Thanks, guver. But makes sense to me that to charge a 18v battery using 12v input is complicated. That's why the manual says:

"• For lithium based batteries rated at 11.1V or greater, the actual amount of current delivered to the battery might be reduced due
to the charger’s maximum power limitation rating. Various circumstances such as battery condition, low input power, etc. can
contribute to such a condition. This is normal."

Thanks, again.