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Overbanking means having a lead-acid chemistry (flooded or AGM) bank oversized compared to available solar power. The solar:battery ratio would be something like 1:2.
Overbanked designs are generally found when a 'dweller wants to run large inverter loads and forgets how much power it takes to recharge the batteries fully every day. Failure to recharge fully leads to battery murder.
Note: Lithium chemistries are not bothered by partial state of charge so overbanking is a non-issue for them.