Why does a rack battery’s breaker trip when the inverter is switched on?

Direct answer: Usually because the inverter’s DC input capacitors are empty and draw a very large current for a few milliseconds when connected. That surge can trip the battery’s breaker or its BMS short-circuit protection. The fix is to pre-charge the capacitors before full connection, by the inverter’s own start-up function, a battery pre-charge feature or a pre-charge procedure, and to follow the start-up order in the manuals. If it still trips, look for a wiring fault.

Why the surge happens. Large inverters have big capacitor banks on the DC side. Connecting them to a low-resistance lithium battery is close to a short circuit until they charge. The battery’s protection cannot tell this from a real fault.

What the batteries list. The 51.2V 100Ah 3U lists a circuit breaker, 100A continuous, and peak currents of 110A for 10 seconds and 150A for 0.5 seconds. The 51.2V 150Ah 4U and 51.2V 200Ah 5U list 100A continuous and a circuit breaker. A capacitor inrush can exceed these peaks briefly, so protection may act.

Start-up order. Many hybrid inverters pre-charge their capacitors from PV or AC, or from an internal circuit, before closing to the battery. Others expect the battery to provide pre-charge. Read both manuals: some say turn on the battery, then the inverter; others say power the inverter first. Ask Redway whether these models have a built-in pre-charge function.

Pre-charge procedure. Where neither device pre-charges, installers use a pre-charge resistor across the isolator for a few seconds before closing it, with a suitable resistor and safe handling. This is specialised work; follow the inverter maker’s method.

When it is not inrush. If the breaker trips after the inverter is running, check for reversed polarity, damaged insulation, a short in a parallel cable, a loose busbar or a load above the battery rating. Reverse polarity can destroy equipment; check with a meter before connecting. See commissioning parallel rack batteries.

Parallel banks. With several modules, the inrush is shared, but only if all are on; switching on one module first can trip it. Follow the maker’s sequence.

Compatibility. Confirm that the inverter is on the battery’s compatible list and the protocol is set; see checking inverter compatibility.