What must match before parallel connection
Matching voltage alone is not enough. Use batteries approved for parallel operation by the manufacturer, preferably of the same chemistry, model, capacity, production state and firmware. Check the maximum permitted battery count and the required BMS communication or master–slave arrangement.
Make the connection as a battery bank
Before connection, compare each battery's voltage and state of charge. Connect each battery to a correctly rated common busbar with similar cable length and conductor size. Each branch needs its own DC fuse or breaker and isolation method. This limits fault current and makes maintenance safer.
Check the complete power path after expansion
Parallel capacity does not remove current limits. Recheck inverter DC current at low battery voltage, each battery's continuous and peak BMS current, charger and MPPT current, busbar rating, terminals and cable temperature. A BMS is a protective layer, not a replacement for DC distribution protection.
Do not mix batteries casually
Different brands, capacities, ages or unknown histories can charge and discharge differently. One battery may disconnect first, leaving the remaining battery to carry a larger load. If an existing bank must be expanded, obtain a compatible expansion method from the battery supplier first.