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Boat Batteries


There are a number of topics which invariably seem to be discussed when a group of boaters get together and one which is fairly high on the list is the batteries.

The whole topic surrounding batteries is subject to all sorts of claims by both manufacturers and boat owners. In my 35 years of cruising and working on pleasure craft I have heard just about every claim it is possible to dream up. Therefore what follows is a straightforward appraisal of what I have found works and what does not work. A great deal of this is for sail boats, however much does also apply to motor boats, such as the type of battery and a method of charging when sat in the perfect anchorage for a few days. (Since originally writing this I have spent years studying batteries and that is now on the battery desulphation page)

The first thing which we need to examine is the way we use our batteries on pleasure boats.

We abuse them is the simple answer. We expect them to sit for long periods without a charge, often running them down to a deeply discharged state. In practice when away from a mains shore supply it is generally true that our batteries are rarely charged much above 80%. Then we want them to charge up as fast as possible. After sitting on a mooring or in the marina, often for weeks, they have to turn a diesel engine by giving out a very high amperage. They often live in a box without good ventilation and inside this box it can become really warm, especially in summer weather. Maintenance such as regular topping up with distilled water, keeping them clean and well strapped down is often overlooked. How often do the terminals get checked and cleaned? We expect to pay the minimum but to get the maximum life from them.

Then on top of all that we have to consider the claims of various manufacturers. Maintenance free - sealed for life - Gel - no spill – AGM - heavy duty - deep cycle - golf cart - mono block - latest technology – starter – leisure - 24 volt – 12 volt – 6 volt – single 2 volt cells – calcium – traction – semi traction - etc. etc. The list goes on and on. Out of all this we have to find a set of batteries that will serve our needs.

Charging is another nightmare. Alternators - alternator regulators - high capacity chargers - car type mains chargers - multi stage chargers - solar panels - wind generators - engine driven generators - suitcase type generators - inverter chargers - towed water generators - prop shaft generators – desulphating chargers -  split charge diodes – split charge relays.

As you will be realising, this is a vast subject. I am not going to delve deeply into each facet but simply give an understanding of ‘what works’?

What the battery does.

In simple terms the battery stores electricity. Exactly how it does this can be found in numerous reference works. For our purposes all we need to know is that the battery has a series of lead plates which are suspended in dilute sulphuric acid. Charge or feed a current one way through the battery and it will convert that electrical current into stored energy. Connect to the battery the other way and we can recover and use that stored energy as direct current to power the various devices aboard the boat.

We want to use as much of that stored energy as we can before needing to re charge. Therefore, deep cycle batteries, which are built more robustly than normal are best to use. There are some ‘heavy duty’ truck batteries which are suitable for us, as on modern trucks which have sleeper cabs they need the ability to deep cycle them. But not all of them. The ones which I have had good experience of are the German ‘Hoppecke’ batteries.  Not inexpensive but lasted very well.

However, one thing that many do not realise is that all batteries are designed to last for a certain number of ‘cycles’. A cycle is a discharge and re charge. This is where manufacturers often mislead. They will state a number of cycles for their battery but many do not state how deep, or in other words how discharged this is. Without obtaining the technical details for each manufacturer all we can do is buy a battery with a stated number of cycles at something like 70 to 80% discharge.  This will totally exclude starting batteries such as normal vehicle batteries as these are designed with thin plates which give high power for a short period, not heavy plates which will give up their power over a longer period and will stand many more deep discharges.

One thing to be certain of is that the less the battery works the longer we may be able to make it last. We can do this by not continually deeply discharging. The best and most economical rates of discharge is about 30% to 50% of capacity. This will give reasonable use and reasonable life from the battery. Therefore if we have a 100 amp hour battery we should expect to use up to 50 amps before re charging. This makes a battery management monitor a necessity. These will tell us how many amps have been used, what the voltage is and the rate at which we are charging. But don’t forget that for long periods we are likely to only achieve 80% of charge. Therefore, in reality our 100ah battery may only be good to supply 30ah, and this must considered when sizing a battery bank.

