Marine Battery Chargers

Choosing the right electrical products for your boat can be difficult.

Battery chargers use a 240V AC (shore-line) to charge your battery bank.

Once the battery bank is charged the charger then acts as a power pack enabling you to use the power output to run your 12V systems without running down your batteries. 

First: The charger inputs a full charge until the batteries reach over 14V

Second: The charger puts in a decreasing charge/current whilst the battery bank equalises

Thirdly: The charger now maintains an input at 13.5V. The batteries having been charged, the full charge is now available to use as a power pack

Fourthly: If the batteries are discharged below 11.5V then the charger resets and the charging process starts again 

How big a charger? 

For leisure batteries the charger rating should be approximately ten per cent of the total amp/hr capacity of your battery bank.

Boat Power supplies

Investing in sophisticated electronic equipment could be pointless if your power system doesn't deliver when and where required.  

Batteries are the critical element in a boat electrical system, but are difficult to choose between.  

There are a number of different types, terminology and prices. but unfortunately they also make it very easy to make an expensive mistake by choosing the wrong one.

There are particular factors that govern what type of battery should be used for a particular job and how it should be charged.  

These factors include the duty cycle, power demand, ambient temperature, depth of discharge, recharge time, electrolyte type and the space and budget available, but the choice isn’t an exact science because some of these factors often conflict with each other.  Getting it right requires knowledge and a little bit of good judgement.

The most important initial decision is to choose the correct battery type:  a battery designed and constructed for use in one application may not be suitable for another.  Starting a diesel engine, for instance, calls for a battery that can discharge very high currents in short bursts – known as cranking duty.  

By contrast, a battery used for services such as lighting or to run a fridge will be discharged more slowly but over a longer period, and considerably more of its capacity is likely to be used before it is recharged.  This is described as deep cycle or cyclic duty, sometimes called traction duty.

A cranking battery has larger, thinner plates than a cyclic duty battery, so if either type were regularly used for the wrong job they would quickly deteriorate.

There are hybrid types, often referred to as leisure batteries, that can be used in both applications but they are inevitably a compromise: whilst perfectly acceptable for vessels that are used intermittently, they don’t always offer the best long-term economy for extensive cruising.  

Another important matter is the capacity required.  For starter batteries this is easy, as most engine manufacturers recommend the size to use.  If not, you will have to find the’ Cold Cranking Amps’ (CCA) drawn by the starter motor and then match the battery to this.  All batteries intended for starter duty have their capacity stated in CCA with a reserve capacity stated in minutes.

It’s more difficult to calculate the capacity needed for service batteries.  First, you need to establish how much energy the battery has to store for a typical duty cycle.  Making this calculation is a subject in itself but it boils down to the average current figure that will be drawn from the battery multiplied by the time intervals between charges, giving a figure measured in Amp-hours (Ah).  

It is important to appreciate that this represents the energy required – not the capacity of battery needed.  To find the capacity required you need to make an extra allowance for the average depth of discharge that you are prepared to accept.  

The life of a battery used for deep cycle duty is usually quoted as a number of cycles rather than a period of time but depends to some extent on how deeply the battery is discharged: very deep discharges shorten the battery’s life significantly, so a good compromise is to aim for an average duty cycle of 50%.  

This means that the amp-hours consumed have to be doubled to arrive at an optimum battery capacity.  You may also have to consider other factors such as high peak currents drawn by equipment such as winches and inverters to ensure the battery can handle heavy discharges without adverse affect.

Having established the correct type and size of battery the problem becomes clouded by the vast array of types and styles of battery available that all claim different capabilities and properties.  

For example, there are choices between traditional flooded or wet batteries (the type that require topping up).  Starved electrolyte, and gel types.  They are all lead acid batteries but with different physical forms of electrolyte.   Each has its own benefits and drawbacks, making them suitable for different purposes.  Good advice is to discuss the pros and cons with alternative suppliers and establish what’s best for you.

Mains Battery Charger

Proper battery charging is just as important as choosing the right battery in the first place.  First, it has to ensure long and trouble-free life with minimum maintenance and second, it has to ensure that there is always plenty of power available for the job in hand.  Too small a battery charger will struggle to get the batteries fully charged; too large a charger will be a waste of money and space, and the wrong type of charger can cause problems by over or under charging.

Charger technology has advanced considerably in recent years, and the use of switch mode design has led to a reduction in size, weight and cost as well as to improvements in efficiency and performance.  

Switch Mode Power Supplies (SMPS) dispense with heavy and inefficient transformers by using high frequency techniques to convert high voltage AC power to low voltage DC power, but although the principle is now used almost exclusively by charger manufacturers, there is still a wide variation in quality and specification.

The ’charge characteristic’ determines how the product applies power to the battery.  The properties of a battery change as it becomes charged, so a good charger will adjust the voltage and current it supplies to the battery accordingly.  It is worth discussing this with individual suppliers and ask them to explain how their charger will get our batteries fully charged.

Engine-driven alternators

As they come, standard alternators are generally low in power output and relatively crude in their characteristics.  This may be acceptable for starter batteries, but it’s less suitable for service batteries because it is likely to leave them under-charged and prone to an irreversible condition know as sulphation.  This leads to a reduction in the available capacity and is ultimately the most common cause of battery failure.  Products know as advanced alternator regulators, charge boosters or battery management systems are available that can be fitted to most alternators to improve significantly the charge characteristics and thus achieve full charge in the batteries.

It should be remembered that these devices only improve the characteristics of the alternator; they don’t enable it to produce and more power.  This may be sufficient in small systems but for quick, effective charging of large service batteries it may be better to replace the alternator with one designed for high output at slower enginerunning speed.

Charging Capacity

It is not difficult to deduce that there is a relationship between the output from the charging source (in Amps) and the time it will take to charge the battery.  A battery of 300Ahrs that is charged by a 30A battery charger will obviously require at least 10 hours to become fully charged.  

In practice, however, the relationship is not quite that simple, because the rules change as the batteries become charged.  It is possible to charge a battery to 80% of its capacity relatively quickly but once this level is achieved; the process has to slow down to avoid overcharging.  It can take as long to put in the last 20% as it does to put in the first 80%.

We also have to allow for the fact that onboard services are likely to be consuming power at the same time as the battery is being charged: in effect, they will be deducting power from that available for charging.  

To make things simple a general rule of thumb is that the charging capacity: a 300Ah battery, for instance, should be charged by a 60A source.  

Higher power will be of little advantage and lower power will result in extended recharge time.  There are exceptions though.  If the batteries are to be charged when the boat is not in use, a smaller capacity can be employed, down to a minimum of about 10% capacity.  Whatever charger you use it’s important to appreciate that the fastest any battery can be charged completely and safely is about 6 hours from flat.

This suggests that any power supply problem needs to be tackled by looking at the system as a whole: for instance, although it may seem logical to add another battery to get more power, you could be making matters worse if the charging system is already over stressed.