I can put things into perspective for you. A couple of weeks ago we headed out to the lake and backed the boat in just as any other normal outing. Went to start the engine before totally backing off the trailer, hit the key and nothing, nada, zilch. Pulled out the water, parked and scratched my head. Checked connections, lanyard, etc - all the typical stuff. Found out I left the main power switch turned on since we last used the boat, which had been a couple of weeks. Luckily I had my lithium jump pack along. Hooked that up to the battery, let it give a bit of charge, hit the boost button and it gave it enough to get the engine started. Knowing my jump pack works, we went on with our day. Ran around the lake for a good 15 mins at a decent rpm and shut the engine off just to see how much charge I had gotten. Engine roared to life as nothing had happened. So we went tubing, cruising, sandbar swimming, etc like nothing had happened. But, we didn't use the stereo when beached and I sure as heck took the jump pack along. Then put it on the charger when we got back home. So long story ..... but yes, the engine can put charge back on the battery when fully depleted.