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:
- availability of current source parameters that would allow to assess its state with sufficient accuracy,
- quantitative data information of these parameters for the studied current source and their statistical spread,
- availability of testing equipment.
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.
So, for example, experimental determination of value relationship between OCV and self-discharge of cylindrical SAFT alkaline batteries and prismatic nickel-cadmium batteries made by JSC “NIAI “Istochnik” allow us to estimate their residual capacity clearly. However, this dependence differ somehow: at OCV = 1.25 V, the first batteries retain 60-65% of capacity, while the latter retain 2 times less.
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 = NRC-IR = NRC-I (RW + Rpol ),
where I is the pulse current, R is the total resistance HIT, RW is the ohmic resistance determined by the resistance of the current-carrying parts of the electrodes, their active masses and the resistance of the electrolyte, Rpol is the polarization resistance reflecting the rate of electrochemical reactions.
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:
- the ohmic resistance RW makes it possible to estimate the degree of discharge of the HIT of various electrochemical systems, as a rule, only for small values of the residual capacitance;
- changes in RW with an increase in the degree of discharge are more significant, the smaller the overall dimensions of the HIT;
- The RW spread of freshly prepared HIT products varies greatly from manufacturer to manufacturer; the smaller it is, the more automated production is and the better process control is carried out;
- the spread RW of freshly prepared hits of a particular type can be commensurate with the change in RW of this current source during the discharge process;
- to assess the degree of discharge of alkaline and lead-acid batteries, it may be more appropriate to record changes in its resistance to alternating current at a frequency in the range of 0.01-1 Hz [1] or the response to a mixed test signal (at a frequency of 1000, 1 and 0.01 Hz);
- after long-term operation, as a result of drying of sealed batteries, redistribution of electrolyte and deformation of batteries, their ohmic resistance significantly increases, which can be used to diagnose battery degradation [2];
- when measuring the AC resistance in the frequency range not lower than 1 Hz, it is possible to estimate the value of the initial voltage drop after long-term storage of lithium cells [3];
- evaluation of the degree of safety of lithium cells is difficult due to the rapid passivation of the anode after a test current pulse, and the spread of the resistance of the passive film increases with storage time [3];
- the possibilities of diagnosing the state of lithium-ion batteries are poorly understood, but it is known that their ohmic resistance increases slightly during discharge, and the passivation of their anodes of different composition is comparable to the passivation of a metal lithium anode in lithium cells;
- for all chemical current sources, it is necessary to accumulate information about diagnostic parameters in a data bank, which will not only describe the typical picture for each electrochemical system more clearly, but also provide clear criteria for making decisions when diagnosing specific types of HIT.
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:
- testing of both individual cells and accumulators, as well as batteries;
- conducting a constant-resistance discharge (for elements);
- conducting multiple charge-discharge cycles with different methods of monitoring the end of both processes;
- providing charge in different modes: at constant current, at constant voltage with initial current limitation;
- sufficiently large charge-discharge currents;
- automated logging of information about the progress of processes;
- measurement of the internal resistance of the current source.
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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