Question on power inverter.
August 6, 2009 3:12 PM Subscribe
Electricity Filter: Roadtrip starts tomorrow, picked up a 400w power inverter to run laptop in car for road tunes, question...
Picked up an inverter with an integrated power switch to avoid the clumsy and awkward need to dig around in the centre console for the 12v cig lighter plug every time we shut off the car. Made the mistake of reading the manual and noticed "When the power inverter is not in use, unplug it from the 12v outlet to prevent a slight discharge of the battery." Really? Isn't the switch enough? If the switch on the inverter is off and the 12v plug is left in what are the chances of clicky-click-click tomorrow morning when trying to start the car?
Picked up an inverter with an integrated power switch to avoid the clumsy and awkward need to dig around in the centre console for the 12v cig lighter plug every time we shut off the car. Made the mistake of reading the manual and noticed "When the power inverter is not in use, unplug it from the 12v outlet to prevent a slight discharge of the battery." Really? Isn't the switch enough? If the switch on the inverter is off and the 12v plug is left in what are the chances of clicky-click-click tomorrow morning when trying to start the car?
First, check what AzraelBrown said. I have an inverter in my car that I leave switched on all the time, because the 12V outlet in the center console is not powered when the ignition is off.
I have a bunch of other inverters and have measured their power-off current draw as less than 30 milliamps in all cases. That would take weeks to run down a car battery. I'm sure there are countless inverter designs out there, though, so the only way to know for sure is to measure yours.
If you have a multimeter with a current measurement (mA) function, and if the cigarette lighter cord for your inverter can have one of its wires disconnected (many have this cord as a separate part that you attach to a pair of ring terminals), here's what you do:
The reason why these things still draw power even with the power "off" is that the power switch doesn't actually disconnect the 12V input. A 400 watt inverter could draw roughly 40 amps from the 12V source, so quite a heavy-duty switch would be needed to switch the input power on and off. Instead, the power switch just shuts down all the inverter electronics, but everything is still connected and so can draw a little power.
posted by FishBike at 3:49 PM on August 6, 2009 [1 favorite]
I have a bunch of other inverters and have measured their power-off current draw as less than 30 milliamps in all cases. That would take weeks to run down a car battery. I'm sure there are countless inverter designs out there, though, so the only way to know for sure is to measure yours.
If you have a multimeter with a current measurement (mA) function, and if the cigarette lighter cord for your inverter can have one of its wires disconnected (many have this cord as a separate part that you attach to a pair of ring terminals), here's what you do:
- Set up your multimeter to measure current (mA) on a scale close to 200mA. You might have to attach the probes to different connectors on some meters to do this.
- Plug in the inverter to a powered 12V outlet and turn it on, then turn it off again and leave it off for the rest of this test. This is just to make sure the input filter capacitors are charged up.
- Disconnect one of the two power wires from the inverter.
- Use the multimeter to bridge the gap you just created in the circuit (touch one probe to the end of the disconnected wire, and the other probe to the terminal you disconnected it from).
- Read the amount of current being drawn with the inverter off, checking to make sure it is not slowly increasing or anything like that.
The reason why these things still draw power even with the power "off" is that the power switch doesn't actually disconnect the 12V input. A 400 watt inverter could draw roughly 40 amps from the 12V source, so quite a heavy-duty switch would be needed to switch the input power on and off. Instead, the power switch just shuts down all the inverter electronics, but everything is still connected and so can draw a little power.
posted by FishBike at 3:49 PM on August 6, 2009 [1 favorite]
nth the bit about the ignition switching your ciggie port. I find it annoying, but I'm sure it's probably saved me in the past.
That said: take a manual, park on a hill, roll-start where necessary.
posted by pompomtom at 4:01 PM on August 6, 2009
That said: take a manual, park on a hill, roll-start where necessary.
posted by pompomtom at 4:01 PM on August 6, 2009
You could just get a cheap 12v extender with a hard switch. Or wire in a relay on your 12v outlet if it isn't already there.
posted by krisak at 4:53 PM on August 6, 2009
posted by krisak at 4:53 PM on August 6, 2009
It depends where the switch is and what kind it is. If it's a mechanical switch on the cord between the plug and the inverter box then absolutely it draws no current when switched off. If it's on the inverter box or not mechanical, there there is quite possibly some non-trivial current draw.
It's possible the manual was translated from Chinese and doesn't entirely make sense or was written to cover more than one product, one of which might not have an off switch.
posted by chairface at 11:44 AM on August 7, 2009
It's possible the manual was translated from Chinese and doesn't entirely make sense or was written to cover more than one product, one of which might not have an off switch.
posted by chairface at 11:44 AM on August 7, 2009
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Also: check if your lighter outlet is already switched via the ignition. if the inverter can be turned on even if the keys aren't in the ignition, you might want to unplug it, just to be safe. If the cigarette outlet is off when they keys are out of the ignition, don't worry about it.
posted by AzraelBrown at 3:32 PM on August 6, 2009