Electrical Measurements of Lithium-Ion Batteries
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Fundamentals and Applications
Electrical Measurement of Lithium-Ion Batter i e s : Fundamentals and Applications
Contents
Introduction .......................................................................................................................................................................... 3
Overview of the lithium-ion battery manufacturing process ........................................................................................ 4
1.
2. Electrode materials and electrode manufacturing process ............................................................................................ 5
-1. Importance of quality testing for electrode materials and electrodes ...................................................................... 5
-2. Quality testing of the dispersion of materials in electrode slurry ............................................................................ 6
-3. Quality testing of electrode sheets during their fabrication process ........................................................................ 7
-4. Testing of electrode sheets for metal contaminants ................................................................................................. 9
3. Cell assembl y ............................................................................................................................................................. 11
-1. Weld resistance testing of terminal (tab-lead) ....................................................................................................... 11
-2. Testing of the insulation resistance befor e e le c tr olyte filling ................................................................................ 14
-3. Measuring enclosure poten tia l (laminated lithium-ion batteries) .......................................................................... 19
4. Finished cells (cell performance testing) .................................................................................................................... 24
-1. Pre-charging and charge/discharge characteristics testing ..................................................................................... 24
-2. Measurement of open-circuit voltage (OCV) d uring the aging process ................................................................ 26
-3. Battery impedance measurement ........................................................................................................................... 28
5. Battery modules and battery packs (performance evaluation at the finished-product level) ...................................... 37
-1. Total resistance testing of battery modules and battery packs ............................................................................... 37
-2. Testing of BMS boards .......................................................................................................................................... 41
-3. Actual-load testing of ba tteries in EVs .................................................................................................................. 46
Conclusion .......................................................................................................................................................................... 49
References .......................................................................................................................................................................... 50
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Electrical Measurement of Lithium-Ion Batteries: Fundamentals and Applications
Introduction
Lithium-ion batteries (LIBs) offer particularly high performance among rechargeable batteries and are used in a
variety of industrial domains. They were primarily used as a power supply for portable devices in the past. In recent
years their applications have expanded to encompass stationary energy storage systems and electric vehicles (EVs),
driving demand for lower-cost LIBs with even higher performance.
Demand for LI Bs for use in electr ic vehicles, incl uding EVs and xEVs ( HEVs and PHEVs) , is growing particularly
rapidly as governments around the world fast-track measures to promote automobile electrification.
Batteries used in EVs must deliver an extremely high level of performance. Examples of automobile characteristics
and the corresponding requirements placed on batteries include:
• Long-distance dri ving: High energy density (high capacity in a compact, lightweight footprint)
• Fast charging: The abilit y to charge using large current s (exceptional high-current characteristics)
• Extended use: Long battery service life (improved performance in the face of repeated
charge/discharge cycles)
• Improved safety: Resistance to combustion (pro tective functionality provided by preve ntion of internal
battery short-circuits, BMS ICs, etc.)
• Lower vehicle costs: Limitations on material prices and high productivity (development of inexpensive
materials and improvements to yields)
For the realization of the battery characteristics shown above, many kinds of measurements and tests are necessary at
each state of the battery manufacturing process to assure the quality of each process. In addition, measurements and
testing are essential in a variety of settings, during not only manufacturing, but also R&D and finished-product
inspections.
Typical measurement and test instrument includes charge/discharge systems, impedance meters, insulation testers,
and high-precis ion voltmeters. HIOKI offers a varie ty of products in the electrical measurement domain that are well
suited to the measurement and te sting of batteries.
This guide introduces ke y considerations in the selection of meas urement and testing equipment that is esse ntial in
evaluating the performance of materials, manufacturing processes, and finished products, mainly with a focus on our
solutions.
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Electrical Measurement of Lithium-Ion Batteries: Fundamentals and Applications
1. Overview of the lithium-ion ba t t ery manufacturing process
First, Figure 1 offers a survey of lithium-ion battery production processes and the types of testing used in each.
