EV Battery Mass Reduction Through Materials and Design

1. Thin Foil Pouching, Hybrid Leads, and Low-Profile Busbars

Mass reduction begins at cell level. A standard 60 Ah pouch wrapped in a multilayer polymer-aluminium laminate dedicates almost 7 % of its weight to sealing and current-lead material. The stack-bonded metal-foil cell architecture attacks this overhead by making the containment skin double as the current collector. Two ultra-thin metal foils are laser-welded edge-to-edge with a sub-millimetre seam, replacing the 3-5 mm heat-sealed flange. Tear-down of a 20-cell stack shows a 12 mm height reduction and a 450 g drop in inactive mass while helium-leak testing reports rates below 1 × 10⁻⁹ mbar L s⁻¹. Because the foils also spread pressure, lighter mechanical restraints are possible further up the hierarchy.

Every copper tab that protrudes from a cell adds grams and galvanic risk. The friction-welded copper–aluminum hybrid lead keeps a short copper stub inside the high-current region, then joins it to a long aluminium ribbon through high-speed rotary friction welding. Metallography reveals a 10 µm diffusion interface free of brittle intermetallics, and salt-spray endurance exceeds 1000 h. Tab copper usage falls by 60 %, trimming roughly 1.4 kg from a 96s2p SUV pack and enabling direct aluminium-to-busbar welding without transition plates.

Envelope height can be tightened further at the interconnect. In the low-profile busbar with an integrated thermistor groove a 2 mm-deep channel milled along the neutral axis houses both the NTC sensor and its ribbon cable. The busbar now sits below the gas-vent plane, which removes a dedicated ABS pocket, shortens the copper trace, and cuts response time to thermal excursions by 25 %. Combined, foil pouching, bi-metal leads and slim busbars strip about 2.5 kg from a 75 kWh pack while setting up the architectural freedom required for the module-less concepts introduced in Section 3.

2. Fiber-Reinforced Thermoplastic and Polymer-Based Battery Casings

Pressed-steel trays with welded ribs can add 60-70 kg to a mid-size EV. Replacing them with continuous-fibre thermoplastic laminates yields rapid cycle times and steep mass savings. The fully thermoplastic composite battery enclosure uses a 1.5 mm carbon-fibre organosheet and over-moulded glass-fibre ribs. A 1200 mm × 1600 mm floor pan weighs 19.4 kg and still meets Euro-NCAP side-pole stiffness targets. Tool recesses shape inlet bosses, M6 captive nuts and honeycomb crush initiators in a single press stroke, eliminating forty steel brackets and a three-metre laser seam for a further 4 kg saving. Post-impact panels can be reheated and reshaped, and ISO 14044 lifecycle analysis records a 32 % GHG benefit compared with aluminium at equal stiffness.

At module scale, aluminium housings remain common because they double as heat spreaders. The thermally-conductive plastic module housing answers with a graphite-loaded PPS compound that reaches 60 W m⁻¹ K⁻¹ through-plane conductivity. A thirty-cell 2170 module weighs 1.3 kg less than its cast AlSi10Mg counterpart yet passes 3 kV dielectric withstand without extra Kapton liners. Specific energy climbs from 177 to 190 Wh kg⁻¹ and the melt-processable compound aligns with the floor-pan over-moulding process, enabling a one-material supply chain. The stiff yet resilient composite shell also tolerates the larger unsupported cell areas inherent in the module-free architectures discussed next.

3. Cell-to-Pack and Monolithic Pack Architectures Eliminating Module Enclosures

Intermediate housings consume roughly 40 % of conventional pack volume. The prismatic-style terminal exposure for pouch cells folds electrode tabs upward, then stitches a continuous stainless frame across an entire row with an 80 cm laser sweep. When this frame bolts to the upper cover it applies a uniform 35 kPa pressure field that halts pouch swelling. Eliminating twelve aluminium module boxes, four silicone pads and 104 fasteners sheds 6.7 kg and frees 11 L of void space.

The monolithic structural battery pack pushes further by potting the cell array inside a load-bearing epoxy frame that incorporates aluminium shear webs. Four-point chassis tests on a 1100 mm span show 25 kN load with only 1.2 mm deflection, enabling removal of a traditional under-floor cross-member and delivering a 9 kg weight cut. Planar copper foils laminated inside the shear webs replace discrete busbars, further cleaning the bill of materials. OEM data shows a 14 % pack-level gravimetric gain and an 8 % material-cost reduction once module housings vanish.

Cell-to-pack solutions require precise, lasting compression; Section 4 explores lightweight methods that deliver that preload without resorting to heavy aluminium bolt plates.

4. Compression, Vacuum, and Insert Techniques for Cell Stack Restraint

Pouch and prismatic stacks need 30-120 kPa compression to maintain electrode contact. The vacuum-assisted cell block bracing encloses the entire stack in a polycarbonate shell, pulls internal pressure down to 200 mbar, and generates an 80 kN clamp on a 600 mm × 700 mm surface. Modal analysis places the first torsional mode at 290 Hz, and four-poster testing shows no rise in cell-to-tab impedance after 300 h. The system saves 9.1 kg compared with a bolted steel frame.

Irregular floor pans create dead space when rectangular modules are used. The polymer block insert for irregular trays fills these voids with low-density, carbon-reinforced polyetherimide foam bonded by elastic urethane. Cells are compressed on an external jig and slid into the tray; inserts wedge between curved walls and the stack, keeping preload uniform while deleting metal end plates. Four litres are freed on the perimeter, room enough for a vapour-cooled electronics corridor.

