This is the photo of my battery installation in AUS174. It s a jumpstart battery pack bought on special for $39.00
We modified it for use on the boat by removing the jump start leads, installing a ON/OFF switch shown on the right.
The unit has a cigarette lighter socket on the right for your external power out connection.
The unit is mounted under the sink and tied to the forward bulkhead using 3 stainless steel saddles . The main advantage over a stand alone battery is that the enclosure houses a battery metre, a low voltage indicator, and a 240VAC charging pack.