Powering netbook from USB

I have an old netbook (Asus Eee Pc, 1001PX) that still works quite well for occasional use. It is powered by a 19V 40W power source. I consider powering it from a USB port using a 5V to 19V current limited step up converter. With maximum input current 500mA to stay within USB specs it would be able to provide only about 2W power. It is surely less than the power needed to run the netbook. But ideally it will slow down the battery drain when the netbook is running and be able to recharge considerable part of the battery overnight. Do you think it may work this way? Will the netbook reject such weak power source? Or is there even a risk something will be damaged by this?

The most battery drain is likely to be the back-light for the display. I doubt you can back feed power to that circuit. Think about the internal voltage regulation necessary to get 5 volts from 19 volts. How can that work in reverse?

Paul

I want to use some other USB power source (phone charger, another computer) to charge the netbook via its 19V charging connector. To have much smaller and lighter but also weaker power source.

So I have tried it using the MC34063. Sadly the netbook is too greedy: it draws a few mAs for about a second (probably start up time of the battery monitoring circuit); after this time it starts drawing "huge" current causing a large voltage drop to ~ 14V. After this it stops charging, voltage goes up again and the cycle repeats.

Guess it's time to do the math again.

Paul

What math? A random USB charger is rated for 500mA@5V = 2.5W. The netbook wants more and I don’t see a way to fix it.

(A long time ago I had a notebook and its power source started failing. It was unable to provide the power the notebook needed yet the notebook was able to work with it, slowly discharging the battery despite being powered. I have hoped for similar result.)

Charging circuits in laptops/notebooks are smart - if the power source is inadequate they will simply
reject it and switch off charging.

40W is what it wants, 40W you have to give it.

There is a small chance it might accept it when in trickle-charge mode with the notebook asleep...