Dewalt 20v Max 1.3ah And 18v Nano

18v tool solar energy storage cabinet lithium battery should i buy 3a or 6a

For the 12v battery, you can use a UPS 9ah battery or small LFP 12v 6ah battery. Both are very lightweight, very cheap ($25 for LeadAcid, $35 for LFP), and would more than satisfy the demand from the charger. Otherwise, a buck converter directly to the charger should work.