The energy storage system is essentially a straightforward plug-and-play system which consists of a lithium LiFePO4 battery pack, a lithium solar charge controller, and an inverter for the voltage requested. Price for 1MWH Storage Bank is $774,800 each plus freight shipping from China.
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.
5 of NFPA 855, we learn that individual ESS units shall be separated from each other by a minimum of three feet unless smaller separation distances are documented to be adequate and approved by the authority having jurisdiction (AHJ) based on large-scale fire testing.