Trickle charging from solar panels or a wind generator is good for keeping things topped up but we often need to charge from the engine or through the mains charger either fed from a shore supply or from an on board generator. In practice a lot of cruisers take their boats into a marina once a month to top up the batteries from a shore supply (and to fill the water tanks while catching up with the washing).

Charging from the engine.

The engine will have an alternator. This alternator will often be designed and have a regulator designed for re charging after engine starting. It will not last long if it is used to regularly re charge deeply discharged batteries, it is simply not designed for this purpose and is being over worked.

Fitting a high capacity regulator to an ordinary alternator should not be done as this will overload and overheat the alternator. The best solution is to fit both a high capacity alternator and high capacity regulator. This will charge the battery quickly and also have other benefits which I cover later. I have used several types of alternator and high capacity regulator. The ones supplied by Driftgate in the UK ( Home Page of Driftgate 2000 Ltd. ) are excellent and sensibly priced. I have also used the well known Balmar alternators ( ) to good effect. I have several Sterling electronic devices, including a high capacity alternator regulator, aboard my yacht but have become unhappy with their recent technical and after sales support. ( However I must say that Mr Sterling’s appraisal of batteries is worth reading. )

When fitting a high capacity alternator we must remember that the drive belt will be put under a higher load. A single 9mm V belt will drive a 50 amp alternator. If a high capacity regulator with a ‘soft start’ is fitted then a single 9mm V belt seems to work OK up to about an 80 amp alternator. ( Soft start means that as the alternator starts spinning it does not immediately go to full charge but slowly increases the rate of charge). Above these charge rates either a twin V belt, a larger V belt or perhaps a serpentine belt will have to be fitted.

Charging from the mains.

When I say charging from the mains I mean either a shore supply or the mains power from a generator. There are various different voltages and hertz around the world but the common ones are 230 volt 50 to 60htz and 110 volt 50 to 60htz.

This 110v or 230v will supply some form of battery charger. There are a number of chargers from lots of manufacturers. What we want is a charger that will work anywhere. By that I mean a charger that is auto ranging. This means that it will automatically work on a supply from around 90v to 260v with a hertz of between 50 and 60. A charger like this can be plugged into virtually any supply around the world.

We want a charger that will supply about 20% of the battery capacity. Most battery manufacturers give the 20 hour discharge rate and this is what most calculations are based on. This is the number of amps supplied over a 20 hour period at which point the battery will be flat or discharged. Therefore for a 100ah battery we want a 20 amp charger.

In a battery, the rate at which energy is drawn affects the overall amount of energy available from the battery. For example , a 100 Ah battery rated at the 20 hour rate means that over 20 hours there are 100 Ah available, or to put it another way you can expect to draw up to 5 Amps per hour for up to 20 hours, 20x5 = 100Ah. Draw more or less amps each hour and the available energy will slightly rise or fall.

We also want a charger that is multi stage. The stages are bulk – absorption – float. In the bulk charge period it is charging as fast as possible. In the absorption stage it holds the voltage higher than normal (usually 14.8) which also helps to condition ( de sulphate ) the battery. Then it drops the voltage to around 13.5 to hold (or float) the battery charged and to supply any power being used aboard. However please read the page on desulphating batteries.

Aboard my yacht we have a bank of batteries rated at 460 amp hours. I have two 40ah chargers ‘stacked up’. These are rather ancient Sterling units but I note that Sterling still sell similar units which can be stacked up. Stacking up simply means that multiple units are wired in parallel. Therefore our two chargers give an 80 amp charge rate. (In reality when the batteries are very low I have noticed that to start with they give 90 amps). These units work between 80 and 250 volts and at 50 or 60 hertz.

Having a charging system like this means that the charger can be supplied either from the shore or a generator.

Type of battery.

Having fitted a high capacity alternator and regulator along with a smart mains charger we now need a set of batteries that will be suitable.

There is only one type of battery that will do. I will say that again!!!

There is only one type of battery that will do!!!

That type is a straight forward, vented, deep cycle, lead acid battery that can be topped up with distilled water.

