Second battery

You may want to read:

12 Volt Bible for Boats, 2nd Edition, by Miner Brotherton: 12 Volt Doctor's Practical Handbook, by Edgar J. Beyn: Understanding Boat Wiring, by John Payne.

You can probably find them in your local library or order them from any number of marine catalogs or over the internet. There is in all likelihood a YouTube video available too.

Good luck and be safe.
Spuncopper
 
I also like to put in a voltage sensitive relay. This goes between the "starting battery" and the 'house battery". These relays isolate the house and starting battery, yet allow the house battery to be charged from the engine as the starting battery comes up to voltage.

There is an added advantage--you don't have to "switch" batteries, to "combine". You also can have the electronics on one battery, and avoid stress (spikes, low voltage, which can be harmful to the electronics. .

The relays are open at rest--(voltage lower than 12.7). When the engine start battery--connected directly to the engine--comes up to 13.7 volts--the relay closes. It stays closed as long as the voltage is over 12.7. If it drops--as it would when the engine stops--it opens.

You want to have your battery in boxes or on trays. They need to be secure. Also you need to have the terminals covered, so there is not a possibility of a short by a tool dropping one them...etc.

Blue seas makes sort of a "kit" with the switch--off, one and combine--and the really. My son's boat has this. Simple combination.
 
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