Connecting second battery

[wetbirks]

I would like to connect my second battery.

In the battery compartment there is a second wire coming from the engine as well as a short wire that looks like it would be a leash between the two batteries. I'm a little confused because they don't have standard terminal clamps on them. I'm not sure how to make in the connection. You can see the loose wires at the bottom of my photo.

Any suggestions on how I complete this connection?

[Antibes]

The larger black should connect negative terminals. Can use threaded or friction connection. The small gauge wires are from the charger one to each threaded post. I don't see the larger positive for the add on battery

Replace the large gauge wire on your primary battery before it causes you fits.

[joe_db]

I am not trying to be snarky - please buy this book!

https://www.amazon.com/Boatowners-Mechanical-Electrical-Manual-4/dp/0071790330/ref=sr_1_2?dchild=1&keywords=nigel+calder&qid=1627326875&sr=8-2

You are trying to learn a lot from a forum here. We all love to help, but some things are not all that easy to pick up with piecemeal questions and some of those things involve dangers to people and property.
Seriously - I worked in this business and I still have a copy. It will save you the purchase price about the first time you use it.

[ronstory]

Joe is pointing you correctly, Nigel C certainly knows what works and doesn't on boat. I have that book as well plus Don Casey's https://www.amazon.com/Caseys-Comple...7327874&sr=8-1

If you opt not to invest in one of the books, at least start here with Rod Collin's website. There is a wealth of knowledge reflecting his experience on that site.

https://marinehowto.com/1-2-both-battery-switch-considerations/

[sastanley]

It looks like your boat originally had a common ground. Ugh. When I rewired I got rid of all this cost saving and ran dedicated wire runs to everything. +1 on Nigel's book & Don Casey's book. It is as valuable as the Moyer Manual.
edit - a common ground from battery to battery will work just fine to connect the 2nd battery, but I personally would not leave it that way.

[wetbirks]

Got the books recommended above. Batteries all wired up! Most wires upgraded, so looking much better.