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I need your brains

Sgt (Ret) Tred

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I need a little help with a project that I'm currently working on to keep my laptop charged for a minimum of two hours.

The plan is to:

Use a CCTV power supply to charge a car battery, the car battery then will keep my laptop charged using a universal laptop charger (which converts 12V to 19V).


The case now is, my laptop needs 120W and the universal charger provide 90W.  


Will the 90W charger damage the laptop in any way? 



Thanks in advance



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The worst you'll get out of that is the laptop running slow or not starting due to an overdraw.


You've basically just constructed an inline UPS, which should be fine. The thing with the wattage is that to get that power output, your transformer is stepping 12V up to 19V and producing a certain amperage.

Plug that into P=VI (I = 90/19) and you get an output current from the power supply of 4.7 amps.

Do the same with the expected wattage (I = 120/19) and EXPECTED input current for the laptop is 6.3 amps.


A significant overvolt may kill receiving hardware.

An overdraw of current at the receiving end will kill things in the middle if there's enough voltage (like water pressure) to push it.

An overdraw of current at the receiving end will only under-power the receiving end if there's not enough voltage to sustain the draw. The voltage will suddenly drop and the equipment will turn off or dip in performance.

A power supply capable of producing more than expected current will not generally (lets not get stupidly specific) harm receiving hardware if the voltages are about equal. This is why people will put 600W power supplies in computers that will only use 300-400W - it's like a safety net for when your computer is drawing max power so it doesn't cause a voltage drop and kill the computer.



So if you can get a power supply that'll go 12V to 19V at 120W or more, you'll be in much better hands. 90 might work, but the computer may fail to remain powered or may perform like crap. A supply with an output of 19V @ 120/150W will ensure power to the laptop.... just make sure you're not overdrawing the battery or putting too much through the supply. If you have a meter, check for significant voltage drop at the battery when the laptop is turned on, and be wary of the supply getting too hot. If it does, remove the equipment. I don't anticipate you overdrawing the car battery... but again, temperature is your indicator. If stuff starts getting hot, watch out.


Some of this may be poorly worded, but the radio systems I work on have inline UPSs EVERYWHERE, so we get pretty familiar with their operation.




Also, this is what I thought of from the thread title:


Edited by Cpl H. Taylor
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