• Determine your power requirements

• Provide adequate power storage capacity

• Provide the proper charging capacity

• Set voltage regulation levels to keep the system in balance

• Provide two independent batteries, preferably an engine side and a house side

Battery Charging 

If you don’t replace what you use, your batteries will eventually go dead, no matter how much battery capacity you have on board. The rate at which you can charge your batteries depends on a number of factors, including how much you discharged them, the temperature, the alternator/charger’s power (in amps) and its output (in volts). Boats may have several ways to recharge its batteries. Two of the most common are by engine driven alternator or by a battery charger using AC from shore power or from an engine driven AC generator set. There are also wind and solar DC power generators.

Alternators

While under way, most boats charge their batteries with an alternator. If your engine running time is minimal, you want to charge as quickly as possible, without damaging the battery. Battery damage begins when the internal temperature becomes too high, causing it to gas and heat up. If it feels warm to the touch, it’s getting too hot. The voltage regulator, which tapers off the charge to prevent overheating, may be defective or improperly adjusted.

Alternators are rated in amps, the rating refers to the maximum output at a certain temperature and rotation speed, You will need about 120% of the energy you used to restore it. Take into account any other power-draining loads you might be adding to the system as you are recharging, such as refrigeration. If you install an oversized alternator you can recharge efficiently while at anchor, with the engine at idle and the alternator operating below its rated speed and output. In general, charging capability should be approximately one third of the battery capacity, plus any additional loads mentioned.

The speed of your alternator’s rotation is a function of engine RPM and pulley size. Once you have determined the maximum alternator output you require, add 25% so you won’t have to operate it at full speed to achieve the required results. Now check how many alternator RPM it takes to reach that output. Then figure the minimum engine RPM at which you will be charging. You need a pulley ratio that gives you maximum required output at your minimum engine speed. Make sure that if you increase your engine speed up to 3,000 RPM thereby increasing your alternator speed to 12,000 RPM you are not exceeding the maximum safe alternator speed.

Voltage Regulators

The voltage regulator maintains voltage at a certain level by matching alternator output with the load and the charge level of the battery. Voltage drops when a load is placed on the power system, or when the battery discharges. The regulator then increases the amperage output of the alternator until the voltage level is restored, and then tapers output to a level that will sustain that voltage.

You should have a regulator that is external, field adjustable, so that you can tailor the settings to your specific power needs and charging patterns. If your engine running time is minimal, you may need to a high setting, like 14.4 volts, to get the fast charge you need without damaging your batteries. If you run your engine for extended periods, 13.8 volts may be adequate. Multi-stage regulators, as well as multi-stage chargers, are highly recommended for gel cell battery applications.

Battery Chargers

Small boat owners who only need battery power when at dockside intermittently for things like lights and bilge pumps, a high-frequency switcher battery charger may work best for you. It varies its charge to keep your battery at a constant 13.4 volts. It should be automatic and have multiple charging capabilities. Larger boats, especially live-aboards, with many 12-volt accessories usually have ferro-resonant chargers that can supply 12 volts continuously to keep up with constant power demands. This chargers need to be fully automatic, self-regulated and ruggedly built to give many years of dependable service. However, the newer microprocessor based high-frequency switcher chargers designed for marine use are now available in larger capacities and have become more reliable in the harsh marine environment. They are known to provide fuller, quicker charging and also extend the life of those expensive batteries.

 The best way to replace the energy you consume from your boat’s batteries is through a controlled multi-staged charging process. Both conventional lead-acid (flooded) batteries and gel sells will charge faster and last longer if they are charged in distinct phases that take into account their chemical and physical properties. The voltage levels required are both very precise and temperature dependent. The acceptance and float voltages for wet cells at 40 degrees and at 90 degrees vary by one volt. The recommended four-stage charging process works as follow:

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. Smart chargers take less time because they deliver constant-current (amperage) output during the initial bulk stage. Conventional chargers taper off amperage output throughout the charging cycle until 14.2 to 14.4 

2. Absorption Stage – To prevent driving up the voltage beyond what the battery can safely accept, the remaining 25% capacity is restored at a gradually decreasing amperage rate, while maintaining the battery at 14.2 – 14.4 volts acquired at the end of the bulk stage. The reduced amperage output gives the battery time to absorb this last 25% of energy without damaging the plates. The battery is considered nearly fully charged.  

3. Float Stage – To maintain battery capacity, the charge amperage decreases to 2-4% of the battery’s amp-hour capacity while maintaining a constant voltage output of 13.2 to 13.6 volts, which is enough to meet ongoing DC power demands and avoid overcharging.  

4. Equalization Stage – This conditioning phase is essential for obtaining the maximum life expectancy from wet-cell batteries. To achieve this, a small constant current is applied until the battery reaches 16 volts. This dissolves the hardened lead sulfate crystals on the battery plates and prolongs battery life.

How do you know extend battery life?

To save generator engine-running time, many boaters charge their batteries to only 80-85% of capacity. Boaters also tend to discharge their batteries below 50% of its capacity. Both of these practices will shorten battery life and give you less than an optimal energy system aboard your boat. A good charger can achieve adequate, but less than a full charge in a reasonable amount of time by bringing your battery quickly through the bulk stage to the absorption phase. It then lets 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 do not usually recharge to full capacity, remember to top off your batteries to fully charge periodically to prolong their life. Be sure to use a fully automatic battery charger with multiple charging capabilities to allow you shorter run time on your generator while charging the batteries. Back at the dock, it also means you can leave the charger on full time without worrying about over charging.

Battery Combiners and Isolators

By separating engine and house batteries, you face the challenge of trying to charge them from a single source like single source chargers or alternators. By using a combiner or isolator a single charge source can be used because they sense the voltage of both battery banks and connect the batteries together whenever one battery’s voltage is elevated. Both banks charge simultaneously and remain combined until the voltage drops, whereupon they are disconnected from one another.

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