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SEPARATOR ELECTRICAL RESISTANCE: SEPARATOR ELECTRICAL RESISTANCE: SEPARATOR ELECTRICAL RESISTANCE: SEPARATOR ELECTRICAL RESISTANCE: HOW LOW CAN YOU GO ?HOW LOW CAN YOU GO ?HOW LOW CAN YOU GO ?HOW LOW CAN YOU GO ?
R. Waterhouse, C. La, E. Hostetler, C. Rogers, J. Kim, and R.W. Pekala
ENTEK International LLC
USA
S. Gerts, M. Ulrich, A. Brown, D. Walker, and D. Merritt
ENTEK International LTD
UK
September 15, 2016
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What is resistance ? Electronic vs. Ionic
How can we influence it ? Separator design and modeling
How do we measure it ? Equipment Test procedures
How low can we go ?
What is the impact on battery performance ?
OUTLINEOUTLINEOUTLINEOUTLINE
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Resistance is the property of a material that impedes the flow of current in the presence of a voltage gradient: I = V/R
Current (I): movement of charge
• Electrons: e-
• Ions: Na+ and Cl- (seawater), H+ and SO42- (lead-acid battery)
Voltage gradient (V): results from a difference in electrical potential (voltage) between two points separated in space.
• Residential: 230V, 50 Hz (gap varies with device), electrons
• Spark plug: 50,000V across 1mm gap, corona discharge
• Lead-acid battery: 2V across 0.5-2.0mm, ions (mostly H+)
WHAT IS RESISTANCE ?WHAT IS RESISTANCE ?WHAT IS RESISTANCE ?WHAT IS RESISTANCE ?
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Separator resistance is a function of the resistivity of the electrolyte (acid) plus the design, pore structure, and composition of the separator.
Resistance of electrolyte within a porous structure (Ω):
R = ρLτ2 / P A
Where ρ = resistivity of the electrolyte, f (wt%, temperature)
L = thickness of the separator (design)
τ = tortuosity of the pore path (structure)P = porosity filled with acid (structure and composition)A = area of the separator through which ions flow
SEPARATOR RESISTANCESEPARATOR RESISTANCESEPARATOR RESISTANCESEPARATOR RESISTANCE
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ACID RESISTIVITY VS. CONCENTRATION AND TEMPERATUREACID RESISTIVITY VS. CONCENTRATION AND TEMPERATUREACID RESISTIVITY VS. CONCENTRATION AND TEMPERATUREACID RESISTIVITY VS. CONCENTRATION AND TEMPERATURE
Operating range for
lead-acid battery
Acid resistivity is a strong function of
both concentration and temperature.
Both must be carefully controlled to get
accurate results.
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SEPARATOR MORPHOLOGYSEPARATOR MORPHOLOGYSEPARATOR MORPHOLOGYSEPARATOR MORPHOLOGY
AGM
PE/SiO2
RUBBER / SiO2 CEDAR
PE/SiO2 PE/SiO2
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The morphology of a PE/SiO2 separator is heterogeneous:
Hydrophilic silica aggregates of different sizes
Hydrophobic polyethylene fibrils
Differences in structure between surface (polymer-rich) and bulk (silica-rich)
Porosity and pore interconnectivity depend upon formulation and process conditions
Broad range of pore sizes and shapes
Not all pore volume is easily filled with electrolyte
• Total pore volume ≠ acid accessible pore volume
PE/SIO2 SEPARATORSPE/SIO2 SEPARATORSPE/SIO2 SEPARATORSPE/SIO2 SEPARATORS
Constricted
Pore
Blind
Pore
Variety of pores
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UHMWPE fibrils
Silica Aggregates
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POROSITYPOROSITYPOROSITYPOROSITY
All separators are
0.15 mm backweb
STD 2.6
Z
YLR
XLR
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TORTUOSITYTORTUOSITYTORTUOSITYTORTUOSITY
0.25 mm
backweb
Tortuosity = 1 if ion path length is equivalent to backweb thickness
Tortuosity > 1 when ion path length is greater than backweb thickness
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Diffusion through a membrane separating two compartment:
Slope of the left-hand-side vs. time can be used to calculate the diffusional resistance• C0: initial concentration in the feed compartment
• Cd(t): concentration of KCl in the diffusate compartment at time t
• A: Separator area exposed to the solutions
• V: volume of solution in one compartment
• Rd: Diffusional resistance of separator
Diffusional resistance is related to tortuosity:
t: separator thickness τ : tortuosity D: Diffusivity ε : porosity
HOW CAN WE MEASURE TORTUOSITY ?HOW CAN WE MEASURE TORTUOSITY ?HOW CAN WE MEASURE TORTUOSITY ?HOW CAN WE MEASURE TORTUOSITY ?
