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I have a laptop with a short battery life that has a proprietary high amperage plug that I need to use in a car while that car is in motion, being driven by someone else. The proprietary plug rules out a standard DC to DC converter to power it, and I have tried, with no avail, to find a compatible DC to DC converter. As such, my only option is to use an inverter. A large inverter, that connects to the car's battery directly is too complicated for me to feel comfortable installing, especially considering the high voltage involved. It is also too expensive to have it professionally installed. This leaves products that provide a 120V house outlet from a cigarette lighter, such as this. My laptop's power connector, however, says that the input is "100-240 V ~ 3.2 A 50-60Hz" and that the output is "19.5 V 11.8 A". I am aware that volts multiplied by amps gives watts, but because there are ranges for the input voltage, I get an input wattage of anywhere from 320W to 768W. If I use the output information then I get 230.1W, although this is likely less accurate with regard to the actual power consumption due to transformer inefficiencies. Two of these values are less than the 400W that the linked inverter says it is able to provide, but 768W is far above it. How can I resolve this ambiguity to know how many watts I actually need to run my laptop? What happens if I make a mistake during that resolution of ambiguity and end up with my laptop trying to draw more power than the inverter can provide? Will it just blow a fuse that I can reset (and then not use that laptop with that inverter again), or does it have the potential to damage other things (ie. shut off the car while it is in motion, or blow a fuse deeper inside the car that is difficult to replace with my next-to-nonexistant knowlege of the inner workings of cars)?

Some other questions give useful information for part of this question, but do not address the car and inverter specific factors, such as what happens to the car and/or inverter if the estimated usage ends up being too low.

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  • \$\begingroup\$ Possible duplicate of Input power rating on power supplies - how does max amp draw apply to 220V \$\endgroup\$
    – winny
    Jun 12 '18 at 10:22
  • \$\begingroup\$ @winny That question only addresses the part of the question represented in the title. It does not address the aspect of the question with regard to what happens if the estimated usage is lower than the true usage. \$\endgroup\$
    – john01dav
    Jun 12 '18 at 10:25
  • \$\begingroup\$ Converting 12V DC up to 120V AC and then back to 19.5 V DC is inefficient and cumbersome, there are boost converter modules available like: ebay.com/p/… that would do the job. \$\endgroup\$ Jun 12 '18 at 10:36
  • \$\begingroup\$ @Bimpelrekkie I have attempted to find a DC to DC converter that is compatible with my laptop's proprietary power connector, but I am unable to find any such converter. i am not comfortable enough with electronics to make such a device myself. \$\endgroup\$
    – john01dav
    Jun 12 '18 at 10:47
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It's not going to damage the car, and the car fuses are usually somewhere really accessible.

My reckoning is that the output number will be accurate, and the input current number will be a maximum at the lowest input voltage; therefore the 320W input number is what I'd expect. The inverter is too small to dissipate much heat, so it's not going to take in much more than the 400W max output.

However, lots of the internet suggests that the max power draw from the cigarette lighter before the fuse blows is 200W. So you may find the fuse blows after several minutes.

You could set the thing up with a suitable 20A current meter on the (powered off) car, measuring at the battery, and see what the actual current draw is.

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  • \$\begingroup\$ "The inverter is too small to dissipate much heat, so it's not going to take in much more than the 400W max output." Is there some product I can buy to place between the laptop and the inverter to immediately blow a fuse if it goes over 400W? If so, would such a product protect the inverter from hazardous outcomes (either damage to the inverter, or dangerous events such as a fire or melting)? \$\endgroup\$
    – john01dav
    Jun 12 '18 at 10:31
  • \$\begingroup\$ The inverter claims "TOTAL PROTECTION - thermal shutdown, reverse polarity protection, over-voltage shutdown, low voltage shutdown and low voltage alarm, replaceable 30A fuse", so I think it's probably OK \$\endgroup\$
    – pjc50
    Jun 12 '18 at 10:39

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