Forklift battery tray manufacturing: stamping, welding, coating

A 48 V traction battery for a 2.5 t counterbalance truck weighs about 1,250 kg and sits in a steel box that is 1,050 mm long, 700 mm wide and 250 mm deep. The tray holds that mass through every lift, brake and dock bump for eight years, and it holds it while being splashed with 37% sulphuric acid at a specific gravity of 1.28. Forklift battery tray manufacturing is a structural job and a corrosion job at the same time, and the two pull on the same 3 mm plate.

Most tray failures do not start at the weld. They start in the middle of a floor panel that was sized as a shelf rather than as a plate, or on the inside face of a wall where acid sat in a hollow because the drain hole was drilled at the wrong corner. Both are drawing decisions, and both are cheap to fix before the fixture is built.

This walkthrough follows a welded tray from plate to dispatch: the load case, the pressing and forming operations, the weld fixture and sequence, the acid-resistant finish, and the inspection points that catch the failures which otherwise appear on a customer's truck.

What load does a forklift battery tray actually carry?

A static load of 16.7 kPa, and a design load of about 25 kPa once the truck's duty cycle is applied. The tray is a plate in bending, not a container.

Take the numbers in order. A 24-cell, 625 Ah battery of the type used on a 2.5 t truck has a footprint that fits a 1,050 × 700 mm tray pan, giving 0.735 m² of support area. Mass 1,250 kg gives a weight of 12.3 kN, so the average bearing pressure is 12.3 ÷ 0.735 = 16.7 kPa, or 0.0167 N/mm².

That average is misleading because the load is not uniform: the cells bear on the floor over most of the area, but a hard dock impact puts a dynamic factor on the whole assembly. A factor of 1.5 is the working assumption for a counterbalance truck in a warehouse duty cycle, so the design pressure is 25 kPa.

Now treat a 1 m wide strip of floor plate spanning between cross members. The section modulus per metre of a 3.0 mm plate is 1,000 × 3² ÷ 6 = 1,500 mm³/m, and the moment for a simply supported strip is wL² ÷ 8.

At 200 mm pitch: M = 25 × 0.2² ÷ 8 = 0.125 kN·m per metre, so σ = 125,000 ÷ 1,500 = 83 MPa, against 235 MPa yield for CRCA — a factor of 2.8. Deflection is 5wL⁴ ÷ 384EI = 5 × 25 × 0.2⁴ ÷ (384 × 210 × 2,250) with E in GPa and I in mm⁴/m, which works out at 1.1 mm.

At 300 mm pitch the same strip sees 187 MPa and 5.6 mm of sag. The stress has used 80% of yield and the sag is visible, so the plate now holds acid in shallow pools between the ribs, the weld toes at the cross members see the load reversal, and the tray starts to crack at the second season.

That is the decision rule for the drawing: 200 mm pitch on 3 mm plate, or 250 mm pitch on 4 mm plate, and nothing wider.

Dimensions, plate and finish at a glance

The values below describe a welded tray for a 24-cell battery, with a lined interior and a coated exterior.

Pressing and forming the pan

Below about 900 mm long, the pan is a single pressing: floor, four walls, corner radii and stiffening ribs in one draw on a mechanical press between 200 T and 300 T, with the blank pierced for drains and hold-down holes in the same tool. Above that length the tool size and the handling weight push the design to a weldment, which is what most 1,050 mm trays are.

The ribs are what make a 3 mm floor work. A formed rib 8 mm deep at 200 mm pitch raises the effective section by roughly a factor of four in the ribbed direction, which is why the drawing should call the ribs as a pitch and a depth rather than leaving them to the fabricator.

Two forming details cause most of the rework on pressed trays:

• Corner radius. A 3 mm plate needs an inside radius of at least 3 mm and 4–5 mm in practice. A tighter corner thins the outside fibre by about 20% at 3 mm thickness, and the thinned corner is exactly where acid sits.
• Springback on the walls. A 250 mm wall formed in 3 mm CRCA springs back 1.5–2.5°, which opens the top of the tray by 13–22 mm across the width. Either form with a coining bottom or plan a welding and gauging step to pull the walls square.

Holes go in after forming, in the weld fixture, because the forming operation moves hole positions by the bend allowance. A hold-down hole punched on the flat and then formed lands 1.5–2 mm from where the drawing puts it, and the battery then sits off centre.

Welding a tray that has to stay square and hold liquid

The tray is welded on a fixture that locates the floor, sets the wall positions and holds the diagonals. Without the fixture, weld shrinkage does the locating: 1.5–2 mm per metre of fillet, applied to a 1,100 mm long wall, closes the frame by 3–4 mm and twists it out of plane.

The sequence that works is the boring one. Tack the cross members to the floor first, then the two end walls, then the long walls, alternating sides. Weld the interior seams in short runs, back-step, and let each seam cool before the next. The exterior seams are welded after the assembly has been checked square on the fixture.

Robotic MIG on the three CLOOS cells suits the long straight floor-to-wall seams and the cross-member fillets, which repeat identically from tray to tray. The corners, the drain boss and the hold-down brackets go to a manual station, where the operator can see the fit. Fixtures for both are designed and built in our own tool room, so a change to the hold-down pattern is a fixture change rather than a supplier negotiation.

Three weld details decide whether the tray leaks acid onto the truck:

1. Continuous seam at the wall base, not stitch welds. Stitch welds leave a gap that holds electrolyte by capillary action.
2. No weld spatter inside the tray. A spatter ball under the lining is a puncture waiting for the first battery change.
3. Dress the interior welds flush where the lining runs over them, or the lining bridges the weld toe and the acid works under the edge.

