Can I fit a second leisure battery in parallel?

QCan I add a second similar leisure battery to the one in my 2020 motorhome to increase my off-site 12V capacity? Presumably, I just link the two with the correct-sized positive and negative wires in parallel? And, does it need to have an in-line fuse?

AMost would do simply as you suggest, that is to link the two positives together with a stout cable and link the two negatives together with another stout cable. This stoutness is determined by your maximum loads.

For example, if you have an 1,800W inverter then these two cables may well be a 35mm2 cross section. No inverter? Then perhaps a 6mm2 cross section.

However, there is a fault scenario that still exists and that is what if one of the batteries suffers from an internal shorted cell?

This sort of fault is mostly associated with wet technology batteries and caused by the lead paste falling off the cell plates over time and forming a build-up in the bottom of the cell box, eventually leading to the negative and positive plates being shorted out. When this happens you then have a 10V battery!

Having a 10V battery connected directly in parallel with a 12V battery can lead to some hefty currents between the two batteries as the 12V battery gets pulled down by the 10V battery.

For this reason, my recommendation is that each battery should have its own in-line fuse and the positive paralleling taking place after these fuses. In the application with the 1,800W inverter, these two fuses will be in the order of

100A rating. Then for the separate outgoing smaller fuse to the “normal” motorhome leisure circuits.

Without that inverter then in-line fuses of 25A would be sufficient, again with the original outgoing fuse to the leisure circuits as fitted by the converter.