If 18V2A is the output parameter of the charger, a 12V lithium iron phosphate battery (above 2Ah, the protection board can last for more than 2A) can be used in emergency, and long-term use is not recommended.
If it is a 12V lead-acid battery, it can be used for emergency, and it will be overcharged after long-term use.
If it is not the above two, it is not recommended.
