Can a 12V LiFePO4 battery be used as a starter battery for an engine?

Direct answer: Not unless the supplier confirms it for that use. Redway’s 12V LiFePO4 pages list continuous discharge currents and short peak currents, but no cold cranking amps (CCA) rating, which is the figure used to match starting batteries to engines. They also list charging only from 0°C, while starting batteries must accept alternator charge in cold weather. Use them as house batteries and keep a starting battery for the engine.

What the pages list. The 12V 100Ah lists 50A or 100A maximum continuous discharge; the 12V 75Ah lists 75A continuous and a 220A peak for 3 seconds; the 12V 200Ah lists 100A, 150A or 200A continuous with 110A, 160A or 210A for 10 seconds. None lists a CCA figure, and the 75Ah page is tagged as a deep-cycle RV battery. A starter motor’s current is set by the engine and starter, and the vehicle maker specifies the battery.

Why the BMS matters. A lithium battery’s BMS limits current to protect the cells. If cranking current exceeds the BMS limit, the battery may switch off during the start, leaving the vehicle with no battery connected to its electrical system.

Temperature. These pages list charging from 0°C to 60°C and discharging from -20°C to 65°C. An alternator will try to charge after a cold start below 0°C, and an engine bay can be hotter than a living area. Both are outside what these house batteries are specified for.

Charging system. A vehicle alternator and regulator are set for the starting battery chemistry. Connecting a lithium house battery directly to an alternator can overload it; a DC-DC charger is the usual way to charge a house bank from the engine. See charging from the alternator with a DC-DC charger.

A practical setup. Keep the starting battery the vehicle maker specifies, add a LiFePO4 house battery charged through a DC-DC charger, and use a battery monitor for the house bank. If you want a lithium starting battery, buy one rated and sold for starting, with a CCA figure, and check the vehicle maker’s guidance. For house-bank sizing see sizing a 12V house battery.