Why does a 12V lithium battery shut off when you start an inverter?

Direct answer: Usually the battery’s BMS is doing its job: the inverter’s start-up or the appliance’s inrush current exceeds the current the BMS allows, so it disconnects to protect the cells. Compare the inverter’s input current, including surge, with the battery’s continuous and peak discharge limits, check cables and connections, and either choose a battery variant with a higher limit, add batteries in parallel, or reduce the load.

Do the arithmetic. On a 12V system, current is power divided by voltage. A 2,000W inverter running at full load draws roughly 2,000 ÷ 12 ≈ 170A, and more once inverter losses are included (our estimate, not a product figure). Many inverters also draw a short charging pulse when switched on, and motors or compressors can start at several times their running power.

Compare with the page limits. The 12V 100Ah page lists maximum continuous discharge of 50A (0.5C) or 100A (1C) depending on version, with no peak figure. The 12V 200Ah and 12V 300Ah pages list versions at 100A, 150A or 200A continuous, with peaks of 110A, 160A or 210A for 10 seconds. A load that needs 170A continuous is above the 100A versions; a start-up spike above the peak value can trip even a larger version.

Check the wiring too. Undersized or long cables, loose lugs and corroded terminals cause voltage drop. Under heavy load the voltage at the inverter falls, the inverter draws more current for the same power and may trip on low voltage before the battery does. Feel for warm connections after a load test and check torque. Our post on cable and fuse sizing for a 12V 200Ah battery and 2,000W inverter works through that case.

Find out which device switched off. If the inverter shows a low-voltage or overload error but the battery stays on, the issue is on the inverter side. If the battery output disappears (no voltage at its terminals) and resets after a delay or a charge, the BMS tripped. A Bluetooth app or display, where fitted, may show the event.

Fixes, in order: confirm the battery variant you actually have (the limit is on the label or quote); reduce the start-up load, for example by not starting the inverter with appliances already on; use correctly sized cables and a fuse rated for the system; add a second identical battery in parallel to share the current (the pages allow up to four in parallel); or move to a 24V system for large inverters. Ask Redway how long the BMS tolerates the peak values and how it resets.