Marine battery basics

Mark Rothfield | Volume 29, Issue 5
The positives and negatives of marine battery care and maintenance.

Contrary to the popular theory espoused in The Hitchhiker's Guide to the Galaxy, the answer to the question of life, the universe and everything is not "42". It's electricity.

An electrical impulse spawned deep in the sinoatrial node spurs the heart to beat. It's the means by which the brain communicates and contemplates, allowing you to read and comprehend this sentence. Even the most guileless of flowers generates electricity to communicate with bees for life-giving pollination. 

Without it they, and we, are nothing. The same goes for your boat. A world without watts is one of mute motors, lifeless lighting and defrosted fridges. Bilge pumps rendered impotent as bricks; depth sounders powerless to warn of shallows; GPS sets saying "GeePerS, we're lost!" No windlass, no thruster, no radio to issue a distress call. 

You are, effectively, all at sea in the modern marine vessel. So a battery deserves love and attention, care and affection... not total disregard. But very few boat owners are 'switched on', so to speak, until they're switched off and plunged into doom and darkness. 

Perhaps we're blasé because the technology is two centuries old, when Alessandro Volta hypothesised that electricity was generated when two dissimilar metals met a moist intermediary. 

Today, though, there are numerous battery types to suit specific roles, priced from the tens to ten thousands of dollars. Yes, they come in the same ubiquitous oblong case, they all deliver the same quantity of volts, but there's a stark difference in construction.

The first commandment of marine battery maintenance is: "Thou shalt not covet thy car battery", for it won't cope with the harsh vibration dispensed by the marine environment. Cheap is nasty in a marine context. 

That said, expensive isn't necessarily best, either. There's a quality variance between popular battery makes, according to the experts, with specialist stores generally only supplying and supporting five or six of the 30-odd brands commonly available. If in doubt, ask for the impartial advice of a qualified marine electrician. 

The second commandment warns: "All batteries art dangerous. "The sulphuric acid (electrolyte) contained within a lead battery is corrosive, causing skin and respiratory irritation, eye damage... even tooth enamel erosion. 

Hydrogen is emitted from lead acid batteries during charging, while a battery immersed in saltwater can create a toxic chlorine gas that's harmful to the airways, eyes and skin. Lithium-ion batteries - the new kids on the block - also have a fiery past, although the use of smart control systems to monitor their temperature and regulate their charging has yielded major improvements. 

Lithium contains sodium and potassium, both highly reactive. Internal damage can lead to a short circuit, triggering a 'thermal runaway' that may lead to combustion. The other critical issue is to charge them within a safe limit. To avoid sparking, disconnect the negative battery cable first and reconnect it last. Loose battery connections can ignite flammable fumes so make sure they're tight. Also, remove your jewellery before working on a battery because a short-circuit current can 'weld' a ring or bracelet and induce severe burns. 

Maintenance tools can be insulated with layers of electrical tape and it pays to place rubber boots on cable connections in case a tool contacts a terminal. For the same reason, use a plastic brush instead of wire bristles when cleaning terminals.

Marine sparkies, like Rohan Collibee, at Lake Macquarie Marine Electrical in NSW, don't take chances: "Lead acid batteries start to vent quite violently when they are being overcharged - put a spark in that environment and, bang, yes it will explode," he says. 

"When I climb into a pleasureboat I let the bilge ventilate for a few minutes and make sure everything is isolated. Battery compartments should always be vented outside the hull." 

The lament of Clive Weatherhead, from Sydney-based supplier, Battery Business is that battery placement is often ill-considered by naval architects. 

"I just wish boat designers would change a few batteries themselves because they wouldn't locate them in the places they do," he says. "When I die and go to heaven, I intend to meet every boat designer I can find and spend some time chatting with them."

Ideally, trailercraft batteries should be located high enough so that they are unlikely to ever come in contact with saltwater. 

Portability becomes a weighty concern when considering safe lifting limits. With single 200-amp batteries weighing up to 70kg, two 100-amp batteries connected in parallel can be a preferred option and will improve handling and placement flexibility.

For boats over 35 feet, 24-volt systems are an option - either two 12s or four 6-volt batteries rigged in series (positive to negative). By doubling the volts you halve the current required, enabling smaller cabling to be run. Remember, series gives more volts for the same amps, parallel gives more amps for the same volts. 

