wendell johnson applications engineer, resistance and solid-state welding
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Reversible Battery Tab Attachments Development of Resistance Heating-Based Process. September 14, 2011. Wendell Johnson Applications Engineer, Resistance and Solid-State Welding Email: [email protected] Phone: 614.688.5172. Rising Importance of Battery Systems. - PowerPoint PPT PresentationTRANSCRIPT
Wendell JohnsonApplications Engineer, Resistance and Solid-State WeldingEmail: [email protected]: 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: [email protected]: 614.688.5172