12V to 110AC Power Inverter efficiency

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Real quick question: I have a cross country trip coming up at the end of the week and the wife wants to bring a heated blanket along. The blanket is rated for 180w max power consumption. That equates to about 15A power draw, but that doesn't include efficiency loss through the power inverter. The inverter I have is rated for 350W continuous and has a 40 amp fuse-- the fuse rating seems high to me, suggesting poor efficiency. Do these small inverters really gobble up that many amps? Curious how many watts one can pull from a cigarette lighter socket, the math suggests 250W (12.8v x 20 amp rated lighter circuit), but some amount of power is lost through the inverter, I just don't how how much. 75% efficiency seems reasonable to me, but that's just an educated guess. Any input / experiences would be appreciated!
 
Just looked at my Camry for you; both 12v outlet covers are marked "12V/120W"... I'd not exceed that by too much. Second the decision to buy a 12v version of the blanket. Not worth any trouble...
 
Originally Posted by EdwardC
this one is 90%: https://www.samlexamerica.com/documents/product-specs/12002-SAM-450-12-1113%20Lrez.pdf I'd guess at 180W at 120VAC, you'll likely pop the lighter socket fuse. I'd pick up a lower wattage 12V blanket and skip the inverter.
Originally Posted by TmanP
Just looked at my Camry for you; both 12v outlet covers are marked "12V/120W"... I'd not exceed that by too much. Second the decision to buy a 12v version of the blanket. Not worth any trouble...
Right on both accounts.
 
120W seems pretty conservative, that's only a 10 amp load. The outlet in question is a 20A circuit with nothing else on it (the always on cigarette lighter outlet with the battery icon). The other one is switched with the ignition and is 10A, I think a 120W limit would be more appropriate for that one. The older vehicles had to have a pretty beefy circuit to support actual cigarette lighters, whereas the newer vehicles' outlets are intended for cell phone chargers and such. I'll probably just fiddle with it in the driveway and see if it holds up. I can put a clamp meter on it and see if I can get an idea of the inverter efficiency.
 
Bad idea, those inverters are designed to charge a laptop or power a similar device, not run a heating device. Not as bad as bringing a toaster or hair dryer, but close enough!
 
Get a 12v blanket..I have one. It's only 40w so it's more of a throw. My daughter gets cold in the car during winter and so it's either I crank up the heat, which I hate, or get her a blanket. I have a 500w inverter and you get what you pay for with them imo. The cheaper ones can have high losses. With the cheaper ones you get a modified sine wave which some devices won't play well with. My inverter can run off the 12v plug in the cabin but it's limited to I think 10amps... for more current I can run heavier guage wiring off the battery to rear mounted terminals on the inverter. I think on that setup I can pull 30amps continuous iirc. Thankfully the fuses are serviceable without cracking the case open. (500w is "peak", I think it's something like 350w continuous) I've never run anything like 180w for as long as you are looking at, so I'd be curious as to how the unit holds up.
 
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Originally Posted by dishdude
Bad idea, those inverters are designed to charge a laptop or power a similar device, not run a heating device. Not as bad as bringing a toaster or hair dryer, but close enough!
i haven't looked at these in a while, but I'd think a resistive load such as a blanket would be easier to support than any kind of power supply, since it won't have the current feedback (power factor) introduced by linear or switching power supplies.
 
^ Power factor is not relevant. It's the average load across the wiring and fuse meant to protect it. Look into how much a suitable 12VDC blanket would cost and compare that to the cost of the inverter, except the inverter is handy for other things too. Efficiency is typically around 90%, but if you buy cheap chinese junk, don't expect good lifespan running it near max rating. You should get one that wires direct to the battery, with an inline fuse near the battery. The better idea is just tell the wife that it's not a good idea to put the extra load on the alternator long term and then buy her a nice sweater. I'm not kidding.
 
Originally Posted by Dave9
^ Power factor is not relevant. It's the average load across the wiring and fuse meant to protect it. Look into how much a suitable 12VDC blanket would cost and compare that to the cost of the inverter, except the inverter is handy for other things too. Efficiency is typically around 90%, but if you buy cheap chinese junk, don't expect good lifespan running it near max rating. You should get one that wires direct to the battery, with an inline fuse near the battery. The better idea is just tell the wife that it's not a good idea to put the extra load on the alternator long term and then buy her a nice sweater. I'm not kidding.
Id argue power factor is relevant for the rating of the fuse. The conversation is a 350W inverter protected by a 40A fuse. But worst case load and worst case input is the design. A battery has enough fault current to clear a fuse. So it's really a continuous overheating of the conductor. 350W needs to be able to support 0.8PF (which is typical) This is 440VA. Make it 90% efficient and it's 486VA. 486VA/12V = 40.5A Because a typical 12v battery suitable to source 350W for any practical amount of time will be able to source a LOT more fault current, so it will easily blow a 40A or higher rated fuse. That's not the main intent here. It's long term overheating and fire hazard. If 0.8PF is not considered, then a 40A fuse would allow the electronics to overheat. Power electronics don't permit high fault throughput - 1.1pu maybe. So this allows a typical worst case load condition and protects the conductor and the switches inside.
 
Originally Posted by JHZR2
Id argue power factor is relevant for the rating of the fuse.
It's not, except for an extremely fast blow fuse, something that is not just a metal strip heating up to blow. In other words no normal fuse ever, only an electronically controlled current monitoring disconnect would ever trip on this. Fuses are about heat buildup. Current over time (interval) is what blows them. Power factor just doesn't factor in, is always based on a far shorter time interval.
 
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I appreciate that the I^2t curve should be considered in the selection of the fuse. But again the battery will clear any practical size fuse rapidly, as a low impedance fault will cause the battery to source up to thousands of amps, so any practical size fuse will clear. It is all about heat in the conductor and power electronics. The amount of continuous current that the conductor should carry dictates the fuse. Notionally the fuse will allow continuous carrying of that level of current, which on the dc side doesn't care about PF, but on the AC side is dictated by VA, not W... Which defines total continuous load on the dc side. A fuse should be selected to allow continuous max current at full power (which should consider a non-unity PF on the AC side), and blow to protect the conductor any higher.
 
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