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Normal car batteries really hate if you discharge them anywhere close to their capacity. You can get deep-cylce lead acid batteries built for that, but at that

• ryukoposting 2y ago
  That's the whole thing, though - shove a large enough lead-acid battery into the boat/car's electrical system, and you won't need to discharge the battery very far at all. 60W on a 100Ah 12V battery is nearly a full day's worth of battery life. Now consider that the real power draw is probably well below the supply's rating, so it's actually 40ish watts on a 1200Wh battery. Now we're pushing 30 hours. Now consider the real sickos of the "rich boat owner" class. These people have large engines in their boats, and those engines have alternators. All cars have those, too. That's a power source already integrated into the boat itself. A simple DC step-up regulator wired into the boat's existing electrical system is a much more elegant solution than a second set of batteries and a second power source to charge those batteries.