Care and feeding of your boat's batteries

Your batteries are critical to engine starts and running all the electrics and electronics found on modern boats, but how do you take care of them to ensure they have a long and productive life? Ralph Skelton takes a look at the care and feeding of your boat's batteries.

MUCH HAS BEEN written about boat batteries, but you talk to the average yachtie and confusion still seems rife. Start talking about batteries and people’s eyes tend to glaze over. Unfortunately, not understanding your batteries can be an expensive and painful. So I’m going to have a go at explaining the care and feeding of your batteries in plain English.

We’re only going to dwell briefly on issues like sizing a battery bank for your boat, or designing a charging system. Most of our readers own and operate some sort of production boat where these issues have been dealt with by very competent designers. Also, on most production boats, to make room for bigger battery banks or more complex charging systems is very difficult, because of the inherent space limitations.

When purchasing a new or second-hand production yacht, one of the considerations should be “what do I want to use this boat for and is the electrical system consistent with that use?” Maybe followed by “if not, what is the potential to change it, and how much will those changes cost?”

Before we start, there are a couple of definitions which you will need to understand, to make sense of what follows.

1. Ampere – usually abbreviated to amp, is a measure of the rate of flow of electricity in a wire. It is similar to litres per hour in an hydraulic system.

2. Volt – is a measure of the electrical force within the system with which we are dealing. It is similar to pressure in an hydraulic system.

3. Watt – is a rate of energy being transferred? One amp flowing at one volt is one watt. So a 24 watt lamp running in a 12 volt system is using 2 amps, or a 24 watt lamp running in a 24 volt system is using 1 amp, i.e. watts = volts multiplied by amps.

4. Ampere hour – is the amount of energy transferred when one amp flows for one hour. To define the actual energy transfer accurately, we also need to consider the voltage at which the system is operating. In a boat, if this voltage is consistent, then it is often convenient to just discuss amp hours. One common application of this principle is that most battery manufacturers give the capacity (or size) of their batteries in ampere/hours. Assuming that their battery is delivering those amps at whatever the rated voltage of their battery is.

5. Cycling – This is the number of times a battery can be fully charged and discharged to 50% of its capacity (within some sensible rules) before its performance begins to markedly deteriorate. At the top end, a deep cycle premium flooded cell battery can last up to 3000 cycles. Lower to medium deep cycle batteries typically have cycle lives of 300-500. Top of the line absorbed glass mat batteries typically rate 600-700 cycles.

Battery choices

Here we begin to get into murky waters because there are lots of conflicting requirements coming into play. As in all such situations, we have to weigh up our particular requirements and try to make a best judgement of what product will best suit our needs.

Firstly, the physical size of batteries have been standardised, and in most production boats it is not easy to fit a battery which is significantly bigger than the one the designer intended. The physical size of the battery does not bear a very close relationship to how it will perform in the long term. Performance is generally related to other features of the battery and to how it is treated during its life aboard your boat.

Most boat batteries are what is called lead acid and they give very reliable service, can store a lot of energy for their physical siz and are relatively cheap to make. Most boat batteries are made up of 6 individual cells in one case. The interconnection between these cells is within the case. The total voltage of the battery appears across the major terminals at 12 volts, i.e. 2 volts per cell. Lead acid batteries come in 3 basic designs. These designs refer to how the electrolyte, which is sulphuric acid, is stored or contained within the battery case. Sulphuric acid is a very corrosive chemical which will cause severe burning of human skin and flesh if left in contact for any period of time. It also generates chlorine gas when mixed in salt water. Chlorine gas is poisonous to humans. Have we got your attention yet??

In flooded cell batteries, by far the most common battery on cruising boats, the electrolyte is loose in the battery and is contained by a small clip-on or screw-on lid. This lid has a vent hole in it to allow for expansion and contraction of the electrolyte with temperature changes. These batteries must be kept upright or reasonably so.

In gel cell batteries, the electrolyte is in a gel configuration, where the sulphuric acid is thickened into a gel. These cells are sealed so that they can be inverted without loss of electrolyte. They do, however, have a pressure release mechanism to allow venting if the internal pressure gets too high. In normal operation no gas escapes from these batteries. The oxygen and hydrogen generated during the operation of the battery recombines within the cell back to water as part of the electrolyte.

In absorbed glass mat (AGM) batteries, the electrolyte is contained in an absorbent glass mat within the cell, which again is sealed, like the gel cell.

Both these latter two types of batteries can perform reasonably satisfactorily upside down for short periods. They therefore comply with yacht racing rules, which specify that any batteries aboard must be able to be inverted without damage. However, they do not give as much electrical output as their flooded cousins of the same physical size. Another problem with them is that is impossible to measure the specific gravity of their electrolyte and hence it’s hard to determine their state of charge (more about this later). Lastly, they are usually significantly more expensive than their flooded cousins. Note too, that like all heavy objects aboard your boat, all batteries must be securely anchored down.

