The parallel connection serves to multiply the capacity of the battery while the voltage remains constant. To do this, all the positives are connected on one side and on the other side, all the negative poles.

Mixed serial and parallel connection:

This type of connection is made in installations at 24V where you want to connect more than 2 batteries at a time. In this case, batteries should always be placed in pairs, ie in pairs. It is not recommended to connect more than 8 12V batteries to each other, as this may damage the installation because of performance losses.

Attention: Never connect the positive and negative poles of the same battery together. This will result in a short circuit and irreparable damage to the battery. Also do not connect several batteries in a circle, for the same reason.
The most important cables of a solar installation are those that are used to connect the batteries to each other and the one that connects from the battery to the inverter. These cables must be sufficiently sectioned.