Batteries on a Boat

Overview

Battery technology can be rather complex, but most boat owners would rather not know too much about how batteries work (or fail to work). Battery discussion can be divided into four major topics: battery construction, installation, application, and charging. I hope this article will help you decide what type is best for your application, and what is needed to maintain them for the longest service life and reliability.

Application discusses what battery configuration is required for the type of boat you have, how it is used, and the kinds of equipment on it.

Construction deals with the battery plates’ design, how the electrolyte is contained, the number of cells, and whether the battery is vented or sealed.

Installation considers the requirements for securing the batteries in a suitable location protected from hazards.

Charging deals with how electrical energy is restored to the battery after being used for powering the various loads.

Applications

System Designs

Batteries lie at the heart of all pleasure craft DC electrical systems, but there is a wide variation in how DC systems are set up, meaning what purpose is assigned to each bank. The vast majority of boats have relatively simple 12 volt systems consisting of banks of one, two or four batteries connected in parallel. In the typical marine system, one bank is used for starting the engine but is also wired to a battery selector switch as position 1. The selector switch may have positions marked 1, 2 or ALL. The other bank of one, two, or more batteries is hooked up to position 2. By selecting ALL, you combine all the batteries. The ABYC standard requires all boats to have a master shut-off switch.

Battery Construction

Unfortunately, batteries are made in so many configurations and types that there is no quick and easy way to tell you about everything. As a guideline for lead-acid batteries, a heavier battery will outlast a lighter one. That’s because it has more lead.

A 12V automotive starting battery is designed to provide a lot of power for the short time it takes to start the engine, but it should only discharge to a 95% charge, or it starts to fail. A deep cycle battery with very thick plates can be used down to a 50% charge without hurting it. Lithium batteries can go down to a 10% charge, but they are a different story (see below).

Most batteries have caps over a reservoir that holds the electrolyte that surrounds the lead plates and allows the battery to charge and discharge when required. This may present problems like allowing the acid (electrolyte) to leak out of the battery when the boat is heeling too much, so gel-cell batteries were designed with gelled electrolytes. Since then, AGM, or absorbed glass mat batteries, were developed to prevent spillage.

Battery Types

All lead/acid batteries are not the same. The basic types are starting or automotive, marine and deep cycle batteries. That last category name has been seriously abused in recent years by marketers of hybrid batteries that are not true deep-cycle, but a cross between a starting battery and a deep cycle. These will have plates that are slightly thicker than starting batteries, but much thinner than real deep cycle batteries. The most important criteria that determine battery type and performance are the thickness and composition of the battery plates, and they are the factors that most affect cost.

Battery service life is primarily determined by how many times it is cycled, and whether it has been designed to withstand frequent and significant discharging. Cycling means each period of discharging and subsequent recharging. Equally important is how far a battery is discharged before recharging.

Lithium Batteries: If you are considering going to a Lithium-based battery system, I would caution against it. Too many have been exploding and burning boats due to overcharging, getting too hot, or taking an impact that will start thermal runaway. For this reason, most insurance companies are reluctant to insure boats with these batteries installed. However, as of 2025, we are reaching the availability of solid-state lithium batteries, which are reported to be safe and stable.

Starting/Automotive: As its name implies, starting batteries are used to start and run engines, not to run electronics for an extended period. Each application requires different characteristics: engine starting needs very high bursts of amperage for short periods, and electronics use smaller amperage over long periods. Starting or automotive batteries have many very thin (0.40″), highly porous plates to provide the maximum surface area to yield that high burst amperage. The downside of this type of battery construction is that it does not tolerate deep discharging well, and will fail after a relatively small number of deep discharge cycles (about 400 versus 2,000 for deep cycle). Starting batteries are commonly found in outboard and many entry-level boats.

These batteries are also frequently inappropriately labelled as “marine” batteries or auto/marine. Automotive batteries are meant to be constantly charged by an alternator so as to avoid deep discharge. Starting batteries are usually rated by CCA (cold cranking amps) or simply CA (cranking amps).

