How to Choose DC to AC Power Supply Inverters
12v Car Battery, Pure and Modified Sine Wave Automotive Converters
Oct 30, 2009
Most electrical and electronic devices run off 115 volts AC (110 to 120 volts, or 220 to 240 volts in some countries). Adapters are available to charge cell phones from a car battery's 12 volts DC. However these need to be purchased separately and will often need to be changed for each new phone.
Using an inverter (often misspelled as "invertor") means that devices can be used in the car without any special converters. Depending on the power rating and type, inverters can cost as little as $25 for a 100 watt model (modified sine wave) and can reach hundreds of dollars for more powerful models. Pure sine wave models (see explanation below) start at about $100 for 100 watts.
Popular brands include Xantrex (Prosine, Trace, Freedom), Coleman, Vector and AIMS.
Cigarette Lighter Versus Clamp-on Inverters
There are two main ways to connect an inverter:
- Through the car's cigarette lighter socket.
- Directly clamping on to the battery terminals.
Inverters with cigarette lighter plugs are convenient but the maximum power they can actually support is limited to 100 to 200 watts (depending on the fuse in the cigarette lighter circuit), no matter what the inverter's rating.
Larger inverters (above 1000 watts) don't have a cigarette lighter plug as they are designed for direct clamping to the battery only. For use with a car, they can be:
- Permanently mounted under the dashboard.
- Clamped as needed with the car stationary and the engine hood up (for example to run power tools). The battery cables need to be long, otherwise the inverter will be balanced on the radiator.
Medium sized inverters (about 500 watts) can have both a cigarette lighter plug and terminals for direct clamping to the battery. However they are still limited to 100 to 200 watts when using the cigarette lighter plug or they'll blow the cigarette lighter fuse.
Pure or True Sine Wave Inverters
These are "real" inverters, generating a sine wave AC voltage that is almost the same as household mains supply.
They should work with almost any electrical device but can:
- Produce noticeable noise in audio equipment.
- Interfere with radio reception.
- Interfere with notebook computers, causing the mouse to become uncontrollable.
Modified Sine Wave Inverters
These are popular because of their low price. Instead of a smooth AC wave, a jagged squarish wave is produced. They might damage, overheat or not work with some types of equipment.
The manufacturer's user guide should be checked for device compatibility issues. Incompatible equipment can include:
- Electric motors.
- Audio and video equipment.
- Computers.
- Uninterruptible power supplies (UPS).
- Battery chargers for cordless tools.
Some high-end inverters have noise filters, making them compatible with more equipment. With cheaper unfiltered inverters, consumers can add a ferrite core snap-on filter to the equipment's AC power cord but the results aren't guaranteed.
How to Choose a Power Inverter
The inverter's power rating (watts) is obviously important:
- The continuous power rating is what the inverter can comfortably supply without overheating. This is what most people would consider to be the "real" power rating.
- The maximum power rating is what the inverter can support for 5 to 60 minutes (depends on the manufacturer), usually 25% more than the continuous watts.
- Peak watts is usually double the continuous watts and can be sustained for only a few seconds: for example to start an electric motor.
Inverters are sometimes advertised with their maximum or peak rating ("1000 watt inverter"), so buyers should read the fine print before buying.
Useful alarms or automatic shut-off safeties are:
- Low voltage protection. Stops inverter from draining battery flat.
- Engine off alarm. Warns that battery isn't being charged.
- Overheating protection.
- Overload protection.
- Over voltage protection. Battery voltage too high.
- Reverse polarity protection. Battery terminals reversed.
The following features are found in larger inverters. They can also be purchased separately as a plug-in.
- Wattmeter.
- Battery voltmeter.
- Output true RMS voltmeter.
- Output ammeter.
Other useful features include:
- Quiet cooling fan. Many are noisy.
- Thermal-controlled cooling fan that switches on only when needed.
- Metal casing for better cooling.
- Easily-changeable external fuse.
- No-load auto-off. Most inverters will still draw some current even if no devices are attached. This can drain the car battery.
- 5 volt USB socket.
- 12 volt pass-through cigarette lighter socket.
The Best Power Inverter
Pure sine wave inverters are the safe choice. Consumers who are on a budget might be willing to take their chances with a modified sine wave inverter, but they should do so knowing the risks.
Occasional use of a 100 watt inverter to charge a cell phone inside a car should be quite safe. Prolonged use of more powerful inverters will need more careful consideration (alternator upgrade, deep-cycle battery). A mechanic or electrician should be consulted.
A jump starter battery pack with a built-in inverter should also be considered as an alternative. The Switching Power Supply website has more information on pure versus modified sine wave rectifiers, and device compatibility issues.
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