Wendell JohnsonApplications Engineer, Resistance and Solid-State WeldingEmail: wjohnson@ewi.orgPhone: 614.688.5172

Reversible Battery Tab Attachments Development of Resistance Heating-Based Process

September 14, 2011

Rising Importance of Battery Systems

Energy sources transition from fossil fuels

According to US DOE 1.2 million plug-in EVs on road by 2015

Li-ion cells with voltage of about 3.7 V will require many cells connected into battery pack (system)

Li-ion cells use aluminum electrode tab and copper is typical of buss material

Aluminum-to-copper interconnects

─ Must be cost effective─ Repeatable made─ Best when repairable

Estimated U.S. Supply of Electric Vehicles from 2011 through 2015

Manufacturer and Model 2011 2012 2013 2014 15 Total

Fisker Karma PHEV 1,000 5,000 10,000 10,000 10,000 36,000Fisker Nina PHEV 5,000 40,000 75,000 75,000 195,000Ford Focus EV 10,000 20,000 20,000 20,000 70,000Ford TransitConnectEV 400 800 1,000 1,000 1,000 4,200GM Chevrolet Volt 15,000 120,000 120,000 120,000 120,000 505,000Navistar eStar EV 200 800 1,000 1,000 1,000 4,000Nissan LEAF EV 25,000 25,000 50,000 100,000 100,000 300,000Smith Electric Vehicles 1,000 1,000 1,000 1,000 1,000 5,000Tesla Motors Model Sev 5,000 10,000 20,000 20,000 55,000Tesla Motors Roadster 1,000 1,000Think City EV 2,000 5,000 10,000 20,000 20,000 57,000Cumulative Total 1,222,200

Battery System Reliability

A battery system ─ Serial string for higher voltages─ Parallel circuit groups for extending

charge storage capacity─ Protection circuits and control─ Interconnects between cells─ Auxiliary systems including cooling

Reliability of a serial string = 90%─ For 50 cells and 50 connections ─ Each with 1/10th of 1% failure

probability Modularity adds replacement

capability Single cell or connection point

replacement is ultimate circuit the in elements of number N

connection or cell singleofy ReliabilitR

systemofy Reliabilit R

notation seriesproduct is

RR

cellith

system

N

icellithsystem

1

Battery System Assembly

Collection of cells and connections

─ View shows 24 battery cells─ Two parallel groups─ Connected serial 12 units

Single or multiple cells attached to a single point

Combinations ultimately lead to dissimilar metal joints – aluminum to copper

Experiences Related to Aluminum Tab to Copper Buss Attachment Range of technologies have

been demonstrated ─ Percussion welding ─ Ultrasonic welding─ Laser welding ─ Deformation welding

Permanent connection─ Can only be applied once to a

joint Development of reversible

attachments─ Ultrasonic tinned aluminum

with lead-free solder─ Then reflow to copper

conductor

Value of Reversible Attachments

Large systems with serial connections can have a low system reliability

─ A 100 serial component system will have a combined reliability of 90% when each component has a failure probability of 1/10th of 1%

Low voltage cells reduce battery performance─ Replaceable cells can maintain battery peak performance ─ Reduce pollution impact by scrapping a single cell versus a

system of cells

Lower scrap reduces system cost and improves buyer acceptance

Multiple Tabs in a Joined Stack

Multiple cells attached to single connection point

Cell tabs can be joined by CHRSW

Four tabs placed between SS caps

Placed in spot welder the current heats SS

The process heats from outside in and cools is reverse

Differing from spot welding the reverse solidification can move contaminants to surface

Four Loose Tabs

SS Heaters on Stack

Consolidated Tab Stack

Peel Test Tab Separation

Process Conditions and Outcome

Aluminum tab stack ─ Four tabs of Alloy 1100─ 0.12-mm thick ─ 12-mm wide by 30-mm long

304 SS tab heaters─ 3-mm thick ─ 12-mm square

100-kVA AC spot welder─ 6-kA weld current─ 1.1-kN welding force─ Weld time 6 cycles or 100 ms─ A 3-cycle downslope after weld

Pre-Tinning Aluminum Tab

Setup Conditions─ SS wire brush 13 × 13 mm─ Heat tab to about 250°C ─ Monitor with contact probe─ Set iron temp to flow solder─ Use SAC 305─ 60-kHz sonic horn set at about

3 W

Process Steps─ Tip the iron with solder─ Contact iron to brushed area─ As tab wets apply sonic

agitation─ Move tip over surface

watching for active surface

Tinning Continued

Single Tabs─ Tab 0.13-mm thick × 13-mm

wide─ Tinned using smaller iron─ Copper buss clips tinned using

rosin core SAC 305

Multi Tabs─ Four joined tabs like above─ Tinned using larger iron and

hot plate─ Copper buss clips tinned using

mechanical brushing

Heating Tool Interface

Tool design─ Close back access 12 mm

above and below─ Pinching action─ High resistance on copper─ Low resistance on aluminum─ Cooling circuit near electrodes─ Consideration for de-soldering

Setup to Reflow Solder

Parameters─ Squeeze Force 378 N (85 lbf)─ On time – 33 cycles (0.5 s)─ Current – 6 kA─ Hold time – 5 s