While not often a problem on sailing boats, the temperature of batteries, both while being charged or discharged, or even just stored, has a significant effect on battery life. The general rule of thumb is that for every 10ºC rise in battery temperature the battery life will halve. This is particularly relevant to motor yachts, where batteries are often in the engine compartment.

AGM batteries can accept much higher charging currents than flooded cells. Again, if you change from flooded cell to AGM you may need to upgrade your charging system otherwise it may destroy itself by delivering more amps for a longer period than its design allows for. Of course the upside of this is that AGM batteries can be recharged much faster than flooded cells if your charge system can deliver the higher charging currents required by AGM batteries.

In short, unless you need to comply with a yacht racing rule, or are expecting to encounter weather which will subject your boat to knock down conditions, flooded cell batteries will give you more electrical energy, longer life, and easier condition monitoring than either of the sealed cell style of batteries, and they’re better value.

Most boats have two batteries or battery banks: one to start the engine, and one to power all the other services, such as lights, instruments, refrigeration, et cetera, often referred to as “house power” batteries. These two batteries have very different requirements. Engine start batteries need to provide high amps for a brief period of time. This is achieved by having a larger number of thinner plates (the components of the cell) providing a larger surface area for the chemical reactions to occur more quickly. Boats with electric anchor winches or bow thrusters often use these engine start batteries to power these devices; the theory being that these devices are large power users for short periods, and are usually employed whilst the engine is running, and delivery charging current to these engine start batteries.

House power batteries need to provide lower amounts of energy, i.e. fewer amps, but for a longer time, often several hours between charging cycles. This necessitates thicker and fewer plates and separators, and these batteries are usually called deep cycle. Buyers need to be wary here, since real deep cycle batteries cannot be distinguished externally from, say, car batteries with fancy labels. Best advice is to only buy from a very reputable dealer or manufacturer.

Assuming that the physical size of your battery is governed by the box into which it fits in your boat, you will still find different electrical sizes available to you. Engine start batteries are generally rated by CCA (cold cranking amps). Here the biggest is not necessarily best because the higher the CCA is, the less number of times the battery can safely be cycled or recharged. We would generally advise, assuming that the physical case size remains constant, that you choose the middle ground in CCA rating for your engine start battery.

House power deep cycle batteries are rated by amp hours, which is a measure of the amount of energy which can be safely stored and retrieved from the battery. Here generally speaking, and again assuming that the physical case size remains constant, more Amp hours is better and, not surprisingly, more expensive.

Sailing boats and motor boats also have different battery requirements. In general terms, motor boats don’t need as much house power battery capacity as sailing boats because the engine is run more often and for longer periods and so charging of the house power batteries is more frequent and for longer periods. Conversely motor boats usually need bigger engine start batteries because they have bigger engines and so require larger cold cranking amps.

Many boats have banks of batteries to provide enough CCA or amp hours. You may ask why not just put one bigger battery? The answer is that at some point, it has to be lifted in and out and batteries are heavy and being in smaller packages sometimes makes them easier to accommodate. However, fewer bigger batteries is better than many smaller batteries. Some boats also use battery banks to provide greater voltage. Most boat batteries contain 6 cells and provide a nominal 12 volts, however, many larger boats operate a lot of their systems at 24 volts which requires 2 x 12 volt batteries to be connected in series.

Even quite small boats will have multiple 12 volt batteries wired in parallel for their house power, which still only provides 12 volts, but the available amp hours is the sum of all the batteries in the bank. If you need to replace a battery in a battery bank it is always better to replace all the batteries in that bank. Generally speaking, this will give a much better outcome than having a mixed set of old and new batteries from different manufacturers.

All lead acid batteries self-discharge; flooded cells at about 1% of their charge every 24 hours and sealed batteries at 1-3% per month. Manufacturers advise that for long life these batteries should not be discharged below 50% of their capacity –so the first step in assuring longevity is to make sure that your batteries are not allowed to be discharged below this 50%.

There are a number of ways of doing this which we don’t have space for in this article, but they include frequent use of your boat, solar chargers, wind powered chargers or battery chargers connected to some sort of shore power. It is good policy to make sure your batteries are fully charged on a monthly basis, this will minimise the likelihood of sulphating occurring.

Sulphating is the build up of lead sulphates between the battery plates and is a normal part of battery operation. When it is freshly formed and soft it re-mixes into the electrolyte during charging. However, if left to harden, it does not re-mix during recharging and begins to degrade the battery performance.

