State of health diagnosis of electrochemical power sources
During the operation of chemical current sources (CCS), the question about the possibility of assessing its technical condition arises quite often. For primary power sources, this is an assessment of their safety and ability to provide the required level of operating voltage. For rechargeable batteries, two questions make sense: an assessment of the state of charge at any time and a forecast of further performance. At the same time, monitoring the state of health of the current source must be non-destructive: without loss of energy or at least with a very small loss.
When considering these questions, we face three problems:
— наличие параметров источника тока, которые позволили бы с достаточной точностью обеспечить оценку его состояния,
— информация о количественной мере этих параметров для исследуемого источника тока и статистическом их разбросе,
— наличие аппаратуры для тестирования.
The same electrical characteristics for sealed current sources both for cells and for power packs are used for making measurements, such as open-circuit voltage and voltage under load, resistance, response to a specific signal that allows to detect influence of the components of the total resistance.
Diagnostics of current sources of different electrochemical systems achieves various successes.
Open Circuit Voltage (OCV) as a diagnostic parameter was used firstly to assess the safety of primary current sources. However, the parameter measured under stable temperature conditions changes very little, and these changes are commensurate with the spread of OCV of newly made cells.
It is also not possible to assess the state of charge of alkaline batteries (with an unknown operational history) using the OCV method at any time, since the OCV value depends on many factors, which cannot be ranked according to the degree of influence.
However, you can evaluate the battery self-discharge level in a sufficiently long period after its charge and recharge it to full SoC ready to work. This requires information about the characteristics of batteries made by different manufactures which is collected during their operation.
Так, например, экспериментально определенные зависимости величин НРЦ и саморазряда цилиндрических щелочных аккумуляторов SAFT и призматических никель-кадмиевых аккумуляторов ОАО » НИАИ» Источник» позволяют четко оценить остаточную их емкость, но зависимости эти несколько различаются: при НРЦ = 1,25 В у первых сохраняется 60-65% емкости, у вторых — в 2 раза меньше [1].
It should also be noted that with battery life to estimate its residual capacity becomes less accurate.
The residual capacity of sealed lead-acid batteries can be determined more precisely, since during the discharge process the electrolyte concentration and its electrical conductivity change linearly and quite significantly. At temperature 25°C, the OCV decreases by 10% during the discharge process until the capacity is exhausted.
Voltage under load seems to be a more promising parameter for assessing the battery SoH, but there is a specific difference in the diagnostic capabilities of primary and rechargeable current sources.
The electrodes of elements of many electrochemical systems undergo serious changes during storage. These changes are especially noticeable in lithium cells, the metal anode of which is passivated the more strongly, the longer the storage time and the higher the temperature.
After applying a test pulse to the battery, we observe a voltage drop. The battery is restored after a certain period, sometimes significant. Further storage again leads to passivation of electrode.
And when designing batteries, ensuring the stability of its voltage over a significant part of the discharge curve was one of the main tasks. Therefore, in modern batteries with thin electrodes, the zone at which the operating voltage varies little is usually 80-85% of the discharge curve in its middle part. And diagnostics of the state of charge in this area is impossible.
The response to a test signal that is short-lived but powerful enough to reveal the characteristics of the current source can provide great opportunities for evaluating its state. For testing, a pulse of direct or sinusoidal alternating current is used, as well as a more complex shape.
The voltage of a chemical current source when a current discharge pulse is applied can generally be written as the equation
U = НРЦ — IR = НРЦ — I (RΩ + Rпол ),
где I — ток импульса, R — полное сопротивление ХИТ, RΩ — омическое сопротивление, определяемое сопротивлением токоподводящих деталей электродов, их активных масс и сопротивлением электролита, Rпол — поляризационное сопротивление, отражающее скорость электрохимических реакций.
When recording the HIT response to a DC pulse, the voltage change can be divided by these two components of its total resistance. On RW there is an instantaneous change in voltage, Rpol provides a gradual change in the HIT voltage to its new stationary state.
The hardware implementation of such measurements is quite simple, the problem is only in the method and speed of recording the response, as well as in setting the duration of the recording period, which depends on both the current value and the state of charge of the HIT.
