Solar voltage should be reduced to 12.6V
Solar energy cannot directly charge 12V lithium batteries. 12V lithium batteries are generally made up of three 3.6 V cells connected in series, for charging. The voltage should be 12.6V. If the charging voltage is too high, it will damage the battery. In other words, you need to reduce the voltage of the 18V solar panel to 12.6V. You can adjust the output voltage to 12.6V to power it. Battery. It is not a big problem if the voltage is lowered, but do not increase it, otherwise it will affect the battery life and damage it.
By connecting two solar cells in series, you can obtain a voltage of around 6V. Then connect a diode then a USB socket (female), which can charge most mobile phones and MP3 players.