Having completed a 12-Volt spreadsheet and selected storage batteries for the boat, the next step in building an integrated electrical system is to choose methods of recharging. The standard methods include: main propulsion engine-driven alternator, shore power via a converter or a generator.
Even small electric-start outboard motors come with alternators and voltage regulators. The stock alternator may suit some sailors' usage needs well but often requires replacement for peak charging system performance.
Alternators too small for the system require wasteful hours of engine time to recharge. Too large an alternator is a waste of money and may harm the batteries.
Here are some thoughts on alternators:
Without some method of monitoring, a charging system cannot be diagnosed or understood. An analog ammeter in the alternator output line and a multi-bank voltmeter cost less than $100 and give the necessary information. A solid-state, digital battery monitoring system has many more features but costs up to $750.
Flooded batteries (often called wet cell or lead acid) take relatively high voltage and low amperage for best performance. Use an alternator between 20 percent and 30 percent of the size of the battery bank to be charged. Thus a 300-amp/hour bank should use between a 60- and 90- amp output alternator set between a 14.2- and 14.4- volt bulk charge. Gelcell batteries accept higher charge rates but at lower voltage. An alternator for gelcells should be sized between 35 and 45 percent of battery bank size and set at a bulk voltage between 13.9 and 14.1. AGM, or Absorbed Glass Mat, batteries may be charged at an amperage up to 40 percent of the bank size with a bulk setting up to 14.3 volts. Gel and AGM batteries are sensitive to long-term overcharging and over-voltage. The voltage regulator must be factory-set for the type of battery, or preferably adjustable. These batteries are expensive and care must be taken to set the output voltage precisely.
As the information above suggests, mixing batteries of different types is a recipe for disappointment. Long recharging times are often a function of the voltage regulator, not the alternator itself. Most stock automotive regulators have very rapid output ramp-down and are non-adjustable. Replacing the stock regulator with a modern three-step or multi-stage regulator may solve a slow-charging situation for under $150. If the alternator has an internal regulator, this will need to first be removed and a field wire brought outside the case.
Charging two or more banks of batteries independently can be accomplished in several ways. Firstly, one or more battery switches gives inexpensive manual control but requires constant monitoring. Secondly, battery isolators (diodes) will charge banks independently at relatively modest cost, but large voltage drops will have to be adjusted out to prevent undercharging. Thirdly, electronic battery combiners are fully automatic with high initial cost.
Recharging batteries by converting 120-volt AC current to DC is practical on boats that frequently return to the dock. A wide variety of battery chargers are available.
As recharging time is usually not a concern dockside, a converter can be selected having smaller maximum amperage than the alternator. Small portable, automotive ferro-resonant battery chargers intended for light use will charge small batteries if the voltage level is observed carefully and the charge terminated when full charge is achieved. These constant trickle-chargers often have a 10-amp maximum charge and will damage batteries if left on indefinitely.
Chargers designed for marine use generally have higher continuous output. Most have automatic shut-off, thermal overload protection, vibration dampening, output ammeter and a built-in 120-volt circuit breaker. Many offer two or three independent battery bank charging via internal diodes. Many offer corrosion-resistant board coatings and an aluminum or stainless case. These features are worth the extra money.
Newer models offer three-stage electronic charge control for fastest re-charge. The initial bulk charge is at very high amperage and increasing voltage. Amperage ramps down and voltage peaks in the absorption stage, ending in a float stage with both low current and voltage. These units also prolong battery life.
Many inverters (changing 12-volt DC to 120-volt AC) have high capacity three-stage converters (120-volt AC to 12-volt DC) built-in for recharging batteries. If you plan to install an inverter, these combine everything in one case, often with automatic switching and options for monitoring and controlling the entire system.
Recharging 12-volt batteries at anchor if the main propulsion engine becomes inoperable is desirable for any cruiser.
Many small boats carry a portable gasoline generator. These have limited charging output, are noisy, require dangerous gasoline as a fuel, and their non-marinized components corrode rapidly. However, they are relatively inexpensive and if the batteries are too flat to start the main engine, it would certainly save the cruise.
Larger boats often have a diesel motor generator. The 120-volt output from these can be wired to replenish batteries through the battery charger. In addition, the motor can be fitted with a 12-volt alternator.
Photographs by Kathy Barron
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