Broadly spe aking, the p rocess b y which lithiu m-ion bat teries are manufactured can be broken down into the following
stages:
• Manufacture of materials and electrodes
• Assembly of battery cells
• Performance testing of finished battery cells
• Assembly of modules and packs (assembled batteries)
• Performance testing of modules and packs (assembled batteries)
The section starting on the next page describes the parameters that need to be evaluated in each process and the
measuring equipment used to obtain them.
Figure 1. Some of th e test parameters by each process
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Electrical Measurement of Lithium-Ion Batteries: Fundamentals and Applications
2. Electrode materials and electrode manufacturing process
-1. Importance of quality testing for electrode materials and electrodes
Battery cell manufacturing process can be broadly divided into material manufacture, slurry production, electrode
fabrication, and battery assembly.
In order to produce batteries that satisfy the desired specifications in a stable manner, it is extremely important to
ensure quality in each stage of the manufacturing process. The mo re defects can be eliminated in upstream processes,
the more production efficiency can be increased. Although there are numerous quality indicators that should be
managed, this section will address the following:
• Slurry production: Material ratio and degree of mixing
• Electrode fabrication Drying conditions and electrode density
• Assembly process Extent of contamination with impuritie s
Strict mana geme nt of eac h proce ss lays the fo unda tion fo r pr ocess c hanges a nd impr ovemen ts durin g the R&D stage
and for high-quality, high-yield, stable p roduction during the manufacturing stage.
Figure 2. Testing during the battery assembly process
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Electrical Measurement of Lithium-Ion Batteries: Fundamentals and Applications
-2. Quality testing of the dispersion of materials in electrode slurry
Lithium-ion battery electrode sheets are fabricated from an electrode slurry that consists of active material,
conductive additives, a polymer binder, and an organic solvent.
To boost battery capacity, it is straightforward strategy to reduce the proportion of conductive additives and to
increase the proportion of active material. On the other hand, it is important to have enough electron conductivity in
order to lower the battery’s internal resistance, necessitating an appropriate quantity of conductive additives. It is
important to optimize the ratio of active material to conductive additives based on this trade-off.
Additionally, several researches in recent years suggest that a uniform dispersion of these materials in the electrode
1-3)
slurry is extremely important in o btaining favorable batter y characteristics
. Ensuring surface contact between active
material particles and electrolyte increases the reaction area, resulting in more favorable battery characteristics. In
addition, an appropriate dispersion of conductive additives, which provides the electron conduction path, is necessary. If
the shearing force applied to mix the slurry is too little, the conductive auxilia r y material will not loosen s ufficiently. On
the other hand, good electron conduction cannot be obtained if the shearing force is too strong that a particle of
conductive additives is broken apart into fine particles. Additionally, if the cond uctive a dditi ves forms clumps, c harges
will conce ntrate t here dur ing the charge/discharge after the battery has been assembled. They are undesirable since the
goal is to facilitate uniform battery reactions across the entire electrode surface. It can be concluded here it is important
to manage the particle size distribution and d isper sibilit y of ac tive mater ial and the conductive add itives in the electrode
slurry.
HIOKI is proposing a new method analyzing electrode slurry using impedance measurement. Thi s method makes it
possible to analyze capacitance components that depend on the electron conductivity of conductive additives, the
particle size and dispersion of active material. It has been difficult to accomplish this with the conventional DC method
or optical testing.
Figure 3. Measuring the impedance of an electrode slurry
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Electrical Measurement of Lithium-Ion Batteries: Fundamentals and Applications
-3. Quality testing of electrode sheets during their fabrication process
The first step in the electrode sheet fabrication process is to apply a thin coat of slurry to metal foil (so-called the
current collector). Next, the so lvent o f the slurry is evaporated by warm air in a drying step. The layer fabricated is
called the composite layer. Then the sheet is pressed with a metal roller to increase the strength of the composite layer
and improve electrical conductivity (“calendaring”). A number of key points must be considered with regard to battery
performance during electrode fabrication.
