I am a relatively new owner, so I am wondering about running music when the boat is anchored and motor shut down. How long can I do that without being concerned about running the battery down and not being able to start the engine?
Might be wise to get a plug in volt meter for the cigarette lighter style power port. That way you can make sure battery doesn't get much below 12vdc. If you have a fish finder, it might also display voltage. Engine running reading is 14.6 vdc approx and then engine off maybe 13vdc then will trail lower as it discharges. Depending on how new and CCA, etc specs of battery, it could be hours if basic radio with no powered subwoofer. Maybe wise to carry a charged jump box (for jumping dead battery) as a safety backup. My newest boat has factory dual batteries so I can select a single one or the 2 together so I could just risk running down 1 and still switch to other to start engine. If you do go too far running down battery, immediately turn off all accessories and try to start after 30 minutes as battery will recover a small amount maybe just enough to get out of a pickle.
I have a single battery and run the radio and depthfinder for several hours and have not had a problem. As the battery gets older, it may become a problem. The suggestion of a jump box is a good one. I take mine with me, whenever I remember to.
We only had 1 battery in our first 2 boats. We always carried a jump pack. My kids ran the battery down in our bowrider playing the radio all day. The battery was so dead the jump pack couldnt start it. Then we found out the internal battery in the jump pack was dead. We were told if you don't use it regardless of how much you charge it they go bad too. Because of this incident that happened back in the 90's we have always been afraid to play the radio while anchored for a long period of time. (Some things you don't forget and they stick with you forever) Our current boat has 2 batteries. But my wife has a bad ass portable Sony Bluetooth speaker that she brings on the boat from time to time and she'll connect Sirius/Yacht Rock radio through her phone and play it while anchored.....Some habits die hard.
The age of the battery will matter too. Their lifespan seems to be somewhere between 3 and 7 years (size of course dictates as well). If it's a brand new battery, you could probably go all day and be fine. A 3 year old one, maybe half that. A 6 year old one, maybe don't shut the engine off!
The jump pack is a good suggestion. The new, small ones that use lithium batteries are great and very inexpensive. At the very least carry jumper cables. With cables ANY boat can help you. Without cables, one of out ten maybe could help you.