Will a Car Power Inverter Drain the Battery?

The answer is: It's complicated...

Adding a power inverter to a car, truck, or RV opens up a world of possibilities in terms of the types of electronics you can use on the road, but nothing in life is free. All that power comes from somewhere, and if it happens to come from the starting battery, that world of possibilities can collapse into a world of hurt with almost no warning.

Family using a power inverter to run a laptop in a van.

Matthew Micah Wright / Lonely Planet Images / Getty Images

While the issue of an inverter draining a car battery is fairly complex, the general rule of thumb is that the inverter won't drain a battery when the vehicle is running, and especially not when it's driving around. However, using an inverter when the engine is off will run the battery down, and it doesn't take much before the engine won’t start back up again without a jump or a charge.

The easiest solution to this problem is to stop using the inverter before it gets to that point. Although, bringing along a separate deep cycle battery for the inverter or bringing a generator with a built-in battery charger are both great options.

Draining the Battery While the Engine Is Running

Whenever the engine in a car or truck is running, the alternator charges the battery and supplies power to the electrical system. The battery is still important since alternators require battery voltage to work properly, but the alternator is supposed to do the heavy lifting when the engine is running.

When everything works properly, the alternator charges the battery if it needs to be charged, powers electrical systems and components like your stereo and headlights, and has power left over for accessories like an inverter.

If the alternator isn’t equal to the task of providing all that power—either because it's going bad or isn't powerful enough—your electrical system may enter into a state of discharge. At that point, you'll notice the charge meter on your dash, if you have one, dip down below 12 or 13 volts, which indicates that power is discharging from the battery.

When that type of situation is allowed to continue for too long, the battery eventually discharges to the point where you don't have enough power available to run all the electronics in the vehicle. At that point, or even before, you will typically experience drivability problems. The engine may even die.

Idling the Engine vs. Actually Driving

It's also worth mentioning that the power curve of an alternator is higher at high engine revolutions per minute (RPMs) than low RPMs, which means an overtaxed electrical system may enter a state of discharge at idle even though it's fine when you're cruising down the highway.

If you find yourself in a situation where the electrical system seems to enter a state of discharge when the vehicle is stopped, raising the engine RPM by applying a little gas may help. However, raising the engine RPM too high can cause damage, so unplugging power-hungry devices from the inverter is often a better idea.

Every situation is different, but you're typically fine to power small electronic devices like laptops, Blu-ray and DVD players, and phone chargers without overtaxing the electrical system. If you need more power, or you also have a high-end audio system with a powerful amplifier, subwoofer, and other components, you may need to invest in a high-output alternator.

Draining the Battery When the Engine Is Off

When your engine isn't running, the battery is responsible for providing power to the electrical system. This is why leaving the headlights on overnight drains the battery down to nothing. The same thing happens if you use an inverter when you're parked.

Some inverters come with a built-in low-battery-voltage shutoff feature, but that may or may not leave you with enough reserve power to operate the starter motor. Since starters require a tremendous amount of amperage in order to crank, running an inverter when you're out camping can leave you stranded.

If you want to use an inverter when you're camping, you may want to hedge your bets by buying an additional deep cycle battery to power the inverter. You can also start your engine to charge up the battery every so often​ or bring along a generator that has a built-in battery charge in case you end up with a dead battery.

How Long Can You Run an Inverter Before the Battery Drains?

The amount of time you can use an inverter to run your electronics depends on how much power you use and the capacity of the battery. If you know the wattage of the devices you want to use and the reserve capacity of the battery, you can plug those numbers into this formula:

(10 x [Battery Capacity] / [Load]) / 2

If your battery has a capacity of 100 amp hours, and you want to use a laptop that uses 45 watts, you can see that you'd be able to get about 11 hours out of your battery:

(10 x [100 AH] / [45 Watts]) / 2 = 11.11 hours

In practice, it's best to err on the side of caution. If you were to actually run a 45-watt load on a 100 AH battery for 11 hours, there's a chance that there wouldn't be enough power left in the battery to operate the starter motor. Bigger loads—like a desktop computer, television, and many other electronics—drain a battery even more quickly.

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