how long can you run 12w on a car battery

[OChristian]

A standard car battery can typically power a 12-watt device for approximately 40 to 50 hours on a full charge, but this varies significantly based on the battery's condition and usage factors. This estimate assumes a common 50-ampere-hour (Ah) battery, where ampere-hour is a measure of electrical charge capacity. For a 12W load at 12 volts, the current draw is about 1 ampere (since Power = Voltage × Current, so 12W / 12V = 1A). The theoretical run time is calculated as Battery Capacity (Ah) divided by Current (A), yielding 50 hours for a 50Ah battery. However, in practice, you should avoid draining the battery below 50% depth of discharge (DOD) to prevent damage, which halves the usable time. Other factors like battery age, temperature, and inefficiencies in the power conversion can reduce this further.

For a quick reference, here's a table showing estimated run times for various car battery capacities under a 12W load. These are theoretical values; real-world times may be 20-30% shorter due to losses.

Battery Capacity (Ah)	Theoretical Run Time (hours)	Practical Estimate (hours, with 80% efficiency)
40	40	32
45	45	36
50	50	40
55	55	44
60	60	48
65	65	52
70	70	56

Always monitor your battery's voltage with a multimeter; if it drops below 12.4 volts, recharge it to avoid stranding. This advice is based on general automotive standards to ensure reliability.