Vacuum shells auto-compensate for cell swell because internal volume contracts as pressure remains low. Foam inserts rely on the stable compressive modulus of their carbon skeleton up to 120 °C. Combining vacuum bracing with cell-to-pack architecture raises pack-level energy density by a further 8 % over legacy end-plate designs.

5. Embedded Cooling, Heating, and Gas Venting Channels within Pack Housing

Thermal management can exceed 15 kg in a 75 kWh battery. The collapsed-expandable gas exhaust conduit hides a flattened silicone tube inside the main cooling plenum. Under normal airflow the tube lies dormant; if a cell vents the resulting 50-60 kPa pressure inflates it, sealing the coolant path and diverting gases to an exterior sill port. No dedicated vent pipe is needed, saving 1.2 kg and avoiding hot-gas impingement on cold module walls.

For routine heat rejection, the stacked thermally conductive cartridges wrap micro-groups of cells in graphite-loaded polymer sleeves. A sintered aluminium wick interfaces with the outer case via an indium pad. The path is 40 % shorter than a conventional cold plate and core temperatures run 3.5 °C lower at 4C discharge, yet total thermal content falls by 3.3 kg.

Liquid passages can be printed directly into the shell. The integrally molded cooling plate enclosure uses Multi-Jet Fusion to embed a lattice of 2.5 mm square conduits within a 4 mm PA12 skin, then electroplates the interior with 20 µm copper. The design survives 3.5 MPa burst tests, removes bolt-on plates and 19 screws, and cuts another 1.8 kg. Integrating coolant, venting and structural load paths in a single wall dovetails with the die-cast structures examined next.

6. Hollow, Die-Cast, and Sandwich Metal Pack Structures with Integrated Reinforcement

Side-impact loads normally pass into separate rocker beams. The one-piece die-cast case with integral cross members forms a 75 mm hollow rib across the full 1500 mm width, merging with the sill at two cast ears. It handles 45 kN lateral crush and drops 11 kg relative to a steel twin-sill beam. Post-impact finite element studies show 28 % of energy absorbed by the battery floor and 42 % by the cast rib, so bolt-on beams vanish along with 48 fixings.

Axial compression walls can also lighten. The metal–resin sandwich compression wall bonds two 0.8 mm 6000-series skins across a 5 mm foamed epoxy core. Bending stiffness reaches 70 % of a 6 mm solid aluminium plate at 40 % of the mass. Corner pockets fill with resin during moulding, fusing panels into a continuous load loop; long-term creep stays below 0.2 mm at 80 °C for 1000 h.

Adding diagonal CFRP tubes as in the triangular frame with diagonal supports closes load triangles between the side sill, die-cast cross member and floor. Tubes only 18 mm in diameter exceed 10 kN buckling load and also carry refrigerant micro-channels. First vertical bending mode rises by 34 %, improving ride quality. Casting, sandwich bonding and CFRP bracing all employ non-fusion joining, so galvanic risk is managed with a 10 µm plasma-electrolytic oxide while self-piercing rivets create the mechanical link. With core structures integrated, attention can shift to auxiliary hardware and fasteners.

7. Consolidated Component Mounting and Reduced-Fastener Pack Assemblies

The embedded high-current connector and switch module locates contactor, fuse and Hall sensor inside a recess in the die-cast bulkhead. High-voltage path length falls from 320 mm to 45 mm, eliminating a 0.6 kg copper busbar and two crimp joints. Sensor grounding becomes cleaner because the shell itself provides the reference.

Fastener count drops further with the dual-purpose cooling/support member. Tongue-and-groove edges on neighbouring cold plates align captured holes so a single M8 bolt locks three plates and the floor pan. Forty bolts and twenty brackets disappear, 0.9 kg of steel is saved, and torque cycles are halved. A nylon-66 sleeve insulates the bolt from the coolant gallery.

Low-power electronics benefit from skeletal mounts. The lightweight rod-based BMS bracket suspends the control board on two 10 mm carbon-pultruded rods without a solid deck. Air flows beneath the PCB, component temperatures drop by 8 °C during fast charge, and the bracket mass falls from 540 g to 110 g. Because brackets locate on bosses that already exist in the composite floor, no extra height penalty arises.

Switchgear integration, captive fasteners and skeletal brackets trim close to 4 kg from a large pack and simplify end-of-life disassembly, paving the way for system-level electrical optimisation.

8. Split Battery Packs and Converter-Free Electrical Architectures

Raising pack voltage cuts I²R loss but forces heavier insulation and pricier SiC devices. The mid-point grounded split pack divides a 600 V battery into two 300 V cartridges tied to chassis ground. Harness can shift from 35 mm² copper to 16 mm² aluminium, saving 1.8 kg and freeing tunnel space. Active balancing keeps state-of-charge divergence within 2 % over 1000 cycles.

Auxiliary loads no longer need a separate 12 V battery. The dual-subassembly pack with a staged interrupter powers low-voltage consumers from one half-pack via a resistor-limited pre-charge path. A lead-acid battery and its DC-DC converter, together weighing 15 kg, disappear. Quiescent current also falls because the DC-DC is idle.

Fuel-cell hybrids carry two DC-DC converters. The converter-free fuel-cell coupling removes the stack-side converter by aligning fuel-cell OCV with the low end of the battery discharge curve. The inverter briefly trims the bus voltage to match the stack before a switch closes. Silicon carbide switches, inductors and cooling plates totalling 5.2 kg vanish, while WLTP efficiency suffers only a 0.4 % penalty.

Integrating cell-level, structural, thermal, fastening and electrical levers allows a medium crossover to shed 75-90 kg compared with a 2020 baseline battery, boosting range by 7-10 % without changing cell chemistry.

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