Wendell Johnson Applications Engineer, Resistance and Solid- State Welding Phone: 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

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 1/10 th 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 1/10 th 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

De-Solder Tool – Cell Interface

Results of a De-Solder Trial Before HeatingAfter 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 Phone: