1

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

2

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

3

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 ?

4

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

5

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.

6

SEPARATOR MORPHOLOGYSEPARATOR MORPHOLOGYSEPARATOR MORPHOLOGYSEPARATOR MORPHOLOGY

AGM

PE/SiO2

RUBBER / SiO2 CEDAR

PE/SiO2 PE/SiO2

7

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

8

UHMWPE fibrils

Silica Aggregates

9

POROSITYPOROSITYPOROSITYPOROSITY

All separators are

0.15 mm backweb

STD 2.6

Z

YLR

XLR

10

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

11

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)

12

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

13

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

14

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

15

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

16

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.

17

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

18

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

19

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++=

20

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

21

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

22

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

23

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.

24

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 %

25

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