After welding, the tray is checked for square on the fixture and leak tested by filling with water for 30 minutes where the specification calls for a liquid-tight tray. That is a cheap test that catches the one tray in fifty with a pinhole at a corner.

Acid, hydrogen and the finish that survives both

The interior and exterior of a battery tray see different environments and need different answers.

Inside, the environment is dilute sulphuric acid mist plus cell tops that get wet during watering. Zinc coatings are the wrong answer: zinc is amphoteric and dissolves in acid, so a galvanised tray loses its protection exactly where it is needed and then corrodes as bare steel. A two-pack epoxy at 120–160 µm is the practical minimum for a coated steel tray; a 1.5–2.0 mm PVC or polyethylene lining is the answer for trays expected to see spill or frequent washing.

Outside, the environment is the truck: washdown, hydraulic oil, brake dust and weather. Polyester powder at 60–80 µm over the seven-tank pretreatment is adequate, and the blast chamber earns its place where the weldment carries mill scale or weld discolouration.

Testing follows the environment, not the habit. Neutral salt spray to ISO 9227 tells you about the exterior and almost nothing about acid resistance, so the specification should include both:

• ASTM D1308 spot test with 30% sulphuric acid, 24 h, assessed for blistering, softening and loss of adhesion.
• ISO 2409 cross-cut for adhesion, class 0–1.
• ISO 9227 neutral salt spray, 500 h, with scribe creep limits agreed at enquiry.

Hydrogen is the other specification that belongs on the tray drawing indirectly. EN 50272-3 sizes compartment ventilation from the number of cells, the capacity and the charge current; for a 24-cell, 625 Ah vented battery on boost charge the requirement works out at roughly 0.25 m³/h per Ah, or about 150 m³/h. A tray with a close-fitting lid, a sealed gasket and no vent path defeats that calculation, which is why most forklift trays are open-topped with a separate vented cover.

Inside a forklift battery tray manufacturing line

Floor plates and wall blanks are either laser cut on the 12 kW fibre machine, which handles a 1,050 mm blank with the ribs marked in one nest, or blanked on the mechanical presses from 20 T to 300 T where the annual volume justifies a tool. Forming is done on the 200 T, 5-axis brake with the 4,450 mm bed, which turns a 250 mm wall on 3 mm CRCA in a single hit and keeps the wall height consistent along the tray — a job that shows up as a rocking battery if the two long walls differ by 2 mm.

Welding runs on the three CLOOS robotic MIG cells for the long seams and at manual stations for corners and brackets, on fixtures from the in-house tool room. Where the tray needs blasting, it goes through the 16 ft × 16 ft chamber; coating runs on the seven-tank pretreatment line and in the 18 ft × 14 ft powder booth with DFT and cross-cut checks per batch. The plant is certified to ISO 9001:2015 and ISO 14001:2015, and trays and material handling parts are built for tractor and forklift programmes including Mahindra's Farm Division.

Inspection points before the tray is released

1. Envelope — length, width and depth to ±1.5 mm, measured on the welded, coated tray.
2. Diagonals — within 2 mm, measured across the top opening on the fixture.
3. Cell well and hold-down positions — ±1.0 mm, checked with the same drill jig used to make them.
4. Leak test — water fill for 30 minutes where liquid-tightness is specified.
5. Lining or film — DFT at five points on the exterior, holiday test on a full epoxy lining at 1.5 kV, visual check for runs and thin edges.
6. Adhesion and acid spot test — cross-cut per batch, acid spot test per coating batch change.
7. Drain and plug — present, clear, at the specified corner, with the plug thread free of coating.

First article inspection covers all seven, and PPAP documentation is available where the truck programme requires it. For a safety-relevant part like a battery tray, the first article record should also capture the weld parameters and the fixture number, because a tray that is right in the sample and wrong in series production is almost always a fixture that moved.

What to put on the tray drawing

• Battery make, model, cell count, capacity and mass, plus the cell footprint to EN 60254-2.
• Tray internal dimensions with the lining thickness deducted, and the clearance to the compartment walls.
• Plate thickness for floor and walls, rib pitch and rib depth.
• Cross-member size and pitch, with the load case if the truck OEM has one.
• Weld symbols and which seams must be continuous and liquid-tight.
• Drain position, diameter and plug specification.
• Lining material and thickness, or the coating system with its acid and salt-spray test requirements.
• Hold-down bracket positions and the clamp force they must resist.
• Tolerances written per feature: envelope ±1.5 mm, cell wells ±1.0 mm, diagonals 2 mm.
• Mass limit, because a tray that is 8 kg over the drawing changes the truck's centre of gravity.

When a steel tray is the wrong route

A stainless tray in 304 is the right answer where the truck works in food, pharmaceutical or cold-store environments and washdown is aggressive. We cut and form stainless, but the cost per kilogram is several times that of coated CRCA, and the lining question disappears, which sometimes justifies it.

A moulded polyethylene tray is the right answer for small warehouse trucks and for batteries below about 300 kg, where the structural demand is low and the acid resistance is bought with the material rather than with a coating. We do not mould polymer.

Aluminium is usually the wrong answer: it corrodes quickly in acid, it has no fatigue advantage at these stress levels, and the galvanic couple to a lead terminal is worse than steel's.

Within those limits, a 3 mm welded tray with the right rib pitch, a lined interior and a specified coating will outlast the battery it carries. The forklift sheet metal parts range covers trays, counterweight covers, overhead guards and chassis brackets; the material handling hub collects the process guides, and welding for heavy-gauge material handling parts covers the 3–6 mm weld details behind this route. Run the flat pattern through the weight calculator before the drawing is released — a tray mass that nobody checked is the most common cause of a late change.