With windlasses and thrusters, it's recommended that a separate battery be housed nearby, with some hefty cabling. It pays to parallel in with the house system through a good solenoid because the last thing you want is for the thruster to fail.

Some basic math is also needed to calculate your DC system's draw, with energy being measured in ampere hours (AH). The equation for two lights that draw 1.5 amps for five hours is simply: 2 x 1.5 x 5 = 15AH. 

You can do the same with watt hours, dividing by 12 to calculate amp hours. For vessels with AC power running through an inverter, the conversion formula is AC watts x hours/10 = AH/day. 

Compile a list of all the electrical components on your boat, calculate the total consumption between charges... then treble it. In other words, if the figure is 150AH, you need a house bank capacity of 450AH.

Aim to never draw more than 40 per cent of a lead battery's capacity between charges. With a 200AH unit, that means drawing fewer than 80 amps on a regular basis. 

To give some different installation examples from Clive Weatherhead, a single battery will suffice for a 15-foot tinnie running basic electronics and lighting. If used regularly and for extended periods, though, it's worth considering a second battery to supplement the starting battery. 

"Most twin-battery installations have a switch, but it's a good idea to add a voltage-sensitive relay between the two batteries that automates the charge when the motor starts," Weatherhead adds. "It costs around $120 in parts and makes life so much easier." 

A 22-foot sterndrive runabout used once a month, on average, needs separate cranking and house batteries, the latter being at least one N200 200AH battery. 

On a 35ft cruising yacht with diesel propulsion and a solar panel, Weatherhead would expect to see 800AH of house battery, though some owners survive with just 600.

Moving up to a 50-foot sportscruiser, you're in the realm of dual cranking batteries per motor and either one considerable battery bank that runs the inverter and DC house or a separate inverter bank and house bank. Bigger trawlers cart up to 20 batteries, providing 200AH each for house.

A battery's lifespan will suffer if you short-change on capacity. A small bank drops voltage quickly when subjected to load and the more you take out of a battery the faster it deteriorates.

Most boaties want all the 240-volt luxuries of home, like air-conditioning, but Rohan Collibee and others prefer to set up boats so they don't rely on generators running continuously.

"You can be more independent with solar panels and wind generators," he says. "I use appliances like 12-volt televisions and 12-volt fridges where possible, and people can charge their laptops and smartphones on 12-volts these days."

By running the boat's motor(s) during high-draw situations - operating the windlass and thrusters - the alternator bolsters the battery bank. They largely run at 14.2 to 14.4 volts under charge, dropping once load is applied.

Most people only replace their existing batteries when they fail or there has been a crisis such as submersion, whereas ongoing vessel usage really needs to be considered - installing new electronics or transitioning from day trips to extended stays, for example.

The third commandment is: "Batteries perform different functions and shalt be chosen accordingly."

Those designed for cranking (starting) have thin lead plates with extra surface-area contact for the acid solution, allowing them to deliver punch fast. They don't store a great deal of energy, however, being designed to discharge no more than two or three per cent before recharge.

A deep-cycle battery, as favoured for house usage, sports thicker plates and stores energy that can be dispersed over a longer period. As the name suggests, they're better at being deeply discharged.

The compromise for smaller boats is a single hybrid battery, roughly halfway between starting and cycling, and generally kept fully charged. From there you move into the different types.

Bargain basement is the traditional 'wet' lead acid type that requires maintenance. They're absolutely fine for the serious boatie, who goes out often and is prepared to check electrolyte levels every few weeks, topping them up when needed.

If that's not you, consider a maintenance-free sealed battery with more acid covering the plates. Over several years that acid will evaporate, so they need more fluid to begin with.

More advanced again is an AGM (Absorbed Glass Mat) battery, which is sealed, but has plates separated by matting that absorbs acid. An AGM can be inverted in the event of a yacht capsize or lain on its side. Most racing yachts require them under regulations, but they should be mandatory for all large cruising vessels, too.

The initial cost disadvantage of 30 to 40 per cent above a wet battery is vastly offset by the longevity. The design life on a good AGM will be eight to 10 years; roughly twice that of a maintenance-free battery. Deep cycling capability and shock resistance are also enhanced.

Next up are gel batteries, in which the acid in the glass mat is replaced with a liquid gel that's pumped into the battery and left to set. They're more expensive to make and buy, are heavier, and are typically better for deep cycling than cranking.