The first co nsider ation is uni formi ty of thic kne ss whe n appl ying the slurr y. If the thickne ss is not unifo rm, t here will
be deviations in battery reactions. Additionally, if there are air bubbles in the slurry, they could burst and cause the
slurry around them to thin. I t is necessar y to measure va riations in the thickness o f the coating in b oth the width wise
and lengthwise directions in o rder to control quality and detect anomalies. Optical micr ometers are used to measure
slurry thickness.
The second consideration is whether the particles in the slurry have been dispersed in a sufficiently fine-grained
manner. If they are not thoroughly dispersed, the particles will not be able to perform their function fully, resulting in
the deterioration of the battery’s performance. Particles sometimes form clumps in the slurry due to poor dispersion. If a
slurry with clumps is applied, the coating can wear away and appear stringy. One technique involves image testing with
a camera to detect this stringy appearance and use it as an indicator for evaluating slurry characteristics.
In addition, it is necessary to increase the mechanical strength o f the dried electrod e composite layer, which is brittle,
by pressing it with a metal r oller. This process also has the effect of embedding the active material in the collector to
improve electrical conductivity. I ncreasing the press force lowers the contact resistance (interface resistance) between
the composite layer and the current collector. However, excessively high press force impedes impregnation of the
electrolyte and increases the battery’s resistance, worsening the battery’s input/output performance. By contrast,
excessively low press force fails to endow the composite layer with sufficient mechanical strength, posing the risk that
it will collapse in the face of repeated charge/discharge. This will lead to reduced batter y service life. Con seque ntly, it is
necessary to set and maintain the appropriate press force.
HIOKI offers the RM2610 Electrode Resistance Measurement System as a means of managing electrode sheets based
on an evaluation of their resistance (Figure 4). The RM2610 determines the volume resistivit y of the composite layer
and the contact resistance (interface resistance) between the current collector and the composite la yer separately, by
applying a current from the composite layer surface and then measuring and calculating the surface potential
distribution created by that current.
The RM2610 makes it possible to evaluate electrodes prior to the assembly of battery cells by using the composite
layer volume resistivity and contact resistance as indicators. Assuring quality during the electrode sheet fabrication
process promises to speed the development work that drives lithium-ion battery evolution and to improve the
production yield rate.
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Electrical Measurement of Lithium-Ion Batteries: Fundamentals and Applications
Figure 4. RM2610 Electrode Resistance Measurement System
Let’s take a look two conventional techniques: 4-probe measurement (Figure 5) and pass-through resistance
measurement (Figure 6) . In the 4-probe measurement method, four probes are placed in contact with one side of the
electrode, and 4-terminal resistance measurement is performed. In pass-through resistance measurement, the electrode
is sandwiched in between plate electrodes, and its electrical resistance is measured using 2-terminal resistance
measurement.
These measurement methods can not measure the contact resistance (interface resistance) or the composite layer
resistance separately. Even so , they do yield resistance values that reflect the electrode’s characteristics, and they are
widely used as qualitative quality indicators in electrode fabrication processes. However, the generally low
reproducibility of measurement makes it essential to carefully manage measurement conditions.
Figure 5. Measurement using the 4-probe method
Figure 6. Pass-through res istance measurement
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Electrical Measurement of Lithium-Ion Batteries: Fundamentals and Applications
-4. Testing of electrode sheets for metal contaminants
The electrode sheet manufacturing process must be painstakingly managed to ensure that materials are not
contaminated with metal powder. Potential sources of metal powder include:
• Metal shavings in the manufacturing area, for example from manufacturing equip ment enclosures
• Burrs from the process in which electrodes are cut to the desired size
• Metal powder that has adhered to workers’ uniforms
Contamination of materials with metal powder can cause internal short-circuits during battery operation. For example,
metal contaminants such as iron, copper, and nickel can dissolve in electrolyte during charging and cause highly
1
branched or dendritic deposits on the negative electrode
. In addition to a reduction in capacity caused by a series of
reactions, contamination in the worst case can cause a large-scale short between the positive and negative electrodes,
resulting in an explosion or other accident (Figure 7).
