What happens in a battery that goes dead is the acid will harden when it no longer has a charge. The longer you go without a full charge on your battery, the more it will harden and it does shorten the lifespan of your battery. This is why maintaining a marine battery is so critical. If the “insides” become hardened, your battery will no longer hold a charge as long as it should. This could result in you taking it off your charger and it immediately losing the surface charge it may have acquired. In the end, you will be out of a battery until you purchase a new one. Wouldn’t you rather spend a few dollars on a gauge instead of spending much more on a new battery?