The reason for this is that we want to fast charge the battery and that means gassing. As the battery charges - the liquid begins to bubble and gives off hydrogen gas. This gas must be allowed to escape otherwise there will be a build up of pressure inside the battery which may lead to an explosion. Also the bubbling is good as it stirs the liquid up ensuring an even distribution of the particles in the liquid. Once charging is complete and the battery has cooled it will be seen that a small amount of liquid will have been lost. This will need replacing at regular intervals by topping up with distilled / de ionised water.

This means that sealed, gel and any other type is unlikely to be satisfactory and despite the claims I would never use anything other than straight forward lead acid deep cycle or traction batteries that can be topped up with distilled water.

During fast charging and de sulphation the voltage in the battery will normally rise to 14.8 volts and can usefully rise to 15.5 or 16 volts to de sulphate. The only type of battery that can take this is a vented lead acid battery.

You will notice on your charger that there are different settings for vented as opposed to sealed and gel batteries. All that the sealed or gel setting does is to reduce the voltage being delivered. This means that the battery will take longer to charge.

It can be seen from this that sealed for life – gel – no maintenance etc. are not good to use on a boat. I have also tried Elecsol AGM batteries. They lasted for just two years of their five year guarantee and I have since read from a battery supplier that AGM are not suitable for our purposes.

Some claim to have a system of recovering the liquid that is gassing off but on talking to a manufacturer they also said that this type of system would be inadequate for our purposes.

There are lots of claims made by battery manufacturers trying to sell their latest style of battery. Do not believe them, I don’t. I just stick to the straight forward deep cycle lead acid type.

The best set I ever had were not the most expensive. They were a set of four x 6 volt x 240ah monoblock semi traction batteries. ( A traction battery is designed to power a vehicle such as golf cart or fork lift) Wired correctly these gave 480ah at 12 volt. I had been thinking that I would buy some single 2 volt cells to make the voltage but when I spoke to the supplier he asked about the areas we would be sailing to. When he found that we may end up in some fairly remote areas he pointed out that should a battery fail, finding a single 2 volt cell may be a problem. But he argued a 6 volt battery can usually be obtained anywhere. He left the choice to us and we decided on the 6 volt version.

When these needed replacing we were first coerced into trying the Elecsol AGM batteries and when they prematurely failed we were in Greece, The Ionian to be precise. The guarantee was eventually honoured but only in the UK despite them promising that it was Europe wide. I had a good look around the Ionian and despite it being a popular sailing area nearly all the batteries offered were just heavy duty lorry types. We tried to have a set of Trojans sent out from the UK but the carriage was excessive. We then found that there was a Trojan supplier in Athens. He did not have the 6 volt type we wanted but did have some 12 volt Trojan batteries which he claimed would perform the same as the 6 volt version. After some awkward discussions regarding price, we eventually bought four for the same price as we could obtain them in the UK. As advertised on the Trojan website they had a 7 year guarantee. This was confirmed 3 times with the supplier who stated that the receipt was our guarantee. With these much respected and guaranteed batteries we felt things would be OK.

Two years and one month later while in Gibraltar preparing for an Atlantic crossing, one cell failed. We then found that the Trojan guarantee is nearly worthless. We eventually obtained a replacement but it took an enormous amount of effort to make Trojan honour their guarantee.

I have compared the weight of several manufacturers batteries and the weight of the 6 volt type that I prefer, which will be similar to the Trojan T105, varies between 28kg and 37kg (61 lb to 81 lb). That is a 20lb difference, on one battery!! Therefore, knowing that I want a battery with good heavy lead plates I guess that the heavier version may be more suitable.

Sulphation and battery desulphation.

Desulphation is something which began to work its way into the sailing vocabulary a few years ago. What happens is that as a battery is used the plates slowly develop a layer of lead sulphate crystals on them. Eventually so much of the battery plate area is covered that the battery is then unable to supply much current. This is a natural process that occurs in all lead acid batteries, but can occur significantly faster if placed in the wrong combination of temperature, poor charging technique, protracted battery disuse, etc. (sound familiar).

This slow reduction of the capacity of the battery is the single biggest reason for batteries needing to be replaced.

Since writing this I have spent several years researching and experimenting with different methods of desulphating the batteries aboard my yacht. This is a vast subject and I have written a condensed history and my conclusions on the other page entitled Battery desulphation. Here Battery desulphation