Let’s take a look at several techniques to detect foreign material directl y which have been commercialized. There are
several types of contaminant detec tion systems:
• Camera-based image testing systems: These systems can perform large-scale contaminant testing relatively
2
inexpensive ly. Although it is difficult to detect metals alone
, some systems offer that capab ility. This method has the
disadvantage of not being able to detect foreign materials that are embedded inside electrodes.
• Detectio n of metal contaminants using X-rays: This approach can detect resins and other contaminant s in addition to
metals. This approach is more expensive than other testing systems in terms of both initial costs and maintenance costs.
• Detection of metal contaminants using a magnetic sensor: In order to detect minuscule metal fragments, the sensor
must be positioned extremely c lose to the DUT. This approach can only detect magnetic metal contaminants.
Since each of the techniques described above has its own strengths and weaknesses, manufacturers must either choose
the system that best suits their obj e c tive or utilize multiple techniques.
Contamination of battery cells with metallic material can be reliably prevented by testing electrode sheets
immediately prior to winding. Alternatively, testing can be performed in multiple processes, making it possible to trace
specific contaminants to specific processes.
1
It is often called simply “dendrite”.
2
Rejecting cells th at contain foreign materials th at do not affect batter y perfor mance as defecti ve is known as overkil l. This p ractice
can worsen the production yield rate.
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Electrical Measurement of Lithium-Ion Batteries: Fundamentals and Applications
Figure 7. Short-circuit in a battery caused by contamination with foreign material
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Electrical Measurement of Lithium-Ion Batteries: Fundamentals and Applications
3. Cell assembly
-1. Weld resistance testing of terminal (tab-lead)
The quality of terminal (tab-lead) welds plays an important role in allowing battery cells to deliver their full
performance (Figure 8). In EV app licatio ns, it is particular ly i mportant to minimize o utput loss and heat ge neratio n. To that
end, it is ideal for welds to have super-low resistance that approaches 0 Ω.
In general, defective and non-defective products are classified based on weld resistance on the order of 0.1 mΩ or less.
Engineers must choose a resistance meter that is ideal for low-resistance measurement with a resolution of 1 μΩ or less.
Figure 8. Tab-leads in a laminated lithium-ion batte ry
The following precautions should be taken wit h regard to low-resistance measurement:
(1) Measurement current
3
First the constant current is applied to t he DUT (device under test
), and then the vo ltage across the DUT is measured.
The resistance value is calculated using Ohm’s law. Instruments known as resistance meters are specifically designed to use
this resistance measurement method. Generally speaking, low-resistance measurement requires a large measurement
current in order to facilitate accurate measurement. If t he DUT has a resistance value of 1 mΩ or less, it is recommended to
use a resistance meter that generates a current of at least 100 mA, and if possible, of 1 A.
(2) Resistance measurement using the 4-terminal method
In low-resistance measurement, the measurement probes’ wiring resistance and the contact resistance of the probe tips
exert a significant influence o n measurement and therefore cannot be ignored. In particular, the contact resistance at the
point of measurement probe contact can reach several ohms or even dozens of ohms depending on environmental
3
The object under measurement is generally called “the device un der test” or simply “the DUT”.
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conditions.
Electrical Measurement of Lithium-Ion Batteries: Fundamentals and Applications
With 2-terminal measurement, the measurement current I fl ows through no t only the r esistance R
the wiring resistance and contact resistance (r
equation: E = I (R
). The resistance value calculated using Ohm’s law from this equation is R0+r1+r2 (Figure 9).