Outboard boats can get away with using automotive cranking batteries so long as there is no heavy power-demanding equipment. This may include navigation equipment like radios, GPS, fish finder, etc., as these normally use little power. Equipment such as live bait well pumps, trolling motors, spotlights, electric downriggers, video chart recorders and so on, demands deep cycle batteries. However, to avoid annual battery replacement, deep cycle batteries will perform best. When charging is completely reliant on engine alternators, automotive batteries do not tolerate deep discharges (greater than 5%) well.

If you are going to be anchored out and running fridges and other high draw items, you cannot do that on automotive batteries. Get deep cycle batteries and preferably something to charge them like solar panels.

BEWARE: cheap automotive batteries will not run your bilge pumps for very long, particularly after engine failure.

Marine: It seems as if every battery manufacturer today sells “marine” batteries, but some marine batteries are deep cycle, others are hybrids, while others are pure hokum. True marine batteries are designed for dual use of engine starting and house service and are therefore hybrids (not true deep cycle). These will have spongy, porous plates that are significantly thicker than automotive batteries. They will be larger and heavier than auto batteries. A true marine battery will tolerate up to 50% discharge, whereas a deep cycle and industrial battery tolerates up to 80%.

Deep Cycle: These batteries are distinguished by having much thicker plates. High-quality batteries will have solid lead plates versus others made of a lead powder composite. Solid or denser and thicker plates are slower charging but have much longer service life.

Deep cycle batteries withstand greater abuse and thousands of charging cycles (down to 20% remaining) and have much greater service life than the other two types. They do not, however, have as great cranking or burst power, being designed to provide power over longer periods of time. Obviously, the deep cycle is the preferred battery type for marine use; its one drawback of less cranking power is overcome simply by increasing battery size or the number of batteries.

Golf Cart: These batteries are generally a quasi-deep cycle, similar to marine, and though not as good as batteries with solid plates, they are better than the auto/marine types. Usually set up in banks of six-volt batteries, connected in series to make 12 V, these have a greater number of plates to provide longer periods of use under a constant power demand and deep discharging. T-105, US2200 and GC-4 are common identifiers.

Gel Cells: The primary difference between gel cells and flooded acid batteries is that the electrolyte in gel cells has been gelled by the addition of silica gel, turning the liquid into a thickened mush. Once hailed as the messiah of marine batteries, gel cells have since revealed their weakness to being damaged by heat and overcharging as these batteries cannot be fast charged by ordinary fast chargers and require much slower charging rates. Gel batteries sustain a far lower number of charging cycles than wet cell batteries, 2,000 versus 500 cycles for gel cells. This makes them less than ideal for marine applications. Additionally, they do not hold up well in hot engine rooms. The added cost has not proved worth the meager benefit of not spilling acid. Despite the common misperception, the gel cell electrolyte does evaporate over time.

AGM Batteries: AGM stands for Absorbed Glass Mat, which contains the electrolyte absorbed in a mesh of boron-silicate glass fibres. Thus, there is no fluid electrolyte to leak or spill, nor will they suffer from freeze damage. There are two big advantages of this type. First, it can be charged with conventional chargers without fear of damage from modest overcharging. Second, water loss is reportedly reduced by 99% because hydrogen and oxygen are recombined within the battery. Further, this type has a modestly lower self-discharge rate of 1-3% versus up to 15% with standard lead-acid batteries. The AGM is a true no-maintenance battery. It otherwise has similar characteristics to the standard lead-acid battery. The downside is the cost of around 2-3 times comparable standard batteries. Thus, the AGM’s greatest benefit is for installations where it is hard or impossible to ventilate charging fumes, such as the interiors of sailboats.

Installation Requirements

Dry Location: Batteries should be installed in a dry location and at a sufficient height above the bilge that a hull-flooding incident will not immediately submerge them and short them out. Salt water conducts electrical current well enough that when a battery is under water, it shorts out quickly and very dramatically.

Be Aware of Gases: Batteries generate hydrogen gas while charging; hydrogen gas is highly corrosive to most metals and particularly rubber products. It is also highly explosive and should be vented overboard. Hoses, wiring, fuel and oil lines should never be located ABOVE batteries, as this gas is lighter than air and will rise.

