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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: wjohnson@ewi.org Phone: 614.688.5172. Rising Importance of Battery Systems.
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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: wjohnson@ewi.org Phone: 614.688.5172
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
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
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
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
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
Results of a De-Solder Trial Before Heating After Heating Close-up After Heating Removed From Tool Solder Bridge
De-Soldering Trials • 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 Johnson Applications Engineer, Resistance and Solid-State Welding Email: wjohnson@ewi.org Phone: 614.688.5172