tVR
2A
C
(t)2CCln
d0
d0 ×−=
−
εD
τtR
2
d×
×=
ln[(C0-2Cd(t))/C0] vs. Time
-0.06
-0.05
-0.04
-0.03
-0.02
-0.01
0.00
0 500 1000 1500 2000 2500 3000 3500 4000
Time (sec.)
ln[(C
0-2
Cd(t))/C
0]
Slope = -(2*A)/(V*Rd)
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Volume of liquid in the feed and diffusate compartments is the same.
Separator samples were boiled in DI water for 10 minutes, and equilibrate at room temperature (~ 22°C)
Three 3” diameter disks were cut from each separator
Conductivity of the solution in the diffusate compartment was measured with time for 1 hour.
Diffusivity was assumed to be 1.90x10-5 cm²/s
Stokes-Einstein radius of K+ and Cl- ~ 1.3A° and 1.2A°
EXPERIMENTALEXPERIMENTALEXPERIMENTALEXPERIMENTAL
Conductivity
Probe
Separator
KCl Feed Compartment:
C0 = 1.0MDiffusate
Compartment
Reference: C. Labbez, et al., Desalination, 141 (2001) p. 291
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TORTUOSITY/DIFFUSIONAL RESISTANCE VS. FORMULATIONTORTUOSITY/DIFFUSIONAL RESISTANCE VS. FORMULATIONTORTUOSITY/DIFFUSIONAL RESISTANCE VS. FORMULATIONTORTUOSITY/DIFFUSIONAL RESISTANCE VS. FORMULATION
0
5000
10000
15000
20000
25000
30000
0.0
0.5
1.0
1.5
2.0
STD 2.3 STD 2.6 LR
Nor
mal
ized
Difu
sion
al R
esis
tanc
e (c
m-1
xsec
/mm
)
Tort
uosi
ty
Tortuosity/Diffusional Resistance vs. Formulation
Tortuosity Normalized Diffusional Resistance
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PORE SIZE DISTRIBUTIONPORE SIZE DISTRIBUTIONPORE SIZE DISTRIBUTIONPORE SIZE DISTRIBUTION
The shift toward larger pore size seen on the LR separator brings about the decrease in tortuosity and increase in porosity.
0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
0.0010.010.1110
Log
Diff
eren
tial I
ntru
sion
(mL/
g)
Pore Diameter (µm)
Pore Size Distribution vs. Formulation
STD 2.3 STD 2.6 LR
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TORTUOSITY/ELECTRICAL RESISTIVITY VS. FORMULATIONTORTUOSITY/ELECTRICAL RESISTIVITY VS. FORMULATIONTORTUOSITY/ELECTRICAL RESISTIVITY VS. FORMULATIONTORTUOSITY/ELECTRICAL RESISTIVITY VS. FORMULATION
Lower tortuosity and higher porosity contribute to the lower electrical resistivity of the LR separator.