Probably the next most important maintenance tip is to make sure that the electrolyte level in flooded cell batteries is kept just covering the plates in each cell. If it needs to be topped up, use good quality distilled water. Electrolyte levels need to be checked and replenished, if necessary, at least once a month.

Another important factor in battery life is to keep the top of your battery clean and dry. A wet or dirty battery will self-discharge much faster than a clean dry one, and while you are looking at the top of your battery, make sure all the electrical connections are clean and tight.

There are really only two indicators of battery condition: one is the specific gravity of the electrolyte, and the other is the no-load voltage of the battery. Only one of these methods can be used to determine the condition of a sealed battery such as a Gel Cell or AGM type, that is the voltage, since the electrolyte is not available for testing in these units.

To measure the voltage of a battery it is essential that it is totally electrically disconnected from any charging source or another battery. Otherwise you are measuring the voltage of these external devices and not your battery. You will also need a fairly accurate digital voltmeter because the difference in voltage between a fully charged battery and a partially charged battery is quite small. (see the table at the conclusion of the article).

To measure S.G or specific gravity of the electrolyte in a battery, it does not need to be electrically isolated, but it should not be being charged at the time of the measurement. The S.G. of the electrolyte is generally the weight per unit volume of the electrolyte compared to the weight per unit volume of pure water. You probably know that 1 litre of pure water weighs 1 kg, so the weight of 1 litre of your electrolyte, assuming the battery is fully charged would be 1.255 kg giving an S.G. of 1.255.

The S.G. of the electrolyte is measured using an hydrometer, which is a glass tube with a rubber bulb on the top and a little carefully designed glass float inside it. The principle is that the glass float floats lower in electrolyte of a low S.G. and higher in an electrolyte of higher S.G. This float also has a little stem on the top of it with numbers engraved on it or sometimes just coloured red, white and green. Where the electrolyte comes up to on this stem gives a very accurate measure of the S.G. of the electrolyte.

So to check the condition of your flooded cell battery follow this procedure:

Take off all the little lids which allow access to the electrolyte. It is considered good practice to arrange them in order so that they go back onto the cell from which they came. The bottom of these little lids will be wet with electrolyte and some care is necessary to prevent that coming into contact with your skin or parts of your boat which are not acid resistant.

Check that the electrolyte in each cell is about 10mm above the top surface of the plates inside the cell. If it is not, top it up with good quality distilled water. If this is necessary, you should replace the caps and wait 24 hours. This allows the introduced distilled water to thoroughly mix with the electrolyte. If you don’t wait, all you will do is measure the S.G. of the distilled water you introduced and get an incorrect answer

Put the nozzle of the hydrometer into the electrolyte and squeeze and release the rubber bulb until the little glass float is floating freely inside the glass tube. Take note of where the level of the electrolyte in the glass tube crosses the stem on the float. This stem should have numbers engraved on it which will tell you the S.G. of the electrolyte in the hydrometer. Some only have coloured bands, which generally will mean that if it is floating with the red sector level with the electrolyte, the cell is flat. If in the white sector level with the electrolyte, the cell is in need of charging, and the green sector level with the electrolyte, the cell is well charged.

No two cells will be exactly the same, so small inconsistencies are not a great problem. However, if one cell is wildly different to the others, it almost certainly heralds a battery failure and probably replacement of the battery is in order. If all cells are discharged, check the charging system over and give the battery a thorough charging.

To get a reasonable idea of the charging system’s health, connect your batteries up to it and start it going. Using a digital voltmeter, now check the actual voltage across the battery terminal. For flooded cell or AGM batteries, you should have a voltage of about 13.7 to about 14.2. The reason for this wide range is variations in charge systems design between engine driven alternators, shore power driven systems, or alternating current systems powered by an auxiliary alternator set, etc.

Whilst we are talking about charging systems, check that any engine driven alternator has a good supply of cool air. Alternators produce much less charge as they get hot.

If at any time you upgrade or replace your batteries with new or better technology, it is important to make sure that your charging system matches your new batteries. This means you need to discuss with the battery supplier how your particular boat works, i.e. is the charging system on your boat consistent with the batteries you are fitting?

Now a little bit about sizing the house power battery bank for your boat. You will need to find out how many amps every major appliance on your boat uses. Then estimate how long these appliances operate in a 24 hour period. As a guide things like a refrigerator, navigation lights, anchor lights, instrumentation, will need to be considered.

Once you have a complete list you need to multiply the amps consumed by the time they operate, which will give the amp hour figure for each item. Adding these together will give total amp hours consumed in each 24 hour period.

Next you need to estimate how often and how your house power batteries are to be re-charged. There are many options here, such as running the main engine to drive an alternator, running an auxiliary engine to drive an alternator, solar cells which are always available, wind driven generators which like solar, are always available, given appropriate weather.