Recording the response to a variable sinusoidal signal.
gives a more complete picture of the polarization resistance. This allows us to test complex models of the equivalent HIT circuit, which reflect a more accurate representation of the electrochemical reactions occurring in the current source.
Measurements of the required accuracy are provided by sequential testing at different frequencies in a wide range. This circumstance and the use of a very small test signal leads to a very complex hardware implementation of such measurements and makes this method of research and testing exclusively laboratory.
A complex test signal can lead to simplification of the test equipment and give good results for diagnosing the HIT state, if such a signal is the result of the addition of several signals that specifically change with changes in the HIT. However, its shape can only be determined as a result of targeted impedance studies in a wide frequency range.
When choosing a testing method, one usually faces not only a lack of sensible recommendations on the use of parameters for testing the HIT of the electrochemical system under consideration, but also the lack of a statistical picture describing the change in these parameters for a particular type of HIT.
Some general information on the systems studied can be presented in the following form:
— омическое сопротивление RΩ позволяет оценить степень разряженности ХИТ разных электрохимических систем, как правило, только при малых величинах остаточной емкости;
— изменения RΩ при увеличении степени разряженности тем значительнее, чем меньше габаритные размеры ХИТ;
— разброс RΩ свежеизготовленных ХИТ сильно различается у разных производителей; он тем меньше, чем более автоматизировано производство и лучше осуществляется контроль технологического процесса;
— разброс RΩ свежеизготовленных ХИТ конкретного типа может быть соизмерим с изменением RΩ этого источника тока в процессе разряда;
— для оценки степени разряженности щелочных и свинцово-кислотных аккумуляторов может быть более целесообразна регистрация изменений его сопротивления переменному току на частоте в диапазоне 0,01 — 1 Гц [1] или отклика на тестовый смешанный сигнал (при частоте 1000, 1 и 0,01 Гц);
— после длительной эксплуатации в результате высыхания герметичных аккумуляторов, перераспределения электролита и деформации аккумуляторов значительно увеличивается их омическое сопротивление, что может быть использовано для диагностики деградации аккумулятора [2];
— при измерении сопротивления переменному току в области частот не ниже 1Гц возможна оценка величины начального провала напряжения после длительного хранения литиевых элементов [3];
— оценка степени сохранности литиевых элементов затруднена из-за быстрой пассивации анода после тестирующего импульса тока, а разброс сопротивления пассивной пленки увеличивается со временем хранения [3];
— возможности диагностирования состояния литий-ионных аккумуляторов изучены плохо, но известно, что их омическое сопротивление в процессе разряда несколько увеличивается, а пассивация их анодов разного состава соизмерима с пассивацией металлического литиевого анода в литиевых элементах;
— для всех химических источников тока необходимо накопление информации о диагностических параметрах в банке данных, что позволит не только описать более четко типичную картину для каждой электрохимической системы, но и обеспечит четкие критерии для принятия решений при диагностировании конкретных типов ХИТ.
From what has been said above, it is obvious that in order to assess the possibility of diagnosing the state of various primary current sources, it is necessary to measure the NRC, the voltage under load (when current is applied) and the resistance to direct current and alternating current at frequencies from 1 kHz to 0.1 Hz. Equipment that can be used to diagnose the state of batteries should also provide the ability to conduct several charge-discharge cycles. With a typically guaranteed operating time of 500-1000 cycles, such tests can be considered non-destructive, but an analysis of the difference in charge-discharge characteristics on these cycles will allow the researcher to better describe the state of the battery.
To conduct comprehensive tests of various current sources, the equipment must provide the ability to:
— испытаний как отдельных элементов и аккумуляторов, так и батарей;
— проведение разряда на постоянное сопротивление (для элементов);
— проведения нескольких циклов заряда-разряда при разных способах контроля конца обоих процессов;
— обеспечения заряда разными режимами: при постоянном токе, при постоянном напряжении при ограничении начального тока;
— достаточно больших токов заряда-разряда;
— автоматизированного протоколирования информации о ходе процессов;
— измерения внутреннего сопротивления источника тока.
For comprehensive testing of batteries and batteries based on them, as well as for testing non-rechargeable chemical current sources, we recommend using battery analyzers of the BA400 series (Canada, LaMantia company).
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