Marine batteries

A salt and battery

As Castrol's TV advert said: "Oils ain't oils." The same is definitely true of boat batteries. The only common factors are an electrolyte, posts and plates. Apart from exotic, extreme-cost dry cells, nearly all boat batteries are lead-acid based. Typical lead-acid battery cells are rated at 2.1 volts, so a nominal 12V battery has six such cells. Easy maths.

Now get your head around this. Lead-acid batteries, be they the traditional flooded type (wet cell) or the later valve-regulated lead-acid (VRLA), gel and absorbed glass mat types, store energy in a reversible electrochemical reaction that takes place between lead and lead dioxide plates and sulphuric acid.

When a battery is supplying current (discharging) there is a chemical change of the lead and lead oxide plates to suspended lead sulphate. In the process each lead plate releases electrons.

When the battery is being fed current (charged) the suspended lead sulphate converts back to lead and lead dioxide.

In theory, this discharge and recharge action should go on indefinitely. Alas, the chemical world isn't perfect: salts fall out of suspension and build up on the floor of the casing; energy is lost to heat; a small amount of lead is lost to electrolytic action; and hydrolysed water (hydrogen and oxygen gases) escapes.

Another issue is the varying density of the electrolyte in flooded batteries that aren't shaken about very much. Stratification of the electrolyte reduces efficiency and needs to be countered by equalising or boost-charging a flooded battery periodically. In this operation the slight overcharge results in oxygen and hydrogen bubbles being produced at the plates and this action stirs the electrolyte.

Still with me? Good. Here's the good news. Of all these battery inefficiencies, hydrogen and oxygen (water) loss and electrolyte stratification have proved the easiest to combat.

The method of water loss replacement in a traditional flooded battery is quite familiar and many of us have squeezed into bilge spaces that were seemingly never intended for human access, undoing sharp-edged cell caps, trying to see the acid level within. Even more fun is struggling with a distilled water bottle spout to top up the cells. Drawing acid into a hydrometer to check electrolyte density is another procedure that's sometimes awkward.

Spilt acid, holes in clothes, streaming eyes and bleeding fingers are sometimes the results of a thorough battery check! Little wonder there has been pressure from boaties for an easier way of storing electrical power (at which point, your Editor at Large is jumping up and down screaming: "hear, hear!" When left on Shorepower his 42-footer demands monthly battery water-level checks).

Enter VRLA (Valve Regulated Lead Acid) batteries, which were developed to overcome water-loss and stratification problems. Although called sealed batteries, VRLA types do have a relief valve to prevent an explosive gas build-up in cases of extreme overcharging. However, in normal discharge and charge operation the VRLA battery does not vent to the atmosphere. Instead, the hydrogen and oxygen gases produced are retained inside the casing and recombined using an internal catalyst, so there is no water loss.

Note that a range of batteries often marketed as low or no-maintenance are in fact semi-sealed flooded batteries not true VRLA. These modified automotive starting batteries have reserve electrolyte storage, but once that's used up, they are finished. Kaput!

GEL AND TELL
Gel batteries are VRLA types with the electrolyte thickened to a jelly state to prevent stratification. Another plus is that a gel battery-case fracture results in a mere oozing of electrolyte, rather than a flood of acid.

To their credit, especially in cold climates, gel batteries can be left in a discharged state for a long period of time and still recover. However, gel batteries require lower-voltage charging than flooded batteries and most gel batteries should never be equalised.

In a gel battery that is being incorrectly overcharged, gas bubbles cause voids that increase resistance and can cause damage through heat build-up.

Warning: Manufacturer's charging recommendations must be followed to the letter when dealing with gel batteries.

Absorbed Glass Mat (AGM) batteries are another VRLA type but with acid suspended in a glass mat to overcome electrolyte stratification and leakage problems.

Like any VRLA battery, an AGM model should not be overcharged. Some high-quality AGM batteries can be equalised, but only under controlled, constant current conditions. Again, manufacturer recommendations for correct charging must be followed closely.

CRANKING AND DEEP CYCLE
Flooded, gel and AGM batteries come in starting or deep cycle types, but the top-quality models can perform both functions. There are also dual-purpose flooded, gel and AGMs that aim for middle ground between true deep cycle types and starting batteries.

But a starting battery is designed to start an engine and power ancillary equipment while the motor is running and the alternator is recharging it. As such, starter motors need a very large current for a very short time. Thus, starting batteries have a large number of thin plates for maximum plate-to-acid surface area.

When a starting battery is discharged, some of the plate is lost and repeated discharging erodes them to the point of failure. A battery that should last for years can fail in weeks.

Conversely, pure deep cycle batteries are designed to be discharged down as much as 70 to 80 per cent. Its heavy plates resist erosion, but there's usually less surface area to deliver the cranking amps of a similar-size and dedicated starting battery.
 
High-quality, dual-purpose, starting/deep cycle batteries combine high cranking ability with good deep cycle qualities. These, I consider, top-shelf boat batteries, either as flooded types or the various VRLA models. However, things get confusing when there are so many cheaper batteries labelled 'marine' that often purport to be superior.

THE MARINE SCENE
A cheap 'marine' battery usually features heavier plate construction than automotive starting batteries. However, note that most budget-priced marine batteries should not be discharged beyond around 50 per cent of capacity.

On the other hand, the more expensive, purpose-designed batteries with clear labelling, backed up with specifications, leave no doubt as to their capabilities.

