Can I charge my PP39L laptop with a USB cable?

you may be getting 5 volts dc but what amps are being generated
if the amperage ( milliamps ) is not high enough , it will not charge

it may show charged but i would get your batterty checked out.

Solar panels produce DC voltage.
some solar panels have the inverter integrated into each solar panel.
I would suggest contacting a local solar panel supplier and asking them if they could add a Battery Charging System to the Solar panels you use. An then what sort of inverter to add to the bank of batteries.
An inverter converts DC voltage to AC voltage to use in the grid.
The AC voltages and DC voltages involved have the possiblity to kill if you much around with electricity.

Naturally due to the size of solar chargers they are going to be a little heavier than a regular charger, but keep in mind that solar chargers are able to charge you device so long as the sun is up.

TAKE BATTERY TO AUTO PARTS STORE FOR A LOAD TEST PROCEDURE, MAKE SURE THE CASE IT IS PUT IN HAS THE DOOR CLOSED FOR YOUR SAFETY, BATTERIES SOMETIMES DO "EXPLODE" AND ACID FLYING IS NOT GOOD. IF BATTERY IS NOT GOOD, I WILL SUGGEST YOUR BUY AN AMPHOROUS SOLAR POWERED BATTERY CHARGING PANEL, THEY COST BOUT 30 BUCKS AND WILL KEEP YOUR BATTERY CHARGED UP OVER LONG PERIODS OF TIME.

How are the batteries for that inverter? Have you got a dead battery in the loop?

If you need further help, I’m available over the phone at https://www.6ya.com/expert/craig_3fa289bf857b1a3c

It should charge just fine connected to your computer as long as your computer is truned on.