Generally, you can connect your batteries in series or in parallel, but you must pay more attentions the following aspects:

1) When you connect your batteries in series, it is very concerned about the battery condition, you must make sure that the battery’s features and capacities are same; generally, you must choose the fully new batteries with same brand, same model, even same manufacturing batch to engage the connection in series, to make sure the individual battery voltage is in an equal state, if the individual battery in a series connection does not balance, it will shorten the working life for certain batteries.

For example: two 6V batteries are connected in series to become a 12V battery, or four 6V batteries are connected to become a 24V battery, or two 12V batteries are connected to a 24V battery; If the battery capacities are different in a series connection, the battery with low capacity will be fully charged firstly, at that time, the battery with higher capacity is still not fully charged, the solar controller just monitors the total voltage of battery series, this will cause an overcharge on the battery with low capacity and generate the heat; also, it will cause an undercharge on the battery with higher capacity and make the battery sulphated.

2) When you connect your batteries in parallel, in theory, you can connect the batteries with different capacity, but you must select the same chemical battery. Since the charging voltage is different for different chemical batteries, for example, Gel battery and WET battery requested different charging voltage.