0
1000
2000
3000
4000
0.0
0.5
1.0
1.5
2.0
STD 2.3 STD 2.6 LR
Ele
ctri
cal R
esis
tivity
(m
Ω-c
m)
Tort
uosi
ty
Tortuosity/ER vs. Formulation
Tortuosity Electrical Resistivity
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PALICO OPERATION: BATH SCHEMATICPALICO OPERATION: BATH SCHEMATICPALICO OPERATION: BATH SCHEMATICPALICO OPERATION: BATH SCHEMATIC
V
I
voltage sensing electrodes (4)
current delivery electrodes (2 each)
( - ) (+)
separator sample
sample holding plates with 32.3 cm² aperture
The Palico system measures separator resistance by sensing the voltage drop between two pairs of
sensing electrodes in response to current pulses delivered by electrodes at opposite ends of the
bath. The difference in voltage drop, with and without a separator in the ionic current path, is
used to calculate resistance.
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Current density and temperature Palico separator ER test: (100ma)/(32.3cm²) = 3.1 ma/cm² 27 °C
Hioki Battery HiTester: (150ma)/(2200cm²) = 0.068 ma/cm² 20-25 °C
Cold crank test: (680000ma)/(2200cm²) = 309 ma/cm² -18 °C
Low leakage current is required to accurately measure separator resistance Barrier resistance > 9 ohm
Multiple piece separator stacks give artificially low values
Soak ER vs BCI test method Time and temperature
LIMITATIONS OF PALICO TEST MEASUREMENTSLIMITATIONS OF PALICO TEST MEASUREMENTSLIMITATIONS OF PALICO TEST MEASUREMENTSLIMITATIONS OF PALICO TEST MEASUREMENTS
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ELECTRICAL RESISTANCE ELECTRICAL RESISTANCE ELECTRICAL RESISTANCE ELECTRICAL RESISTANCE ------------ PALICO MEASUREMENTPALICO MEASUREMENTPALICO MEASUREMENTPALICO MEASUREMENT
Electrical resistance of a PE/SiO2 separator is traditionally measured using the Palico Low Resistance Measuring System:
• RSES: the areal resistance of the Separator-Electrolyte System (SES)
• RPalico: The resistance value in milliohms measured by the Palico instrument times the area of the aperture (32.3 cm²)
• RE: The areal resistance of the electrolyte that would occupy the same volume as the SES
V’1 V’2
I
V1 V2
I
R’ = (V’1-V’2)/I = R1 + RSES + R2
With Separator
R = (V1-V2)/I = R 1 + RE +R2
Without Separator
RPalico = (R’ – R) = (RSES – RE)
R1 RSES R2 R1 RE R2
RSES = RPalico + RElectrolyte
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SEPARATOR ER MODELSEPARATOR ER MODELSEPARATOR ER MODELSEPARATOR ER MODEL
V2
Rmajor
Relyte,bw
Rbw
Rminor
Relyte,minor
V1
R SES
With knowledge of acid and separator resistivities, one can calculate overall resistance for the
separator-electrolyte system (SES) with different separator profiles by using a parallel-series circuit
model.