0+r1+r2
and r2), with the result that the observed voltage E is given b y the follo wi ng
1
o f the DUT, but also
0
The 4-terminal method is u sed to resolve this problem. With 4-terminal method, separate pairs of current-applying and
voltage-measuring electrodes are used. The measurement current I flows through the resistance R
voltage mea surement terminal’s r
result, the voltage E measured is exactly equal to the voltage E
measured without being affected by r
or r4. Consequently no voltage occurs across the r3 and r4 portion of the circuit. A s a
3
of the DUT, allowing the resistance to be accurately
0
, r2, r3, or r4 (Figure 10).
1
but not through the
0
Based on the above, it is necessary to choose a resistance meter of the 4-terminal method when mea suring low resi st a nce
values on the order of milliohms.
Figure 9. Resistance measurement with the 2-terminal
method
Figure 10. Resistance measurement with the 4-terminal
method
(3) Influence of thermal electromotive force
Thermal electromotive force is a potential difference that occurs across the junction of different metals. When
measuring the resistance, thermal electromotive force occurs at the junction of the measuring probe and the DUT, which
becomes a source of error. The influence of the thermal electromotive force V
value R
of the DUT is small, because the voltage RX IM to be measured is small in such cases. One technique to
X
is particularly large if the resistance
EMF
eliminate the influence of ther mal electromotive for ce is to conduct the measurement twice, one the measurin g curre nt
in the positive direction and one in the negative directio n. The influence of thermal electromotive force can be removed
by means of a calculation.
By subtracti ng the voltage measured with the current in the negative direction from the voltage mea sured with the
current in the positive directi on, it is possible to obtain a resista nce value that is immune to the influence of t herma l
electromotive force (Figure 12, Equation [1]). The Resistance Meter RM3545 provides an offset voltage compensation
(OVC) function for canceling the influence of ther mal electromotive force.
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Electrical Measurement of Lithium-Ion Batteries: Fundamentals and Applications
Figure 11. Error caused by thermal electromotive force
Figure 12. Cancellation of thermal electromotive force
using the OVC functio n
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Electrical Measurement of Lithium-Ion Batteries: Fundamentals and Applications
-2. Te sting of the insulation resistance befor e e le c tr olyte filling
When the insulation of the components, between which must be i nsulated, is insufficient, the deficiency may cause a
lowering in the battery’s service life or an accident involving fire. The primary causes of the deficienc y of t he insulation
resistance are contamination with metallic material and separator tears.
4
Principal parts of the battery that must be insulated include the electrodes and the electrodes and enclosure
.
Figure 13. Insulation Tester ST5520 to measure the insulation resistance of battery cells
In order to ensure sufficient insulation resistance, it is essential to perform insulation resista nce te sting of battery cells
before the electrolyte filling. Insulatio n resistance meters are used to perform insulation resistance testing. Insulation
resistance meters are one type of resistance meter that has been specifically designed to measure high resistance
4)
values
.
Insulation resistance meters apply a high voltage to an insulator, measure the flowing current, and calculate the
corresponding resistance value. These instruments equip highly sensitive ammeters that can accurately detect minuscule
picoampere (pA) and femtoampere (fA) currents.
Because the measurement signals in insulation resistance measurement are minuscule, measured values are highly
susceptible to external noise or leakage currents. It is essential to prepare a suitable measurement environment. The
stability of measured values is also important.
Conventionally, the term “insulation resistance meter” refers to an instrument that is capable of measuring resistance
values up to around 10 GΩ. Insulation resistance meters that can measure even higher resistance values (typically
12
values on the order of TΩ = 10
Ω or higher) are known as super megohmmeters to distinguish them5. This guide
differentiates broadly between the two types of instruments in its explanations by using the terms “insulation resista nce
meter” and “super megohmmeter.” Since the two types of instrument differ significantly not only in terms of the
4
It is particular ly important to identify batteries with insulation defects between the negati ve el ectrode and the enclo sure as defective.
A more detailed explanation is provided in the chapter “Measuring the enclosure potential”.
5
Super megohmmeters ar e sometimes known as super-megg er s or picoammeters.
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Electrical Measurement of Lithium-Ion Batteries: Fundamentals and Applications
performance but also cost, it is advisable to choose the best instrument based on the pass/fail judgment criteria o f the
process.
