While not a perfect analogy, it is useful to think of voltage as pressure. Just as applying too much pressure to a balloon for too much time can ruin a balloon, applying too much electrical pressure (voltage) ruins batteries. Batteries are charged by applying a higher voltage (electrical pressure) than a battery normally has. That forces a chemical reaction in the battery that stores energy in chemical form that is later turned into electrical energy when power is required.
For this process to occur effectively, charging voltage must be applied for the appropriate time and must be adjusted up (for cold batteries) or down (for warm batteries). A charger’s failure to properly manage these three variables - VOLTAGE, TIME, and TEMPERATURE, results in batteries that are chronically undercharged or overcharged. This delivers less energy for the boater’s safety and comfort and results in expensive premature failure of batteries.