Rugged Container: Regardless of type, it is highly recommended that batteries be mounted in rugged, covered plastic boxes specially designed for this purpose. Batteries need to be safely held so they don’t tip over and spill sulphuric acid into the boat; this acid very damaging to all organic materials (clothing, wood, skin) as well as most metals. The box will also contain the inevitable acid leaks. I was called to an aluminum fishboat to examine an electrical problem and found both 8D batteries had melted their cases and spilled acid in the hull. This boat came very close to burning to the waterline, and there was over $100,000 damage.

What this means to the surveyor: the batteries are in a box, which will contain any spilled acid, and they are held down so that a pull equal to the weight of the battery will not move it more than 1 inch. The posts must be protected from a metal object falling on them to prevent a short, and any explosive vapour coming from the battery while charging must have a way to escape outside the hull. Any fuel lines should be kept away from the batteries.

Charging

All batteries will naturally discharge themselves over time at a rate of anywhere from 1% to 15% per month, depending on type. These batteries should not be left uncharged month after month but should be maintained and charged regularly. The total discharge will damage a battery so that it will never take full charge again.

Most inboard-powered boats are fitted with shore power systems and battery chargers to keep the batteries charged. Up until recently, all battery chargers were the ferro-resonance type capable of “trickle” charging, that is, supplying a very low charge rate sufficient to keep the batteries up to snuff. The problem with those older chargers was that they had a bad tendency to overcharge and boil the entire electrolyte away, which damages and eventually ruins the battery. Overcharging is deadly to gel cells. The introduction of electronic, 3-stage chargers in recent years has been a vast improvement in battery maintenance because these chargers are able to sense when the battery cannot take any more charge and then shut off. Therefore, if you have an old charger and are having premature battery failure problems, you’d best replace the unit. Symptoms of overcharging are hot batteries and unusual fluid loss.

Never use a non-marine designed battery charger in a boat, i.e. one that is designed for automotive use. They are usually cheaper, but they aren’t designed to be electronically safe in a marine environment.

There is really no such thing as quick charging when talking about completing a full charge. A quick charge may bring a battery up sufficient charge (75%) to start an engine, but full charging takes much longer at lower amperage to complete the final 25%.

Charging Considerations for Automotive Batteries

If you use an automotive battery to start your boat and run the engine only far enough to get out in the bay then turn it off to drift or sail, your engines alternator will not have time to recharge the amount of power used to start the engine. The battery will be discharged somewhat. When you start the engine to come back into the marina the same thing happens, further discharging the battery.

If you plug into shore power, and have a battery charger, it will top up the battery although it takes a few hours. If you are on a buoy, and don’t have solar charging, the battery stays discharged. Do the same the next day and it gets worse. Remember an automotive battery will only take a couple of instances of a dead battery before they are ruined. It would be better to use a larger deep-cycle battery, as they will start your engine as well.

Battery Testing

The problem with any simple method of testing batteries is that it is only good for proving the negative. That is, you can prove that a battery has low power or is bad, but without a load tester, you can’t prove the overall condition. If you have wet cell batteries, using the hygrometer is useful under controlled conditions, like before charging, when the electrolyte is well mixed. After charging, the electrolyte tends to concentrate near the top and give false readings. But with sealed batteries, all you can do is test the voltage, which will only tell you the present state of charge, not the likely remaining useful life, or how long the battery can send electric current. You might get a very short burst of 12 V DC, but it soon drops off.

The voltage on a fully charged battery should be about 12.7-12.8 volts. If it’s higher, the charger is still operating. Disconnect the charger and wait an hour to see what is actually in the battery. Batteries will usually fail to start an engine at 12V or less. This is dependent on the age of the battery. A new but depleted battery may only fail to start at a voltage as low as 11.5 volts, and an old battery often won’t start an engine at 12.2 V. Using a depleted battery in any mechanical situation puts a lot of strain on all the electrical equipment it powers and will cause things like the alternator to wear out prematurely.

With the engines running, the helm voltmeters are reading through the alternator and are showing the charge rate, not battery state. Read these meters before starting the engines. These meters should then be verified for accuracy by checking the batteries directly with a multimeter.