A battery bank is a group of two or more batteries, connected in parallel or series.
By connecting the batteries in parallel, the amperage rating (Amp Hours) increases while the voltage remains the same. For example, three 100Ah batteries on 12V, connected in parallel will result in 300Ah battery bank on 12V.
By connecting the batteries in series, the voltage increases, while the amperage rating stays the same. For example, three 100Ah batteries on 12V, connected in serial will result in 100Ah battery bank on 36V.