If you own a small boat and you only need battery power at dockside intermittently for things like lights and bilge pumps, an SCR (silicone-controlled rectifier) charger may work best for you. It varies its charge to keep your battery at a constant 13.4 volts.

Live-aboards and larger boats with many 12-volt accessories usually have ferro-resonant chargers that put out 12 volts continuously to keep up with constant power demands.

Experts agree that the best way to replace the energy you consume from your boat's batteries is through a controlled, multi-stage process. Both conventional lead-acid (flooded) batteries and gel cells will charge faster and last longer if they are charged in distinct phases that take into account their chemical and physical complexities. The recommended three-stage charging process works as follows:

1. Bulk Stage: This first stage provides a constant amperage bulk charge of 25-40% of the battery's capacity (in amp hours, Ah) up to about 14.4 volts (14.2 for gel cells). This bulk charge will restore about 75% of the battery's total capacity. It takes less time than tapering chargers, like ferro-resonant and SCR chargers, because the smart charger delivers greater current to the batteries which can accept greater current loads when they are discharged.

2. Absorption Stage: The remaining 25% of capacity is restored at a decreasing rate. Maintaining the battery at 14.4 volts (14.2 for gel), the amperage is steadily reduced. The battery is considered fully charged when it will accept only 2-4% of its amp-hour capacity at 14.4 volts.

3. Float Phase: The charge amperage has declined to 2-4% of the battery's capacity, and voltage drops to 13.5 (13.8 for gel cells). This maintains the battery without losing electrolyte from gassing.

4. Equalization: Applies to lead-acid batteries only. This fourth, manually triggered stage prevents lead-acid batteries from aging prematurely by applying a small, constant current until the battery reaches 16 volts, which dissolves the hardened lead sulfate crystals on the battery plates and prolongs battery life.

How do you know when you're fully charged? To save engine running time, many boaters charge their batteries to only 80-85% of capacity. But you can maintain a better, balanced energy system aboard your boat and prolong battery life by installing an amp-hour meter or percentage meter. (The meter will tell you how much battery capacity you have discharged. If you follow the experts' advice and never discharge below 50% of capacity, the amp-hour meter will let you know when it's time to begin charging. When the battery is fully charged, the amp-hour meter will read "0." Your charging time will be more efficient, and you'll rest assured you have enough power to start your engine.

An amp-hour meter will also let you monitor your charging so you can decide how much time you want to spend in the absorption phase to gain how much battery capacity. A good rule of thumb for achieving adequate, but less than full charge in a reasonable amount of time is to bring your battery quickly through the bulk stage to the absorption phase, and then let the amperage decline to about 10% of your battery capacity; i.e., charge a 200-Ah battery until it accepts about 20 amps at 14.4 volts (14.2 for gel cells.) If you don't usually recharge to full capacity, remember to top off your batteries to full charge periodically to prolong their life, and to "zero", or reset, your amp-hour meter when your battery is fully charged.