A car battery is not a deep cycle battery. It was built to deliver massive current for a few seconds to crank the engine, then get immediately recharged by the alternator. Running accessories with the engine off drains a battery that is not designed for this — and going too deep risks permanent damage and a no-start situation.
Why Car Batteries Are Not Built for Runtime
Car batteries (SLI — starting, lighting, ignition) use many thin lead plates packed closely together. This design maximises surface area for high-current bursts (300-800 amps for 5-10 seconds during cranking) but makes the plates fragile under sustained discharge.
Deep discharging an SLI battery causes the thin plates to warp, shed active material, and develop permanent sulphation. One deep discharge to 20% SOC can knock 20-30% off a car battery's remaining lifespan. Three or four deep discharges may render it unable to start the engine at all.
The calculator above defaults to 30% DoD — that is the maximum you should pull from a car battery if you want to start the engine afterward. Even at 30%, you are shortening the battery's life with every cycle. For regular accessory use with the engine off, install a dedicated deep cycle battery with an isolator.
Common Car Accessory Power Draws
| Accessory | Typical Watts | Runtime on 60Ah (30% DoD) |
|---|---|---|
| Dome light | 10W | ~17 hours |
| Radio (engine off) | 15-25W | 7-11 hours |
| Headlights (halogen) | 110-130W | ~1.3 hours |
| Headlights (LED) | 30-50W | 3.4-5.7 hours |
| Phone charger | 10-18W | 9.5-17 hours |
| Dash cam (always-on) | 3-5W | 34-57 hours |
| Heated seats | 40-60W per seat | 2.8-4.3 hours |
| 12V air compressor | 120-180W | ~1 hour |
| Car camping (fan + lights + charger) | 50-80W | 2.1-3.4 hours |
The most common way people kill their car battery: leaving headlights on. At 120W, halogen headlights drain a 60Ah battery to 30% in about 80 minutes. By the time you notice, the engine will not start.
Frequently Asked Questions
Check the <a href="/electrical/amps-draw-calculator">amps draw calculator</a> to convert any accessory wattage to amps for fuse sizing.
Car batteries are the wrong tool for sustained accessory power. Use the calculator above to know your limits, stay above 30% discharge to protect your ability to start the engine, and add a dedicated deep cycle battery for anything beyond occasional short-duration use.
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Written and maintained by Dan Dadovic, Developer & Off-Grid Energy Enthusiast. On the energy side, Dan has hands-on experience with residential solar panel installation, DIY battery bank construction, off-grid power systems, and wind power — all from building and maintaining his own systems..
Disclaimer: Calculator results are estimates based on theoretical formulas. Actual performance varies with temperature, battery age, load patterns, and equipment condition. For critical electrical work, consult a licensed electrician.