Of course, it's possible to use deep cycle batteries for all functions on some craft, provided the battery bank is sized to provide enough engine starting power. It's also possible to use cheaper starting batteries or marine ones on small craft that don't have much electrical equipment or which mainly draw on power when the engine is operating.

Let me surmise by saying "you get what you pay for" in batteries. And, after such a big investment in your pride and joy, the last thing you want is a flat battery, right? Indeed, a bank of top-shelf batteries at your beck and call is a wise investment in turnkey boating pleasure. All aboard!

THE SHORT ANSWER
The traditional flooded lead-acid battery is still the most popular choice of battery because it is the lowest-priced. However, there are definite quality differences between brands and place of manufacture. Many of the traditional brands that used to be locally made are now produced offshore and it's sad to note a definite downward shift in quality.

"We've had some formerly reliable brands giving up after only a year or 18 months," says Anthony Daube of Fleet Marine Electrics, "But we've had good service life from US-made batteries and they're the ones we're recommending at the moment. They're up around the $280 mark, compared with $180 or less for cheaper imports, but we find they're lasting three or four times as long."

The next step up the quality chain is to gel-filled or AGM, no-maintenance batteries. These designs eliminate the possibility of spilled electrolyte: the gel won't pour out and in AGM batteries the electrolyte is absorbed in a glass mat.

"However, we've found that gel batteries can be easily damaged by an accidental overcharge and once the plates have buckled the battery is useless," counters Daube.

"Wherever there's a risk of overcharging we recommend the AGM type rather than the gel type.

"Spiral wound batteries are another top-quality choice and have the advantage of being able to mount in any position.

"They're available as cranking or deep cycle types, have very low internal resistance and are our preferred power supply for electric bowthrusters.

"The downside of top-quality VRLA batteries is a retail price that starts around $500, but we find that they last typically eight to 10 years," Daube adds. And if you have ever had battery issues, you'll agree that getting the best pays dividends, helping ensure happy times afloat, and is therefore money well spent.

» BATTERY BUYER'S GUIDE

Many boaties are confused by marine electrical systems, so Allan Whiting asked an independent bright spark in the marine electrical world to clarify the selection process and make buying a battery easy.

"Fact is, there's no one, simple solution to different boat electrical needs," explains Anthony Daube, principal of Fleet Marine Electrics, a flourishing marine electrical company based on Sydney's Pittwater, with branches at d'Albora Marina, The Spit and Empire Marina at Bobbin Head.

Servicing literally thousands of boats each year, battery-related issues top his list of pre-season fixes. But it's not a one-size-fits-all situation. Different types of boats require specific battery set-ups and finding the right fit needs a quick check of the way the boat will be used.

RUNABOUTS
Most runabouts are used for inshore or close offshore fishing where most - or all - of the electrical demand occurs while under power as the battery is being charged.

"For this application, either a deep cycle battery or a battery primarily designed for engine starting is acceptable," said Anthony Daube, "But if there's going to be a lot of stop-start engine operation, the cranking battery is preferred.

"Alternatively, if there's a need to use some electrical equipment when the engine isn't charging, say while running bait pumps or deckwashes while drift fishing, then the deep cycle type is better.

"But the most critical requirement is that the chosen battery has sufficient cranking power. And, as with any marine situation, a charged back-up battery is desirable (in other words, dual batteries are best)," Daube said.

TRAILERSAILERS
Usually dayboats powered by pull-start outboard motors, trailersailers and traileryachts have few electrical demands, perhaps just the need to run radios, a stereo system and navigation lights in the short-term.

"A deep cycle battery is the preferred fitment here, because there is only minimal charging capability from small outboards," Daube says.

"A deep-cycle battery will tolerate discharges between home-based or in-vehicle charging opportunities. And an in-vehicle battery charger is a good way of charging the boat battery, while the vessel is being towed," he said.

MEDIUM-SIZED YACHTS
Small and medium yachts have more electrical demands, but usually limited space for battery banks when compared with large yachts or cruisers.

"On the positive side, a yacht auxiliary doesn't need much cranking power from its battery bank, so the batteries can be deep cycle types. These suit most yacht demands better than starting batteries, since you typically run cabin, navigation and deck lights, and electronics and sound systems," Daube explains, adding that "solar panels are popular, too, as a means of recharging batteries while the boat is moored.

"Fact is, most sailors don't use their engines for long enough periods to keep their batteries charged," he said.

SAIL AND POWER CRUISERS
Now things get interesting. Larger craft usually require a combination of starting batteries and deep cycle house batteries. But the correct connection and switching system is as important as the right battery choice.

"In respect of the smaller cruisers we've considered thus far, dual batteries with a four-position rotary battery switch is a sufficient setup," Daube advices.

"Rotary switches are cheap and work well with the same type of battery, but they're not suitable for electrical systems that combine cranking batteries and deep cycle batteries.

"Mixed-type systems require selective switching, using a three-key control panel and a voltage-sensitive relay. The top-shelf battery control system we use goes further, with a toggle-switch control of large solenoids to make and break starting circuits. This reduces the voltage drop and weight of long cables running between the batteries and the control panel.

"A typical cruiser layout we do consists of a single starting battery and four six-volt, deep cycle house batteries, with series/parallel switching. A multi-stage alternator charges the house batteries first, via a 90amp, voltage-sensitive relay.

"Another factor we take into account is that larger vessels usually have on-board generators to supplement the charging capability of the propulsion engines," Daube adds.