Lithium-ions are the pinnacle, offering a substantial (up to 70 per cent) weight and space saving, but adding complexity and cost, at least initially. They have three times the lifespan of traditional batteries and boast ultra-fast charging and discharging - down to 20 per cent and up to 100 per cent.

A good battery monitor is essential to determine battery health. A 12-volt system should be sitting at 12.8 volts or above when idle, and if you get an inexplicable change in voltage - even to just 12.4 - the alarm bells should sound.

One crook battery can cannibalise the good batteries in a bank, as the charger will innocently try to force all of the batteries to get to 14.4 volts - with one out of action its mates will rapidly be gassed off. Also, when not under charge, the good batteries attempt to compensate for the weak link and their voltage drops dramatically.

A battery has a 'memory', of sorts, in that it will 'sulphate' if not cycled. Once sulphated, it will produce resistance and confuse the charger into thinking it's full, yet when the battery is subjected to load it soon collapses.

How often do your hear that a power vessel got to a fishing spot then wouldn't start again? Sadly, you can't clutch-start or call roadside services, although emergency 'jump-start' products are available. Sometimes it's because the alternator isn't charging the battery, so it's always best to have a voltmeter installed so you always know it's charging properly. Ideally it should read in the high 13s or just over 14 when running.

Another reason is that boaties run livewells off the cranking battery, unaware that a pump can draw 13 amps or more, which will flatten a starting battery in hours. That's why it's worth having a separate deep-cycle battery.

With a battery switch you should run initially on '1' then switch to 'Both'. When you shut down the motor, switch to '2' and stay on that. Battery 1 will thus be peachy keen when you restart. Opinion is divided on whether you can or should use the battery switch when the engine is running. If in doubt, only switch over when the engine is stopped.

Moored boats are most at risk of flat batteries due to inadequate charging. A combination of solar and wind generation covers all the bases when you're away from shore power. Some boats use solar for the 12-volt bank and wind for the 24-volt bank.

Hard-wired bilge pumps operate even when the switches are off and electrical systems are often plagued by phantom current drains as well. Lead acid batteries also have a tendency to drain themselves, more so than lithium-ion, over time.

If batteries haven't been kept fully charged over winter, then do so immediately, ensuring they come up to full voltage in reasonable time. Left to sit after being exhausted, the battery risks being irreparably damaged.

The current thinking ('scuse the pun) is that your deep-cycle charger should be able to produce 20 per cent of your battery's capacity in charge output per hour; it used to be 10 per cent. A charger must have an appropriate algorithm for the battery type since some can be overly brutal.

Reconditioning is possible if the battery isn't too badly sulphated. It undergoes a 24-36 hour desulphation program then is subjected to a specialised charge.

If a battery has dropped a cell, there's nothing that can be done. Disposal must be performed by an EPA-approved recycler. Most battery suppliers offer a free drop-off service. Battery Business, for example, recycles two tonnes a week, with around 97 per cent of battery components being recycled.

AGM batteries are recommended as the minimum for most marine applications, and a rudimentary Google search shows a generic 12-volt 200AH AGM deep-cycle battery selling for as little as $350, but these are generally lower quality and we're advised that quality units carry much higher prices.

With a 200AH Lithium-ion costing $2800 or more, it appears a hard argument to make. But when you consider the relative costs compared to capacity (usable amp hours) - along with greater longevity and potentially a reduction in fuel consumption from a generator - the balance can tip towards the latter.

Whichever way you go, get your mix of batteries right and you'll have years of service from your power source. Get it wrong and, frankly, it's lights out.

  • Make sure your batteries are securely tied down with strong straps and saddles.
  • Keep battery terminals and connections greased to prevent corrosion. Make sure you tighten all bolts to cables.
  • Use hot water to clean any acid spills away from the top of the battery, which can otherwise cause discharge.
  • Don't mix battery types as they charge at different rates. Replace like-for-like or the entire bank.
  • If a battery fails in a big bank of batteries, replacing only one can cause it to age and quickly deteriorate to the same condition as the rest of the bank. As a rule, it is preferable to replace the whole bank at the same time.
  • Lead acid batteries prefer temperatures around 25 degrees C and their capacity plummets as the mercury drops below 15 degrees. Cold garage floors can be a battery's worst enemy.
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