Setup─ Anvilloy contact with Cu─ RWMA-2 contact with Al─ Electrode contacts are 10-mm

diameter by 3-mm thick

Reflow Trials

First four trials with 4-tab tab stacks─ Trials to produce assemblies for desolder testing─ More assemblies made with hot plate reflow

Trials after No. 4 were with single tab─ Additional assemblies for setup made using pinch tool

Trial

NumberImpulses Time On Time Off

Squeeze Force

Electrode Tinning Practice

Results

Number Cycle Cycle N Note No. Note No.1 2 20 2 378 1 3 Appears aluminum surface damaged 2 1 20 ---------- 245 1 3 appearance ok solder wetting only on edges3 1 20 ---------- 245 1 3 added solder tab between part flow and wetting4 1 20 ---------- 245 1 3 arc between electrodes and parts

Reflow trials with single aluminum tab and modified tinning practice5 1 30 ---------- 245 2 4 Pull test tears tab at copper edge see note 6.6 1 30 ---------- 245 2 4 Light pull tension OK then desolder shows wetting7 1 30 ---------- 245 2 4 Pull test tears tab at copper edge see note 6.8 1 30 ---------- 378 2 5 Pull test tears tab at copper edge see note 6.9 1 33 ---------- 378 2 5 Tensile test tab pulled at right angle failed at 11.7 lbs

10 1 33 ---------- 378 2 5 Tensile test tab pulled inline failed at 17.3 lbsAll trials used 6 Kamps setpoint resulting in about 5.5 Kamps1. Anvilloy both top and bottom2. Anvilloy contacting copper and RWMA-II contacting Aluminum tab3. Aluminum tabs tinned all together and Copper tinned with solid solder aided with mechanical brushing4. Aluminum tabs tinned one at a time using smaller ultrasonic tinning iron, Copper tinned using rosin flux core 5. Same as Number 4 procedure but adding drop of rosin flux core to aluminum tab after tinning6. After pull test, remaning material heated on hot plate and separated revealing well wetted interface

Solder Trials with New Pinch Tool

Temperature Trace for Reflow

Top trace probes were both in solder joint

─ Blue trace front of center─ Purple trace near tab edge─ Temp below 100°C in 1.5 s

Lower trace probe on Al tab 2 mm away from joint

─ Temperature remains below 50°C

─ Max reached at about 3 s

Temperature Trace ReflowAl to Cu Solder Joint

0

50

100

150

200

250

300

350

400

450

0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5

Time Seconds

Deg

rees

C

Temperature 2 mm away on AlTemperature at Joint

Temperature Traces Reflow Solder JointAl Tab to Cu Buss

0

50

100

150

200

250

300

350

400

450

500

0 1 2 3 4

Time Seconds

Te

mp

era

ture

De

gre

es

C Thermocouple in Joint Away From TabThermocouple in Joint Near Tab

Results

Pulling tab axially or at right angle to bond plane

─ Single tab – failure at edge─ Axial load 77 N (17.3 lb)─ 90-degree load 52 N (11.7 lb)

─ Multi tab – failure tear of tabs at CHRSW bond edge

Removal of a Single Cell or Connection

Cut away cell from tab with special shear or knife tool

Engage the junction with resistance heating tool

Clamp device to separate tab from buss

De-Solder Tool – Cell Interface

Results of a De-Solder Trial

Before Heating After Heating

Close-up After Heating

Removed From Tool Solder Bridge

De-Soldering Trials

Trial

NumberImpulses Time On Time Off Current

Squeeze Force

Electrode Note

Result

Number Cycle Cycle Kamps N Number1 2 10 3 5.5 289 1 Separated2 2 16 3 5.5 289 1 Separated3 2 17 3 5.5 289 1 Separated but arced4 2 16 3 5.5 289 1 Separated5 2 16 3 5.5 289 1 Separate with several solder strings6 2 17 3 5.5 289 1 Separate with several solder strings7 2 16 3 5.5 289 1 Separate but melt flow around electrode tip8 2 16 3 5.5 289 1 Same as 7

Use Copper tab to ensure full surface wetting9 2 16 3 5.5 289 2 Using copper tab could not separate

10 3 17 5 6 289 2 Same as 911 3 17 5 6 289 3 Separated

1. Anvilloy both top and bottom electrodes2. Anvilloy contact with copper RWMA-II with tab 3. Same as 2 but cooling to electrode off during cycle on before

Desolder Trials with Pinch Tool

De-soldering or de-attaching tabs with a mechanical clip─ Clip was attached to the tab with tool clamped─ Assembly heated without cooling of heads joint heated to flow point─ Pinch jaws opened immediately on heating cycle completion─ Separation of lower jaw pulls tab away from copper buss

Conclusions

Multiple aluminum tabs could be effectively joined into a stack using CHRW

Ultrasonic solder deposition could be used to pre-tin both aluminum and copper components

Reflow soldering facilitated the dissimilar Al-Cu attachment Resistance de-attachment was also demonstrated using a

wedging device Tool must be specifically designed to fit in a confined space A single new battery cell can be field replaceable and attached

to the buss by using solder joints Next steps are

─ Demonstrate with real battery system ─ Advance the design of tab removal mechanics─ Demonstrate function with other tab configurations

Questions?

Wendell JohnsonApplications Engineer, Resistance and Solid-State WeldingEmail: wjohnson@ewi.orgPhone: 614.688.5172