How many amps is required to charge a laptop battery with solar power?

Question

I want to build a solar panel with a bunch of solar cells that I have and all together produce 18v and 1.25amps. My battery is 18v and 3.2 amps. So am I correct by assuming that it will take roughly 3 times longer than normal to charge my laptop battery under perfect solar conditions?

Answer 1

19 volts is the usual voltage for laptop power bricks, but maybe yours is really 18.  That 3.2 amps is generally the maximum power drawn, when the laptop is up and running - disks, LCD backlight, and all.  So if you want to charge the battery while the laptop is off, 1.25 amps at 18 volts should be enough.

However, you will need to be certain that your panel will really produce 18 volts and 1.25 amps at the same time.  A panel sold with an open circuit 18 volts is generally intended to run at 14 volts or so, to charge a 12 volt battery.  At 18 volts, the current will be minimal - if you can even get that high on a warm day.

Can't hurt to try, I suppose.  Hook it up, and measure the voltage when connected to the laptop.  If it stays at 18, you're good to go.  If it drops, you need more panel.

Answer 2

You will need a higher voltage than the battery to recharge it.

As the applied voltage increases current will begine to flow into the battery in the reverse direction from when it discharges. The amount of current used during charge will depend upon how much additional voltage above the battery potential is applied, the internal resistance of the battery, and the abiiity of the charger to supply the current at the rated voltage.

The amp rating of the charger is a capability of the charger to supply upto the rated current while being able to supply the rated voltage.  

If your battery demands more current than your charger can supply the charging voltage will be pulled down to a lower potential.   Conversly if the battery doesn't demand much current only the amount of current required by the battery will be drawn from the charger.

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