12 Volt Electrical Systems

• determine your power requirements

• provide adequate power storage capacity

• provide the proper charging capacity

• set voltage regulation levels to keep the system in balance.

Determining Your 12-Volt Power Requirements

First, calculate your daily (24-hour) average power consumption for all of the electrical loads you place on your system. List all of the appliances and their amp draws. If amps are not listed on the appliance, you can figure amps with the following formula:

amps = watts

              volts

Next, estimate the normal daily usage for each in hours, so that you have a list of appliances and their daily draw in amp-hours (Ah). Now total them all up.

What Size Battery Do You Need?
Let's assume your daily power consumption totals 100 Ah. A 100-Ah battery won't do it. Why? Because battery capacity is determined in part by the intervals between battery charges, and the discharge level. A 100-Ah battery would meet your daily energy requirements, but would have no reserve. And a battery should never be fully discharged. It must be able to store and deliver the full 100 Ah between charges.

Automotive batteries are made for starting engines, with the quick release of a big burst of power. They discharge only about five percent, and are immediately recharged by the alternator. They cannot handle the repeated deep discharges typical of marine use or the constant pounding they receive at sea. Conventional wet (liquid electrolyte) batteries or deep-cycle gel cells are best at withstanding the deep discharges, recharging abuse, and the physical pounding of the marine environment. Look for batteries with the greatest number of life cycles at 50% discharge and do not mix gel cell and lead acid batteries - use one or the other.

You will double the life of your battery if you don't discharge it below 50% capacity. Consider also that batteries recharge rapidly only up to around 70-80% of capacity. If you don't want to spend a lot of time recharging that last 30%, plan on using only about 30% of the battery's full capacity; i.e., the capacity between the 50% you're discharging down to, and the 80% you recharge to. Add in another 20% to account for the fact that no battery operates at 100% over its full life, and give yourself a little power in reserve. Given all this you will need a 400-Ah battery to meet your 100 Ah daily energy habit. In general, a battery rated at four times your daily usage will be adequate.

Engine Starting Batteries

Do yourself a favor by reserving an adequately-sized, fully charged battery solely dedicated to starting your engine. A deep-cycle battery can be used for this purpose, as long as it provides enough cold-cranking amps (the measure of how many amps the battery will supply to the starter motor for 30 seconds continuous at 0°F). Then use a separate bank of deep-cycle "house" batteries to supply the rest of the boat's electrical needs.

Or, you can use two banks of deep-cycle batteries, each with enough cold-cranking amps to start the engine. Parallel the two banks with a dual-purpose battery isolation/selector switch for tough engine starts and then alternate between the two banks for "house" use.

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 your boat.

The rate at which you can recharge your batteries depends on a number of factors, including how much you discharged them, the temperature, the alternator's power (in amps) and its output (in volts).

Most boats charge their batteries with an engine-driven 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. Use a voltage regulator to taper off the charge to prevent this.

Alternators are rated in amps; the rating refers to the maximum output in one hour at a certain temperature and rotation speed. Typical automotive alternators can't run continuously at full load in the warm temperatures found around boat engines, or in warm climates. A suitable marine alternator will be rated at full continuous load at temperatures up to 200°F.

You will need about 120% of the energy you used to restore it. And to prolong the life of your alternator, plan on running it at less than full output. 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 battery capacity, plus any additional loads as mentioned.

The speed of your alternator's rotation is a function of engine rpms 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 bore to achieve the required results. Now check how many alternator rpms it takes to reach that output. Then figure the minimum engine rpms at which you will be charging. You need a pulley ratio that gives you maximum required output at your minimum engine speed. If you need 110 amps, and it takes an alternator rpm of 4,000 to generate it, and your engine is running at 1,000 rpms, then you need a 4:1 pulley. Make sure that if you punch your engine up to 3,000 rpms, thereby increasing your alternator speed to 12,000, you are not exceeding maximum safe alternator speed.

Voltage Regulation

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 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.