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Hopefully I'm asking the obvious; but my computer is important so I would like more opinions. According to the question Laptop battery: is voltage really important to respect? It seemed their consensus was that modern laptops will accept a wide range of voltages into their charging ports. And second, I have a Lenovo T420. According to their currently suggested universal charger, it may output anywhere from 11.5 to 18 volts; and the original charger I have outputs 20 volts. So rather than this nonsense of inverting 12 VDC (nominal) from the car to 120 VAC and then rectifying it back down to 18 VDC, would there be any harm in interfacing my computers charging port directly with the car's DC wiring?

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    like it said there is a high probability that the laptop would function ok on the power supplied by the charged 12V battery , and/or the running automobile with its alternator and charging curcuit doing about 13-16v depending on the car. That still leaves how critical the power going to a computer motherboard can be, and how sloppy a cars power can get, with spikes, flyback, droops when the starter kicks in and interferances. It would be nice to at least have a full cleanup of that power before shoving it into a high tech device. I bet the cars computer uses a dedi connection and has cleanup.
    – Psycogeek
    Nov 12, 2015 at 8:49
  • Thank you for the insight. I'm starting to investigate methods of filtering to provide a nearly constant voltage. Maybe with a switch mode style regulator.
    – chew socks
    Nov 12, 2015 at 9:55
  • That's exactly what you're buying when you buy a laptop power adapter that will run on DC input, except that it has the added advantage of being designed and intended to work with your laptop. Unless you know of something wrong with it there is no good reason for using anything else. Nov 12, 2015 at 19:42
  • I really wouldnt do that - a car battery is capable of producing massive amounts of current in a short amount of time - if anything goes wrong in your laptop, and as you could easily end up with a fire. As @vojtech says, your best solution really is a Lenovo car adaptor. Nov 15, 2015 at 13:23

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In short: Don't do that. The laptops are rather picky about the input voltages and a 20V Lenovo laptop will only work with a 20V Lenovo power supply. And the car 12V can go down to 6V while starting the engine and up to 15V while charging in high revs. It'll fluctuate wildly, have all kinds of EMI and spikes in it. That's all bad for sensitive electronics.

In addition, the both the round and rectangular Lenovo charge connectors have 3 contacts. Outer barrel surface is ground, inner barrel surface is +20V, pin is data. The data pin is used to identify the power brick to the laptop, such that the laptop knows how much current it can draw.

The easy solution is a Lenovo car adaptor. It's not too expensive, does a direct DC/DC conversion from 12V to 20V, and filters and regulates the voltage, also providing the proper information on the data pin. Non-Lenovo, "universal" car adapters may work, depending on how close they get to providing what the laptop expects.

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According to the question Laptop battery: is voltage really important to respect? It seemed their consensus was that modern laptops will accept a wide range of voltages into their charging ports.

There may be some truth in that but what I expect you will find is that you will need the input voltage to be higher than the battery voltage for things to work correctly (i.e. for the battery to be charged and for the laptop to draw it's power from the input instead of from the battery).

According to their currently suggested universal charger, it may output anywhere from 11.5 to 18 volts

While I don't know for sure I strongly suspect that the charger outputs different voltages depending on which laptop you connect to it.

So rather than this nonsense of inverting 12 VDC (nominal) from the car to 120 VAC and then rectifying it back down to 18 VDC, would there be any harm in interfacing my computers charging port directly with the car's DC wiring?

Car electrics are known for being spiky, personally I wouldn't connect any device to car wiring that was not specifically designed for the job. Even if you think you will only use it with the car not running sooner or later you are likely to forget and start the car with it still connected.

Thank you for the insight. I'm starting to investigate methods of filtering to provide a nearly constant voltage. Maybe with a switch mode style regulator.

While with sufficient electronics experience you could build such a thing yourself I don't really see the point when it's readily available off the shelf.

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  • In addition to the low voltage during starting there is often one heck of a spike when you release the key (from the collapse of the field in the starter solenoid). Nov 12, 2015 at 19:44
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Yes, there could be harm. The car's "12 volts DC" is very unstable. It can easily reach almost 15 volts during charging and may have far higher brief spikes. While the car is running it will also have a strong AC component (this shows up in poorly designed audio circuits as "alternator whine").

This is not what you want to feed to your laptop.

By the way, I don't for a second believe the specs you quoted on that universal adapter. I strongly suspect that the "11.5 to 18 volts" rating is the allowable DC input spec, not the output.

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  • I hadn't considered noise in the line but I can see how that would be problematic. For sake of argument, if the engine was not running then I cannot think of a reason for there to be noise in the power supply.
    – chew socks
    Nov 12, 2015 at 9:53
  • I may have misunderstood it. You can check the specs yourself, I provided a link.
    – chew socks
    Nov 12, 2015 at 9:53
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    The universal charger can indeed produce a range of output voltages. The reason is that it is universal and each Lenovo laptop generation has its own input voltage requirements, be it 12V, 16V or 20V. The universal charger most likely selects the appropriate voltage based on what connector you attach to it and also based on the data communication with the laptop. It doesn't mean that each Lenovo laptop will be able to accept the whole range of voltages the universal charger can provide.
    – Vojtech
    Nov 12, 2015 at 12:17
  • I did read the linked specs. I think they're miswritten. Nov 12, 2015 at 19:38

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