At the end of the wire

Modern batteries are still, from the outside, the simple box they always used to be, but "getting a feel for what's going on inside is something of a black art", says John Monks of Taplin.

Further, there are many different kinds of loads on board these days, so it really does make sense to do a bit of research to match the demand of a system with what's at the end of the wire.

Mr Monks adds: "There's a lot more to think about now. For example the old Westerlies and Moodies used to run off a simple electrical circuit, but boats generally now require something much more sophisticated."

Andy Cooper of Barden agrees. “Most people use electronic equipment, although it has to be said that they don’t necessarily constitute such a big draw. On most leisure boats the biggest drain comes from domestic ware, such as fridges, cookers and air-con with a different set of needs for things like windlasses.”

So, since it’s been realised there are two or three very different jobs for batteries to do, there’s been some interesting trade-offs.

Mr Cooper explains that the starter motor wants a high cranking current, giving a short burst of power to the engine to get it moving. "It charges quickly from the alternator and is ready to do the whole thing again at short notice. It has a very shallow cycle and therefore lasts many more years than domestic batteries. Since it isn't really being drained in the same way as the auxiliary battery, it can be much smaller."

Tuck it away
This means you can take the starter battery and tuck it away near the engine so there’s no huge length of wiring to cause a drop in power. Further, there’s often extra space available to install a higher capacity dedicated deep cycle battery, explains Mr Cooper.

Starting batteries have thinner plates to give a larger surface area for good cranking performance – and there have been developments: Varta for example, uses a diagonal grid structure on its plates so current has a direct run to the terminal.

Optima use spiral cells to get a larger surface area which, together with a very high lead content, speeds up charge acceptance and gives good cyclic performance.

However, if someone is just doing a day's sailing around local waters every so often, then there may be nothing much gained by swapping over to separate start and auxiliary circuits.

There are dual-purpose batteries around which have been tailored for both starting and cyclic performance, important for general use. For example, Manbat’s sealed XV31MF batteries are still in essence a leisure battery but they use thicker plates, which incorporate a calcium formulation.

This gives them the ability to withstand the demands of auxiliary applications in addition to cold crank duties: the battery benefits from around 30% more starting power than traditional wet cell batteries.

Dedicated sorce
But many boats would do better with a dedicated source for the power equipment, often either gel or absorbed glass mat (AGM) batteries. These have developed from their lead acid counterparts and both varieties store their charge efficiently, as well as not degrading as easily as the common wet cell battery.

Further, there is very little chance of a hydrogen gas release, which is unhealthy at least, and which can, in some circumstances, lead to an explosion.

The AGM batteries are however more sensitive when it comes to charging. “You can’t bang as much voltage into them on the whole as with conventional flooded leisure batteries," says Myles Pemberton of Manbat, “but provided that care is taken to match the charger to the battery specification there’s much less chance of sulphation.”

He adds, while some normal chargers will work adequately, for optimum performance and service life, gel and AGM batteries should always be used in conjunction with a multi stage, voltage controlled automatic charger.

So, although the wet cell variety is more likely to gas through overcharging if you don't have an automatic cut off, if you overcook any variety you lose electrolyte, and it’s impossible to simply top up a sealed gel or AGM unit with distilled water as you can with the wet cell kind.

However, there have been some simple but effective innovations that counter this issue. For example, Manbat’s XV products have a labyrinth that re-condenses any freed evaporation, feeding it back into the cells.

On the other side of the equation, the margin below which batteries run into trouble is variable. It’s about halfway for most wet cell batteries, with the more specialist kinds often being able to be taken a bit below this, although it’s not particularly healthy for any of them to go too low. And being continuously cycled down to 30% will be worse for the battery than stopping at the halfway mark, says Mr Pemberton.

Normal charge
Further, Peter White of CTEK adds that a normal charge off the alternator might simply not take the battery up to its capacity. “Only charging a standard battery up to 80% capacity means that the battery will be cycled more often than necessary which will increase stratification, drastically reducing capacity,” he says.

