into two states: internal resistance in the charging state and internal resistance in the discharging state.

1. Charging state internal resistance refers to the measured internal resistance of a battery when it is fully charged.

2. Discharge internal resistance refers to the internal resistance of a battery measured after it has been fully discharged (to the standard cut-off voltage). In some cases, the internal resistance in the discharged state is not firm, and the measured result is much higher than the abnormal value, while the internal resistance in the charged state is relatively firm, and measuring this value has practical significance. Therefore, in the measurement process of batteries, we use the charging state internal resistance as the measurement standard.

2、 Internal resistance cannot be measured correctly using individual methods. Or you might say that in high school physics class, there is a method taught to calculate the internal resistance of a battery using a simple formula and a resistance box... However, the algorithm taught in physics textbooks to calculate using a resistance box has too low accuracy and can only be used for practical education. In practical use, it is basically helpless to adopt it. The internal resistance of a battery is very small, so we define it in units of micro ohms or milliohms. In certain measurement situations, we request that the accuracy of measuring the internal resistance of the battery must be kept within plus or minus 5%. Such a small resistance value and such a correct request must be measured with instruments.

3、 The current battery internal resistance measurement methods used in the industry. In industry utilization, the correct measurement of battery internal resistance is carried out through equipment. Next, I will talk about the battery internal resistance measurement methods used in the industry.

1. Direct current discharge internal resistance measurement method. According to the physical formula R=V/I, the testing equipment forces the battery to pass through a large constant DC current for a short period of time (sometimes 2-3 seconds) (currently using a large current of 40A-80A), measures the two true voltages of the battery at this time, and calculates the current internal resistance of the battery according to the formula. The accuracy of this measurement method is relatively high, and if it is accurate, the measurement accuracy error may be kept within 0.1%.

But this method has obvious shortcomings:

(1) Only large capacity batteries or rechargeable batteries can be measured, and small capacity batteries cannot withstand a high current of 40A-80A within 2-3 seconds;

(2) When a battery is subjected to high current, the electricity inside the battery will generate a biochemical phenomenon, resulting in chemical resistance. Therefore, the measurement time must be very short, otherwise the measured internal resistance value will have a large error;

(3) High current flowing through the battery can cause certain damage to the internal electricity of the battery.

2. Exchange pressure drop internal resistance measurement method.

Because the battery is actually equivalent to an active resistor, we apply a fixed frequency and current to the battery (currently some use a frequency of 1KHz and a small current of 50mA), and then sample its voltage. After rectification, filtering, and a series of processing, the internal resistance value of the battery is calculated through the operation and discharge circuit. The battery measurement time using the exchange voltage drop internal resistance measurement method is short, sometimes around 100 milliseconds, and it is almost completed with just one press of the measurement switch. ha-ha. The accuracy of this measurement method is also good, with measurement precision errors ranging from 1% to 2% in some cases.

The advantages and disadvantages of this method:

(1) It is possible to measure almost all batteries, including small capacity batteries, using the exchange voltage drop internal resistance measurement method. This method is commonly used to measure the internal resistance of laptop battery cells.

(2) The measurement accuracy of the exchange voltage drop measurement method is likely to be affected by ripple current, as well as the possibility of harmonic current interference. The anti-interference ability in the circuit of this measuring instrument is a test.

(3) Measuring with this method will not cause significant damage to the battery itself.

(4) The measurement accuracy of the exchange voltage drop measurement method is not as good as that of the DC discharge internal resistance measurement method. In some applications of online monitoring of internal resistance, only DC discharge measurement method can be used, and exchange voltage drop measurement method is reluctantly adopted.

3. Component errors in testing instruments and issues with battery connection wires used for testing. Regardless of which method mentioned above, there are some easily overlooked issues, which are the component errors of the testing instrument itself and the problems with the testing cables used to connect the battery. Because the internal resistance of the battery to be measured is very small, the resistance of the circuit needs to be taken into account. A short connecting wire from the instrument to the battery also has resistance (probably at the micro ohm level), and the contact surface between the battery and the connecting wire also has contact resistance. These factors must be properly adjusted for errors within the instrument at that time.     

https://www.senyafactory.com：Portable Power Station Factory  Portable Power Station OEM  Portable Power Station Manufacturer