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I am going on a long road trip with a buddy soon and I was going to use a power inverter that plugs into the cigarette lighter socket of his car so I could use my laptop while we drive. I'm trying to match up specs to make sure everything will work safely together but I'm not sure if I'm reading the labels correctly.

All of the reviews for the inverter swear up and down that this will work, but when I dug into the actual specs for the devices involved I'm no longer sure. My assumption is that I should be matching up the input and output ratings so that car out >= inverter in and inverter out >= laptop in but the numbers don't seem to make sense that way. The cigarette lighter fuse is 10A but the inverter in draws 28.5A so shouldn't that be way too high? But if most lighter fuses are in the 5-20A range how does this device have so many positive reviews? Instead, if I'm supposed to be matching outputs does that mean the laptop's 10.3A vs the inverter's 2.6A is a nonstarter?

So, in short:

  1. Would this setup work?
    • Cigarette Socket: ???V 10A ???W out
    • Inverter: 11-15V DC 28.5A in, 110-120V 60Hz 2.6A 300W out
    • Laptop: 100-240V 3A in, 19.5V 10.3A 200W out
  2. What is the correct way to read these specs so I can figure this out on my own in the future?

Sources for the above numbers.

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  • Most auto 12V outlets are fused at between 10A and 20A. So this is going to be marginal. Worst case it won't work and will blow the fuse. Chances are good, however, that you'll be OK since the HP supply will probably not draw the full rated capacity.
    – jwh20
    Commented Sep 22, 2022 at 20:01
  • Can you purchase this inverter before the road trip to try it out immediately? Available locally for immediate purchase to test it, regardless of price and returnable? You have nothing to lose by trying this inverter or others before counting on it for a road trip. Better to know before the trip begins. As it is, the input current may be a misprint of the decimal in the wrong place, more likely to draw 2.85 amps, not 28.5A. Drawing more amperage than typical dc power outlets would blow the fuse powering the outlet. Inverter output meeting/exceeding laptop adapter input seems correct.
    – F Dryer
    Commented Sep 23, 2022 at 20:27

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All power ratings are max ratings on the devices. The DC converter 300W output suggests it will power a device up to 300W - the laptop PSU is certainly below that.

However the DC converter max output will pull current relative to what is needed - in theory a max of 200W is required (by the laptop charger). That would roughly equal to 2/3rd of what the max amp the DC converter requires on its input. Going by the numbers, if you charge a full empty laptop battery - where the laptop charger will output it's max power - the amp at DC converter will require can be up to 2/3 * 28.5 = 19A.

Pulling 19 amperes from a cigarette lighter socket is too much (I've seen the comment about certain car models may have a max of 20 amp fuse) in my opinion. I've seen a couple of European cars, all had a max of 10amp fuse on the cigarette socket.

These are my options for this setup:

  • You could operate the DC converter connected directly to a car battery. Checked the link, this only has a hardwired cigarette lighter input, you'd need a cigarette socket adapter with battery clamps: Car Battery Clipon Cigarette Lighter Socket Adaptor 12v

  • An alternative could be to search for a different, but compatible HP charger, which has a max power output of about 100W - with this a cigarette lighter socket can be a viable input source to the DC converter, but the laptop charging time will be increased considerably (you may need to check if the specific laptop can use lower powered chargers, I have a hp zbook 15 G5, it comes with a 120W charger, but was able to use a lower power charger with it)

  • use a dedicated wire connected to the battery, which is appropriately sized (in diameter, for the high amp) and fused

  • I also checked the reviews, it's worth purchasing and testing before your trip (looks like it's not of the highest quality)

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  • Thank you for the detailed explanation. I'll look into a lower wattage charger. On the laptop charger it says its maximum input is 3A, would that mean it can't draw more than 3A from the converter?
    – 5E4ME
    Commented Sep 26, 2022 at 18:35
  • I wish I could edit comments after 5 minutes... Just to make sure I'm correctly wrapping my head around this, is the math something like: 200W/19.5V = 10.2A charger output -> 200W/120V = 1.7A inverter output -> 200W/15V = 13A car output But with a 100W charger it becomes 100W/19.5V = 5.6A charger output -> 100W/120V = 0.8A inverter output -> 100W/15V = 6.7A car output and since 6.7A is below the car's 10A fuse I'm good?
    – 5E4ME
    Commented Sep 26, 2022 at 18:47
  • @5E4ME yes, something like that. but as suggested, you want to try this out before real-life usage, as the reviews for the particular converter weren't very good. (it may fail internally etc). I want to add that the cigarette socket connector usually has an internal fuse - the knurled part is a nut. (while unscrewing, keep holding it, as there's a spring behind). internals of cig.lighter connectors: cig.lighter male adapter
    – PeterM
    Commented Sep 27, 2022 at 10:10

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