Cooling Plate Assembly

Overview

The cooling plate is the thermal backbone of a liquid-cooled HV Battery Pack. It is a flat, fluid-carrying metal panel that the Battery Module stack presses against, pulling heat out of the cells during fast charging and hard driving and, when paired with a heater, warming them in cold weather. Lithium-ion cells live longest and deliver their rated power only inside a narrow temperature window — roughly 15 to 35 °C for best life, with hard limits near 0 °C for charging and 50 to 60 °C at the top. The cooling plate is what keeps every Li-ion Cell, 21700 in that window and, just as importantly, keeps them all at the same temperature.

Construction / how it's built

A cooling plate is fundamentally a sealed network of internal channels carrying coolant beneath a flat contact surface. Several constructions are common. Roll-bonded plates are made from two aluminum sheets printed with a graphite pattern where channels should form; the sheets are roll-welded together everywhere except the pattern, then inflated with high-pressure air so the unbonded regions balloon into channels. Extruded plates use a multi-port aluminum extrusion (MPE) with parallel bores running its length, capped and manifolded at the ends. Stamped/brazed plates join a stamped channel sheet to a flat cover sheet in a vacuum-brazing furnace.

The coolant — almost always a 50/50 mix of water and ethylene glycol for freeze protection and corrosion inhibition — enters through one port, snakes through the channel network in a serpentine or parallel-manifold layout, and exits the other port into the vehicle's thermal loop. Channel geometry is tuned so coolant velocity stays high enough for turbulent, high heat-transfer flow without driving pressure drop past what the system pump can supply.

Between the plate and the cells sits the Thermal Interface Pad or a dispensed thermal interface material. Cells are never perfectly flat or co-planar, and the plate is rigid; the gap pad — typically 1 to 3 mm of silicone loaded with ceramic filler — fills the air gaps that would otherwise act as insulators, conducting heat across the interface while absorbing tolerance and vibration. Without a good interface material, the best cooling plate in the world is throttled by the air film at the contact.

Key specifications explained

Flow rate (8 to 15 L/min) and pressure drop (<30 kPa). These two trade against each other. More flow removes more heat and improves uniformity, but raises pressure drop and pump power. The plate is designed so the pump can deliver the target flow within its head limit.

Heat rejection (1 to 5 kW). During a DC fast charge a pack can dissipate kilowatts of waste heat from internal resistance. The plate, coolant, and downstream chiller must carry that away continuously or the charge rate has to be cut. Sustained 5 kW rejection separates fast-charge-capable packs from those that derate.

Temperature uniformity (<5 °C cell-to-cell). This is arguably more important than absolute temperature. If one Battery Module runs hotter than its neighbors, it ages faster, drifts in capacity, and drags down the whole series string, since pack capacity is limited by the weakest cell group. Good plate design keeps the spread small by managing where the hot inlet and cool outlet coolant contact the cells.

Material (aluminum) and thickness (4 to 12 mm). Aluminum is chosen for its high thermal conductivity, low density, and low cost. Thickness balances structural stiffness and weight against the volume the plate steals from the cells.

Manufacturing & assembly

For roll-bonded plates the process is sheet printing, roll bonding, inflation, then trimming and port brazing. Extruded plates are cut to length, the bores deburred, and end manifolds welded or brazed on. Every plate is leak-tested — usually with pressurized helium or air-under-water — because a coolant leak inside a high-voltage pack is both a thermal failure and a Pack BMS (Master) isolation fault waiting to happen. Ports get quick-connect or threaded fittings, and the contact face is cleaned and flatness-checked before the Thermal Interface Pad is applied during pack assembly. The plate is then bolted into the floor of the Pack Enclosure and the modules are clamped down onto it.

Role in the pack

The plate connects to the vehicle's thermal management loop, which typically shares a chiller and pump with the cabin and power-electronics cooling. The Pack BMS (Master) requests heating or cooling based on the temperatures reported by every Module BMS Slave Board, and the vehicle's thermal controller adjusts coolant valves and pump speed. During fast charging the loop runs at full cooling; in winter it routes warm coolant or runs a heater to bring cells above the safe charging threshold before allowing charge current. The plate also doubles as a structural floor element, adding stiffness to the Pack Enclosure.

Variants & alternatives

The main alternatives are air cooling, simpler and lighter but limited to low heat loads and poor uniformity; immersion cooling, where a dielectric fluid bathes the cells directly for excellent uniformity at the cost of complexity and fluid mass; and refrigerant-direct cooling, which evaporates refrigerant inside the plate for very high heat flux without an intermediate coolant loop. Among liquid plates, the choice between roll-bonded, extruded, and brazed comes down to cost, channel-density needs, and pressure rating. Cylindrical-cell packs sometimes use the plate only at the cell bases, while prismatic packs may sandwich plates between rows of cells. The plate also pairs with the Nickel Busbar and module layout, since the cooled face must align with the cells' best heat-rejection surface.

A further design axis is where the plate contacts the cells. Cooling the base of a cylindrical Li-ion Cell, 21700 is mechanically simple but slow, because heat must travel the length of the cell to reach the cooled end, and the cell's jelly-roll conducts heat far better radially than axially. Side or tab cooling reaches the hottest part of the cell faster but complicates the Module Housing and the Thermal Interface Pad geometry. Designers also decide whether one large plate spans the whole pack floor or several smaller plates serve groups of modules; a single plate gives the best uniformity and the simplest plumbing, while split plates ease assembly and let a damaged section be replaced. Whatever the layout, the plate must coordinate with the heater used in cold climates, since the same surface that pulls heat out in summer must inject heat in winter to bring cells above the freezing-point charging limit. Increasingly the plate, the Pack Enclosure floor, and the structural crash members are merged into one bonded aluminum assembly, so the thermal designer and the crash engineer share the same part and must agree on its geometry, a constraint that did not exist when cooling was an afterthought bolted under the cells.