Great question! Odoo does not natively support applying the same tax multiple times (e.g., 4× battery eco-tax) directly. Here are two practical workarounds:

Option 1: Create product-specific tax variants (recommended)

Create separate tax records for each scenario:

• "Battery Eco-Tax x1" = fixed amount (e.g., €0.10)
• "Battery Eco-Tax x2" = fixed amount (e.g., €0.20)
• "Battery Eco-Tax x4" = fixed amount (e.g., €0.40)

Go to Accounting → Configuration → Taxes → New, set the Tax Type to "Fixed" and enter the correct amount. Then assign the appropriate tax to each product.

Option 2: Use a tax with a fixed amount per unit

If the eco-tax is per battery unit, set it as a fixed tax (e.g., €0.10 per unit). Then in the product's Bill of Materials or sale order line, adjust the quantity to reflect the number of batteries. However, this only works if you're selling batteries directly.

Best practice: Option 1 is the cleanest solution in standard Odoo. Simply create one fixed-amount tax per battery count scenario and assign it to the matching product. This keeps your tax reports clean and avoids complexity.

Note: If you need fully dynamic eco-tax calculation based on battery count with a single tax rule, that would require a custom module.