Now in general terms, you need to remember that you cannot take more amp hours out of your batteries than you put in. Bigger amp hour capacity only means you can have a longer period between charging cycles. If you have an appropriate continuous charge from say a solar source or an auxiliary alternator, you will not need a big house power battery. Conversely if you only charge from a shore based power supply, you need enough house power battery to supply all your amp hours for however long you may be away from shore.

Is lithium-ion the answer?

Andrew Wilson of Mastervolt makes the case for lithium-ion batteries, a top end option.

The standard lead-acid battery has barely changed since it was designed in 1859. Although new variants such as AGM and gel batteries have been developed, improvements in terms of power density, lifespan, size and weight have found their bounderies. This is why the technical experts at Mastervolt felt the time was ripe for change.

Lithium-ion technology

Used primarily in smaller devices such as mobile phones and laptops, lithium-ion batteries are appreciated for their limited size and weight, low rate of self-discharge and lack of ‘memory effect’. We asked ourselves whether this technology could be put to more robust use? How about on a large, seagoing yacht, an advanced sailboat or a van with sensitive equipment onboard?

After some extensive R&D,  Mastervolt has now developed the first-ever high-capacity lithium-ion battery which can easily and safely be deployed in such demanding environments. This advance is sure to revolutionise the way yacht owners, yards and contractors see things in terms of batteries.

More efficient and effective

Two elements determine the amount of useful power that a battery can deliver: The charge/discharge efficiency and the extent of the authorised depth of discharge (DOD), a factor that can dramatically lower the lifespan of a battery. The Mastervolt lithium-ion battery is up to 15% more efficient than a traditional lead-acid battery, allowing:

• shorter charge times;

• shorter running hours when charged by generator;

• more power from a battery of the same dimensions:

• a lead-acid battery, for instance, has a recommended DOD of 50%; this means that you can only use up to 200 Ah from a 400 Ah battery;

• a lithium-ion battery with a DOD of 80% means that those same 400 Ah provide some 320 Ah instead – almost 60% more!

The design in Mastervolt’s characteristic colours is fitted with two handy integrated handles. The battery poles are easily accessible as well as fully protected, meaning no additional shields or covers are necessary. The handles include a number of recesses for a correct and convenient installation of the cables.

Masterbus compatible

For the best possible performance, the Mastervolt lithium-ion battery is equipped with MasterBus communication. To charge the MLI 24/160 lithium-ion battery you have to use a MasterBus enabled battery charger. Besides a quick and safe charge process this gives some more advantages:

• a more efficient charging process adds around 5% more capacity;

• consumption and available capacity are perfectly attuned to each other;

• MasterBus system integration gives you a good overview at any time of how much your battery is charged.

Faster speed thanks to lower weight

When speed and sailing performance matter, a lithium-ion battery–weighing 70% less than a comparable lead-acid battery has major benefits. A fast yacht with an average 800 Ah set onboard, for example, will shed 600 kg and enjoy a considerable performance gain.

Easy installation

A lithium-ion battery is smaller than an equivalent lead-acid battery. In fact, the MLI 24/160 replaces two separate 12 V lead-acid batteries, making the footprint 70% smaller. This means it requires less space, can be placed almost anywhere, and is easier and faster to install.

Discharging and the memory effect

The lithium-ion battery is not affected by the so-called memory effect, which means it can be used for any applications throughout its lifespan. Large or small discharges, cyclical and non-cyclical use are never a problem.

Versatility

Its high capacity makes lithium-ion technology well suited to vessels and vehicles with electric and hybrid propulsion. The MLI 24/160 can be connected in parallel unlimited and up to 250 V DC in series without restrictions.

Safety

The lithium-ion battery is equipped with a Mastervolt Battery Management System, including bidirectional active cell balancing. This allows:

• optimal safety without any restrictions during charging and discharging the battery to produce more power;

• series or parallel wiring of batteries;

• longer lifespan.

Lithium-Ion batteries remarkable weight and space reduction

• ultra-light; 70% less weight;

• ultra-small; smaller volume and footprint;

• 360° installation angle; can be placed in any position.

Installation

• ideal replacement for lead-acid batteries;

• fully MasterBus compatible;

• completely isolated battery poles;

• unlimited parallel wiring;

• series connectable.

Safety and environmental concerns

• comes with Mastervolt Battery Management System for optimal safety and yield;

• maintenance-free;

• no gas formation.

Lifespan

• very long lifespan (< 2000 charge cycles);

• no memory effect;

• 90 to 94% charge efficiency (compared to 70 to 83% for lead-acid batteries).

Use

• shorter charging times;

• can be used as a starter or service battery thanks to very high peak capacity (10xC);

• no capacity loss with fast discharging; equal Ah capacity at C1, C3 and up to C20; • the high allowed DOD gives more and lasting current.