HIOKI’s insulation resistance meters include the Insulation Tester ST5520, and its super megohmmeters include the
Super Megohmmeter SM7110 series. The following points should be taken into consideration in order to choose the
best instrument for a given set of requirement s.
(1) Insula tion resi stance value measurement range
It is necessary to choose an instrument that is capable of measuring insulation resistance values that are greater than
the insulation pass/fail judgment threshold. It is particularly important to check the measurement range when the
judgment thr esholds ar e high, on the order of several gigaohms or greater.
• If you require accuracy on the order of several percent with judgment thresholds from several megaohms to several
gigaohms: ST5520
• If you require higher accuracy with judgment thresholds from several kiloohms to several hundred teraohms: SM7110,
SM7120
(2) Voltage output performance
Choose a model that offers optimal voltage levels that are applied during insulation resistance measure ment.
• 1000 V or less: ST5520, SM7110
• 2000 V or less: SM7120
In addition, exercise care with regard to the capacitance of battery cells. Some battery cells have large capacitance
values, several hundred picofarads or several microfarads, or even greater. When measuring such cells, the applied
voltage may exhibit overshoot. When overshoo t occurs, it will take time for the set test voltage to be output in a stable
manner. This phenomenon may influence cycle time, as described below. In addition, a voltage exceeding the set
voltage is applied during the overshooting, raising the risk that the DUT could be damaged.
HIOKI’s insulation resistance meters and super megohmmeters are designed to limit overshoot even when measuring
the DUT of large capacitance. They are well suited to use in the insulation resistance testing of batteries.
(3) Cycle time
In order to efficiently test large amount of battery cells, it is important to minimize cycle time. The following
insulation resistance meter specifications help determine cycle time:
a. Current capacity (current limitations)
To measure the insulation resistance of the capacitive DUT like battery cells, the DUT must be charged
previous to the measurement. Current capacity refers to the limit of current that can be applied to the DUT.
Product specifications for instruments use terms such as charging current, current limitation, current limiter,
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Electrical Measurement of Lithium-Ion Batteries: Fundamentals and Applications
measurement current, or rated curre nt to e xpres s this qua ntity. A low current capacity means that more time will
be required to charge the DUT before the measurement. It is therefore important to use an instru ment that offers a
sufficiently large current capacity relative to the required cycle time.
In insulation resistance measurement, a constant voltage is applied to the DUT. When this voltage is applied,
charges accumulate between the electrodes, and between the electrode and the enclosure’s insulation layer
time [s] needed to charge the DUT can be calculated from the capacitance [F] of the DUT, the current [A]
flowing to the circuit, and the applied voltage [V] as follows:
∶ charge time, ∶ capacitance of the DUT, ∶ test voltage, ∶ charge cur rent
6
. The
The capacitance C o f the DUT and the te st voltage V are determined by the process in question. Consequently,
the time needed before the me asureme nt is in versel y propor tional to the current that can be ap plied to the DUT,
or a current capacity. In other words, in order to shorten the cycle time, it is necessary to use an insulation
resistance meter with a high current capacity.
The current capacities of HIOKI insulation resistance meters are as follows:
ST5520: Max. 1.8 mA
SM7110, SM7120: Max. 50 mA (the maximum value is settable as “current limit”)
b. Discharge function (charge ab sorption function)
The discharge func tio n ser ves to d ischa rge the accumulated charge in the i nsul at ion la yer of t he D UT after the
measurement. Inadequate discharge could lead to electric shock or cause damage to the DUT due to residual
charge. Consequently, after the completion of testing it is necessary to absorb the charge with a discharge
function. Most insulation resistance meters provide a discharge function, of which there are two variants:
resistance discharge method and co nstant-current di scharge method. When testing a capacitive DUT suc h as a
battery cell, the constant-current method can complete discharging significantly faster.
[Time required for resistance discharge method]
6
The phenomenon in which the insulator is charged is known as dielectric absorption.
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