You can typically run a 100-watt AC device (like a small TV or laptop) for 10 to 20 hours on a healthy, fully charged car battery before it's too drained to start the engine. However, this is a best-case scenario and depends heavily on your battery's health, capacity, and the power draw of the device. The biggest risk is a dead battery, leaving you stranded.
The primary factor is your battery type. A standard starting battery is designed to deliver a large burst of power (cranking amps) to start the engine, not for prolonged, low-power discharge. Using it for accessories can quickly damage its plates. A deep-cycle battery, found in RVs or as an auxiliary battery, is built for this purpose and can be discharged much more deeply without harm.
The actual runtime is a simple calculation. Car battery capacity is measured in amp-hours (Ah). A typical mid-size car battery might have a 50Ah capacity. To find watt-hours, multiply amp-hours by the battery voltage (12V): 50Ah x 12V = 600 watt-hours. This means it can theoretically power a 100-watt device for 6 hours (600Wh / 100W = 6h). However, you should never drain a starting battery below 50% charge to avoid damage, effectively halving your safe runtime to about 3 hours.
| Device (approx. power draw) | Safe Runtime on 50Ah Starting Battery (to 50% discharge) | Safe Runtime on 100Ah Deep-Cycle Battery (to 50% discharge) |
|---|
| Laptop (60W) | ~5 hours | ~10 hours |
| Small TV (100W) | ~3 hours | ~6 hours |
| Box Fan (50W) | ~6 hours | ~12 hours |
| CPAP Machine (30W without humidifier) | ~10 hours | ~20 hours |
| LED String Lights (10W) | ~30 hours | ~60 hours |
For safety and battery health, always start the engine every 1-2 hours to recharge the battery if you're using it for extended periods. The most reliable solution for frequent use is installing a dedicated deep-cycle auxiliary battery with an isolator to protect your vehicle's starting system.
