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Battery internal resistance and measurement method

2019/11/22


  Different types of batteries have different internal resistances.For the same type of battery,the internal resistance is different due to the different internal chemical characteristics.The internal resistance of battery is very small.We usually define it in milliohm units.Internal resistance is an important technical index of battery performance.Under normal circumstances,the battery with small internal resistance has strong discharge capacity of large current and weak discharge capacity of the battery with large internal resistance.

  In terms of the schematic diagram of the discharge circuit,we can separate the battery and the internal resistance,and divide it into a power supply without internal resistance and connect it in series with a small resistance.At this time,if the external load is light,the voltage distributed on the small resistance will be small;otherwise,if the external load is heavy,the voltage distributed on the small resistance will be relatively large,and a part of the power will be consumed on the internal resistance(which may be converted into heating,or some complex reverse electrochemical reactions).The internal resistance of a rechargeable battery is relatively small when it leaves the factory.However,after a long-term use,due to the depletion of the electrolyte inside the battery and the decrease of the activity of the chemical substances inside the battery,the internal resistance will gradually increase until the internal resistance is large enough that the electric quantity inside the battery cannot be released normally.At this time,the battery will be"dead".Most of the aging batteries are useless because of the excessive internal resistance.Therefore,we should pay more attention to the capacity of batteries rather than the capacity of charging.

  I.internal resistance is not a fixed value.The trouble is that when the battery is in different power state,its internal resistance value is different;when the battery is in different service life state,its internal resistance value is also different.From the technical point of view,we generally divide the battery resistance into two states:the internal resistance in charge state and the internal resistance in discharge state.

  1.Internal resistance in charged state refers to the measured internal resistance of the battery when the battery is fully charged.2.Discharge state internal resistance refers to the internal resistance measured after the battery is fully discharged(when discharging to the standard cut-off voltage).In general,the internal resistance in the state of discharge is unstable,and the measured result is much higher than the normal value.However,the internal resistance in the state of charge is relatively stable,which is of practical significance.Therefore,in the process of battery measurement,we take the internal resistance of charged state as the measurement standard.

  2.The internal resistance can't be measured accurately by general methods.Maybe you will say that there is a method to calculate the internal resistance of battery with simple formula+resistance box in high school physics class But the accuracy of the algorithm of resistance box calculation in physics textbook is too low,so it can only be used in theory teaching,and can not be used in practice.The internal resistance of battery is very small.We usually define it in micro ohm or milliohm units.In general measurement occasions,we require that the measurement accuracy error of battery internal resistance must be controlled within plus or minus 5%.Such a small resistance and such a precise requirement must be measured with a special instrument.

  3.In the current industry application of battery internal resistance measurement methods,the accurate measurement of battery internal resistance is carried out by special equipment.Let me talk about the battery internal resistance measurement method applied in the industry.At present,there are two main methods of measuring the internal resistance of battery in the industry:

  1.DC discharge internal resistance measurement method.According to the physical formula r=u/I,the test equipment forces the battery to pass a large constant DC current in a short period of time(generally 2-3 seconds)(at present,a large current of 40a-80a is generally used),measures the voltage at both ends of the battery,and calculates the current internal resistance of the battery according to the formula.The accuracy of this method is high.If it is properly controlled,the measurement accuracy error can be controlled within 0.1%.

  However,this method has obvious shortcomings:(1)it can only measure large capacity batteries or batteries,and small capacity batteries can't load large current of 40a-80a in 2-3 seconds;(2)when the batteries pass through large current,the electrodes inside the batteries will polarize and produce polarization internal resistance.Therefore,the measurement time must be very short,otherwise the error of the measured internal resistance value is very large;(3)the electrode inside the battery is damaged by large current passing through the battery.

  2.AC voltage drop internal resistance measurement method.Because the battery is actually equivalent to an active resistance,we apply a fixed frequency and a fixed current to the battery(at present,we usually use a 1kHz frequency and a small current of 50mA).Then we sample its voltage,and calculate the internal resistance of the battery through the operation and discharge circuit after a series of processing such as rectification and filtering.The battery measurement time of AC voltage drop internal resistance measurement method is very short,generally about 100ms.The accuracy of this method is also good,and the error of measurement accuracy is generally between 1%and 2%.

  Advantages and disadvantages of this method:

  (1)almost all batteries,including small capacity batteries,can be measured by AC voltage drop internal resistance measurement.This method is generally used to measure the internal resistance of notebook battery cell.

  (2)the accuracy of AC voltage drop measurement is likely to be affected by ripple current and harmonic current interference.This is a test for the anti-interference ability of the measuring instrument circuit.

  (3)using this method,the battery itself will not be damaged.

  (4)the measurement accuracy of AC voltage drop method is lower than that of DC discharge internal resistance method.

  3.The component error of the test instrument and the problem of the battery connecting wire used in the test.No matter which of the above methods,there are some problems that are easy to be ignored,that is,the component error of the test instrument itself and the test cable used to connect the battery.