minorR'
1
bwR'
1
majorR'
1
TR'
1++=
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ER CONSIDERATIONSER CONSIDERATIONSER CONSIDERATIONSER CONSIDERATIONS
162 x 0.80 x 0.25 GE STD 2.6 LR Comments
Palico ER (mΩ cm2) 92 55 - 40.2%
Electrolyte (mΩ cm2) 100 100
Sep-Elect-System (mΩ cm2) 192 155 - 19.3%
162 x 0.80 x 0.15 GE STD 2.6 LR Comments
Palico ER (mΩ cm2) 62 38 -38.7%
Electrolyte (mΩ cm2) 100 100
Sep-Elect-System (mΩ cm2) 162 138 - 14.8%
* Electrolyte resistance = 1250 mohm-cm x 0.08 cm = 100 mΩ cm2
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R[SES] VS BW THICKNESSR[SES] VS BW THICKNESSR[SES] VS BW THICKNESSR[SES] VS BW THICKNESS
STD
XLR
LR
AGM (0.8 mm)
Advanced
Development
approach
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BATTERY IMPLICATIONS FOR ER MODELBATTERY IMPLICATIONS FOR ER MODELBATTERY IMPLICATIONS FOR ER MODELBATTERY IMPLICATIONS FOR ER MODEL
Assume each cell has 6 positive and 6 negative plates
5” x 6 “ electrodes (194 cm2)
161 x 1.0 x 0.15 mm separator; STD 2.3; 210 mohm cm² at -18°C
Cell resistance from SES = 210 mohm cm² / (11 x 194 cm2) = 0.1 mohm
Battery resistance attributable to separator system = 6 x 0.1 mohm = 0.6 mohm
CCA: 600 amps x 0.0006 ohm = 0.36 Volts
If STD 2.3 is changed to LR separator, voltage drop = 0.23 volts
If separator is removed and acid only between plates, voltage drop = 0.21 volts
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INFLUENCE OF TEMPERATURE ON BATTERY RESISTANCEINFLUENCE OF TEMPERATURE ON BATTERY RESISTANCEINFLUENCE OF TEMPERATURE ON BATTERY RESISTANCEINFLUENCE OF TEMPERATURE ON BATTERY RESISTANCE
Electrolyte Resistivity vs. Temperature (38 wt% H2SO4)
0.0
1.0
2.0
3.0
4.0
5.0
6.0
-40 -30 -20 -10 0 10 20 30 40 50 60
Temperature (°C)
Ele
ctr
oly
te R
esis
tiv
ity
(Ω
-cm
)
Battery Resistance: Electronic vs. Ionic
R = 0.4776x + 2.2915
0
1
2
3
4
5
6
0 1 2 3 4 5 6
Electrolyte Resistivity (Ω-cm)
Ba
tte
ry R
esis
tan
ce
(m
illi
oh
m)
Electronic (metal)Ionic (electrolyte, separators)
21°C0°C
-18°C
-29°C
Reference: James Klang, BCI 121st Convention, Las Vegas, May 3-6 2009
Electrolyte resistance varies strongly as a function of temperature while the
resistance of the metallic components is nearly constant.
A linear regression of battery resistance versus electrolyte resistivity gives the
electronic resistance as the y-intercept as resistivity goes to zero.
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RELATIVE CONTRIBUTIONS TO BATTERY RESISTANCERELATIVE CONTRIBUTIONS TO BATTERY RESISTANCERELATIVE CONTRIBUTIONS TO BATTERY RESISTANCERELATIVE CONTRIBUTIONS TO BATTERY RESISTANCE
Ohmic Contributions of an Automotive Battery: 70°F
0 10 20 30 40 50 60 70 80 90 100
Total Electronic
Top Lead
Terminals
Intercells
Plates
Positive
Negative
Total ionic
Separators
Electrolyte
Trpped gas
% o
f Tota
l Resis
tance
Ohmic Contributions of an Automotive Battery: 0°F
0 10 20 30 40 50 60 70 80 90 100
Total Electronic
Top Lead
Terminals
Intercells
Plates
Positive
Negative
Total ionic
Separators
Electrolyte
Trpped gas
% o
f Tota
l Resis
tance
Reference: James Klang, BCI 121st Convention, Las Vegas, May 3-6 2009
Electronic resistance is much greater than ionic at room temperature.
Ionic resistance becomes significant at low temperature.
6 % 10 %
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ER, the resistance to ion flow through the separator, is one of the most important characteristics of the separator, especially for high rate uses such as SLI and start-stop vehicle applications.
Separator ER is a function of design, composition, and pore structure.
ER is a difficult measurement to make and requires attention to detail.
ENTEK has developed a simple, but effective, model for ER that is predictive for different geometries and can be included in battery level models used by our customers.
The separator contributes approximately 5% of the battery resistance at room temperature and 10-15% at low (-18°C) temperature.
SUMMARYSUMMARYSUMMARYSUMMARY
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