Sizing battery connection cable for containerised storage

Start with the duty cycle

An energy storage system charges and discharges continuously. The cable current is bidirectional, the conductor temperature cycles with it, and the terminations see the same swing. Sizing on a steady-state current alone understates the thermal duty.

Take the maximum continuous current from the PCS or rack datasheet, then apply the enclosure ambient and the grouping factor. A cable rated 105 °C in free air is not rated 105 °C in a closed container with five neighbours. Construction and current data for the rack connection are on the energy storage battery cable page.

Derating factors to apply

• Ambient. Container internal temperature in summer, not site average.
• Grouping. Number of loaded cables in the same tray or duct.
• Installation. Free air, tray, conduit or buried.
• Terminal rating. The lug and the busbar often limit the current before the cable does.

Apply the factors to the tabulated current, then check that the resulting section still satisfies the voltage drop limit for the run length.

Voltage drop on a 1500 V DC rack

At 1500 V DC the percentage drop for a given current and section is smaller than at low voltage, but the runs inside a large BESS can still be tens of metres between rack and PCS. A 1 % drop at 1500 V is 15 V, which is usually acceptable; a 3 % drop is a design problem and a heat problem at the same time.

Where two cables are paralleled, they must be the same length, the same construction and the same route. Mismatched lengths share current unevenly and one cable runs hotter than the calculation.

What to specify, and what to ask for

Specify the voltage rating, the conductor class (Class 6 for tight rack dressing), the sheath (LSZH in a container), the temperature rating and the fire performance. Ask for the current rating table, the derating factors and the conductor resistance for the section you are buying.

If the rack layout is fixed, ask for cut lengths with lugs and heat-shrink fitted. It removes a field termination step and a common source of hot joints. BMS and EMS communication links are covered by the energy storage monitoring cable, and the full DC-side range is under energy storage cables.