There’s also the problem of sulphation, where the plates inside could start to form crystals, which lowers the available contact area of the internal plates, curtailing the cells’ lifecycle.

His point is that by using a charger such as the D250S Dual while on the move, boat owners are able to keep the batteries in good condition and maintain 100% charge – he says the D250S can extend the life of the battery by as much as three times as well as maximising capacity.

However, whether the AGM or gel variety of battery comes off best for marine applications is a hotly debated topic. James Hortop of Merlin is convinced the AGM batteries are “the way forward”. He says the AGM variety take a charge better than the gel kind, and give more flexibility.

“AGMs are a better type of battery, they can be used to give the kind of cold cranking power that gets on board windlasses and power driven items going as well as running the domestic ware.”

Other people, however, say it’s really horses for courses, as the gel variety is quite similar, and further, some say they can take being discharged to a lower level.

Few quirks
Nick Holland of Mastervolt asserts AGMs do have a few quirks if they are discharged very deeply. “If an AGM battery really gets run down it needs waking up very gently”.

This means, he says, putting in a low level current for a few hours, then stepping it up in phases till it reaches a normal charging pattern, “or you risk ruining the battery”. He adds, however, that it’s something that all Mastervolt chargers will do automatically.

There are other factors to take into account when considering battery types. "You find the batteries on board get placed in all sorts of locations and you simply might not want a flooded lead acid battery posing the threat of gassing underneath, for example, a bed,” says Mr Cooper.

Further, there are other safety considerations. Older wet lead acid batteries need a bit of maintenance, such as topping up with distilled water from time to time – and people have been caught by acid splash back into the eyes.

Other issues on the choice of battery come into play around how the boat is generally used. "Firstly it is obvious that if the person is likely to be spending a lot of time aboard, out of the marina, the boat will want extra domestic power. Added to this, if room is particularly tight, you might not easily be able to get at the battery to do a bit of maintenance. And if it is in a sensitive location, you might want to change over to a completely sealed unit," says Mr Cooper.

But it seems you can’t simply bank on AGM versions giving you a much longer life than the traditional kind, as there can be a gulf-like difference in quality and manufacture. Mr Hortop adds there are only a few “true, deep cycle batteries” on the market – adding that the Enersys and the Lifeline range are good examples. The pure lead acid AGM varieties have a much lower internal resistance than many others, leading to a deeper and faster charge. “It can be as much as 20% quicker than others,” he adds.

Read the labels
However, Mr Cooper also makes the point that you do have to read the labels pretty carefully to get an accurate picture of what power a battery is going to give you.

Discrepancies in labelling is an issue that Manbat is trying to highlight. Although there has been a move toward clear labelling of the power output of batteries in the form of EU Directive 2006/66/EC, mysteriously marine and leisure batteries are exempt from it.

This directive aims to clear up the more unscrupulous labelling practises where, for example, a manufacturer could call a battery an L75, leading most people to assume it was a 75Ah battery – in other words, you’d get 75 hours at the standard discharge of 1 Amp per hour.

Mr Pemberton explains: “In some cases what would appear to be a 75Ah battery could in fact be only, say, 65Ah if they’ve measured the output at a lower Amp per hour discharge – with the real information being buried in the small print. Further, this standard measurement already has a 10% error margin built into the rating description, and an unreliable manufacturer could easily aim for the bottom end of this range, even though they have already taken a slice off the standard measurement. Therefore a unit labelled L75 could conceivably only be giving you 55Ah output when measured correctly."

Mr Pemberton went on to say: "The first question that should always be asked when purchasing a battery should always be, ‘What is the battery’s capacity measured in Ampere hours at the C20 rate?’ The answer to this question will counter any attempts of misinformation right from the start.”

He goes on to say: “There are many different batteries and brands out there with a large variation in quality. A price-driven marketplace often means that quality comes second to price and to make a cheaper battery usually means the quality and performance drop.”