Here is a quick overview of an efficient, modern boat charging and battery system, with a few tips on how to wire things so you’ll know what they are and be able to work on them later.
Most pleasure boats have a 12-volt DC (direct current, the same as in a car) electrical system that powers devices, such as lights, radar, autopilot, fresh water and bilge pumps, etc., any time and a 120-volt AC (alternating current, the same as in a home), powering outlets, battery charger and high-demand devices such as microwaves, as well as tv’s and other electrical devices commonly available in 120V AC. Grounds on both are common at some point and an isolation transformer is needed to ensure a clean AC ground, entering the boat, from the dock.
Batteries are of three basic types: Wet cell batteries are the cheapest but require regular filling with distilled water and spill if not upright, thus not suitable for sailboats, which often aren’t upright. Sealed wet cell batteries are also inexpensive, maintainance-free but will spill if not upright. Gel cells are maintainance-free, have a slightly lower maximum charging voltage and slightly lower cranking amperage but are more durable for deep discharging, won’t leak and are roughly 1.5 times as expensive as wet cells. They were revolutionary when they came out in the early 1990s but are eclipsed by today’s AGM batteries. AGM (absorbed glass mat) batteries are amazingly durable, resistant to vibration, won’t leak, can be mounted on their sides, have excellent cranking amperage, high maximum charging voltage, are durable for deep discharging and are twice as expensive as wet cells.
Battery quality varies considerably: Buy cheap and often or buy quality and rarely. Cheap batteries generally last two years and high-quality batteries can last a decade, if properly charged and not stressed. Beware of house-brand batteries sold by retailers. The best batteries have many thin, convoluted lead plates squeezed together with thin glass mat in between. They are much heavier than normal batteries and provide greater storage as well as faster and deeper charging and discharging. The factors in choosing a battery are their footprint (their size), amp-hours (total electrical storage), cold-cranking amps (CCA or peak discharge), warranty and cost. I look at the size of the compartment, then the size of batteries I can fit in it and then budget and battery details. Replacing all batteries at the same time avoids the weakest constantly draining the others and bringing newer battery lifespans to the level of the oldest. This, corroded cables and ancient/underpowered chargers are the usual reasons battery banks fail.
Battery chargers need to be set for the type of battery: Wet, Gel or AGM. They need to be at least a three-stage charger: Flood stage, to quickly build the charge of a battery to maximum voltage; fill stage, to fill the battery at a variable amperage, while maintaining a stable amperage and a maintenance stage, which trickles small amperages and keeps the battery topped up. Marine chargers are mandatory, as others will rust to death amazingly quickly and not being made for the marine environment, are often subject to chassis shorts and fire hazards. They should charge at least one amp for every ten amp-hours of each battery bank they charge with each charging lead: If you have three 100 amp-hour batteries, you will need a 30-amp charger with three 10-amp charging leads or if the batteries are in one bank, a 30-amp charger with a 30-amp charging lead. Modern marine chargers have excellent capabilities and measure the state of the batteries they charge regularly as they charge, for maximum efficiency. They run much cooler and quieter, are smaller and charge so much better that they are well worth buying to replace chargers that are 20 years old or older: They will extend your battery life that much.