A battery tray is a flatness-critical, sealing-critical structural part. Cells stack on it, a lid seals to it, and the whole assembly has to survive vibration and thermal cycling. What makes it difficult is not the material removal — it is holding a large, thin-walled part flat and leak-tight after machining and welding.
The same part can be produced several ways, and the choice is decided by volume and geometry rather than preference. We quote whichever is actually cheaper for your quantity.
| Approach | How it works | Best at | Watch out for |
|---|---|---|---|
| Machined from plate | Thick plate machined down into a tray with integral walls | Prototypes and low volume; complex internal features | Material utilisation drops below 20%; large plates move when machined |
| Extruded body + machined ends | Extruded aluminum section forms the tray body, machined end plates and flanges | Prismatic trays at production volume | Die cost; sealing surfaces need machining after assembly |
| Welded fabrication + machined faces | Plate walls welded, stress relieved, then sealing faces machined | Large trays and complex shapes | Weld distortion; the sealing face must be machined after welding, never before |
| Die cast + machined | Cast tray with machining allowance on critical faces | High volume (10,000+) where tooling amortises | Porosity can cause leaks; sealing faces need machining and often impregnation |
These are the decisions that account for most of the cost difference between two parts of the same overall size.
| Feature | Recommended | Why it matters |
|---|---|---|
| Floor flatness (cell contact area) | 0.5mm/m typical; tighter if cells are glued | Cell stack height and thermal contact depend on it |
| Sealing flange flatness | 0.1mm over the flange; 0.05mm for gas-tight | Gasket or adhesive needs uniform contact |
| Wall thickness | 2–3mm; 1.5mm only with welded construction | Thin machined walls distort after stress relief |
| Floor thickness | 3mm minimum under cell load | Thinner floors drum and lose contact area |
| Datum strategy | Datum the cell-contact floor, not the outer flange | Cell position matters; the flange follows |
| Drain and vent features | Radiused corners, no sharp internal corners | Sharp corners trap electrolyte and crack under vibration |
| Fastener inserts | Specify insert type and pull-out requirement | Threaded inserts in thin aluminum need a defined boss |
A battery tray that is 0.3mm out of flat over its cell area causes uneven compression in the cell stack. That shows up as reduced thermal contact, uneven aging and, in the worst case, a cell that is mechanically stressed from day one. The flatness specification is not bureaucratic — it is the functional requirement.
The machining consequence is that you cannot simply clamp a large thin tray and cut it. The part has to be supported over its whole footprint, machined with stress-relieved stock, and often rough machined, stress relieved, then finished. On a large tray that two-stage process is normal, not a sign of a difficult shop.
| Cost element | Share of price | What to change if it is too high |
|---|---|---|
| Material utilisation | 15–30% machined from plate | Hollowing a big block is expensive — extrusion or welding wins at volume |
| Cycle time | 30–45% of part cost | Large floor areas need many passes; pocketing dominates |
| Stress relief / re-machining | 10–20% | Two-stage machining on large trays, plus heat treatment between |
| Welding and leak testing | 10–20% (welded construction) | Leak testing is a real cost and a real schedule item |
| Inspection | 5–10% | Flatness over a large area, plus leak test records |
The material and machining cost of a battery tray is secondary to the construction method. A tray machined from plate at 20% material utilisation cannot compete with an extruded or welded body at production volume — the difference is not a few percent, it is multiples.
Machining from plate makes sense for prototypes, for low volume, and for trays with internal features that cannot be formed any other way. Above a few thousand units the conversation should be about extrusion or casting, with machining reserved for the sealing faces and cell-contact floor.
A tray seals either with a gasket against a machined flange or with adhesive. Gasket sealing needs flatness and a defined compression; adhesive bonding needs surface preparation and cure control. Either way, leak testing is part of the process — specify the test method and acceptable leak rate at the RFQ stage.
Aluminum trays in contact with steel fasteners or copper busbars create galvanic couples. The usual responses are insulating coatings, plated inserts, or isolation washers. Decide the coating before the drawing is released, because it affects masking and thread allowances.
If the tray conducts heat away from the cells, the floor surface finish and flatness both matter — as does the thermal interface material. Specify the requirement in thermal terms (contact area, flatness, roughness) rather than leaving it to be inferred from a general tolerance.
For cell stacking with a thermal interface, 0.5mm per metre is a common specification, and tighter if cells are bonded directly to the floor. The sealing flange is usually controlled separately at 0.1mm or better. Confirm the number against the cell module design rather than adopting a value from another project.
Machined from plate for prototypes and low volume, extruded body with machined ends for prismatic trays at mid volume, and die cast for high volume where the tooling amortises. Machined trays win on flexibility and internal features; they cannot compete on unit cost at production volume because material utilisation is so low.
By starting with stress-relieved plate (T651), rough machining, stress relieving, then finish machining. Supporting the whole footprint during machining matters too — a large thin floor clamped at the edges will deflect. For large trays the two-stage process is standard and should be quoted as such, not treated as a problem.
Pressure or vacuum decay testing is normally done by the assembly partner rather than the machine shop, but the machined surfaces must be capable of sealing. If leak testing is part of your incoming inspection, specify the acceptable rate so the machining tolerances can be set to achieve it.
Include the quantity, whether the finish is visible, and which fits or seals are critical. You get a line-item quote within 24 hours and DFM notes on the features driving your price.