How-to: Replacing an Electrical System

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If you’ve bought an older boat with a battery bank like this, you’ll no doubt be looking at upgrading (left); Some charge splitters cause a voltage drop—others, like this one, do not (right).

If you’ve bought an older boat with a battery bank like this, you’ll no doubt be looking at upgrading (left); Some charge splitters cause a voltage drop—others, like this one, do not (right).

Although a fiberglass sailboat can seemingly have a half-life of 50 years or more, its electrical system most certainly does not. Up until the 1990s, few production builders used tinned wiring in their installations, and most owners had fairly basic requirements, allowing boatbuilders to get away with providing rudimentary electrical systems.

Now, the cruising life is more complex, and sailors want more home comforts, which means more electrical gear on board. This, in turn, demands a greater battery bank capacity and improved charging systems. Heavy-duty items such as inverters can really test a feeble electrical network and often result in arcing connections, blown fuses and even cable meltdown.

As a result, electrical system upgrades are often high on the to-do list for owners of older—and sometimes not so old—cruising boats. It’s far from unusual for 30-plus-year-old boats to be offered for sale with their original systems, so let’s approach this series from the point of view of someone who’s just acquired such a vessel and wants to bring the entire electrical system up to today’s standards. Let’s start with the heart of the electrical system—batteries and how they are managed.

Where do I Start?

A VSR is added between the two battery banks to control charging (left); VSRs are simple but effective devices for charge control (right).

A VSR is added between the two battery banks to control charging (left); VSRs are simple but effective devices for charge control (right).

First, you should carry out an audit of the total electrical capacity you will require between charges. This can vary widely with sailing styles. If you plan to regularly go off-grid for a few days, for example, this figure can be quite high. Daysailors and weekend cruisers, on the other hand, not so much. At the same time, the larger your battery banks, the greater the expense and weight, so it’s all a careful balancing act.

It’s always best to assume the worst. I’ve lost count of the people who’ve told me they can’t understand why their batteries are always going flat when all they run is a few lights and a laptop—conveniently forgetting their fridge is continually on and that they run an electrically pumped diesel heater all night in the cooler seasons.

Any economies you can make while working out what you need are always better than providing more and more power for an ever-increasing inventory. Examples include changing all your cabin lights to energy-efficient LEDs and making sure your fridge is well insulated.

There are several energy consumption worksheets available online where you can input the loads specific to your boat and determine the necessary battery bank capacity. Even if you are not presently planning to change your DC electric system, these worksheets are a very useful tool for analyzing your electrical loads and needs. One such can be found on the West Marine website.

The Power Supply

Your existing system will most likely have a dedicated engine start battery and a house bank comprised of one or more deep-cycle batteries linked together. You will have to decide whether to keep the same setup and just renew the components or expand your battery bank’s capacity. Given the expectations of today’s cruisers and the power needed to meet them, most sailors will opt to expand their house bank.

Know the difference between batteries connected in parallel or in series

Know the difference between batteries connected in parallel or in series

The house bank can be a combination of batteries wired in series or parallel, depending on your particular needs. One popular setup is a house bank comprised of 6V deep-cycle lead-acid batteries linked together to produce 12V or 24V overall. The main reason to use 6V batteries is their ready availability (they are commonly used in golf carts) and their portability. A pair of 6V batteries do, however, take up slightly more space than a single 12V one, so finding room for them might be an issue.

If you connect batteries in series, the output voltage is increased while the capacity (Ah) remains the same. Connecting them in parallel increases the bank’s current capacity while the voltage remains the same.

For instance, you might have 8 x 6V/100Ah batteries joined together in series pairs to effectively give you four 12V/100Ah batteries. The four 12V banks could then be connected together in parallel to give you a 12V/400Ah house bank.

Which Battery?

Li-ion batteries (left) are increasingly popular; AGMs like the one right provide years of service

Li-ion batteries (left) are increasingly popular; AGMs like the one right provide years of service

The principal choice of marine service battery for many years has been the deep-cycle lead-acid wet cell battery. These flooded batteries are simple to install and maintain, relatively cheap and available all over the world.

In the past few decades, however, battery technology has improved considerably and several new types have been introduced with varying benefits. Primary among these are AGM and gel batteries, which have non-liquid electrolytes that make them leak- and maintenance-free. They also have a longer lifespan and never need to be topped up with distilled water, as is the case with lead-acid batteries. More recently, there’s also been a new kid on the block—lithium-based (Li-ion) batteries, most commonly LiFePO4. Initially, these were very specialist and vastly expensive, but now it’s quite common to see them on the chandler’s shelves and if you are looking at a total refit, now is the time to consider buying them for your house bank.

Li-ion batteries are much lighter than lead-acid and offer deeper discharging and more service cycles, but they also have their limitations. They have a higher cell voltage than lead-acid and much stricter charging tolerances are required to properly protect them.

Although a dedicated mains Li-ion battery charger is a voltage-limiting device similar to those for lead-acid batteries, it utilizes a different regime. The first, “bulk” stage can be faster as it can take a higher current, but once it’s around 70 percent full, the second or “absorption” stage can take longer.

Unlike lead-acid, Li-ion batteries don’t need to be fully charged, nor indeed should they be, as doing so can overstress the cells and make them unstable. For this reason, rather than having a third, “float” level as you do for lead-acid cells, the charge parameters on a multi-stage Li-ion charger are usually preset to switch off at the end of the absorption stage and only cut back in when the voltage has dropped to a predetermined level.

It’s also important, whatever the charging source, to ensure the cells in a Li-ion battery receive a balanced charge by regulating the charge to each cell. This requires a lithium battery management system (BMS) to ensure the procedure is carried out correctly and safely. That said, many of the latest LiFePO4 batteries now come with a BMS built-in, so provided your charger is outputting the correct voltage ranges they should be fine.

A fairly typical house bank on an older boat, with a pair of deep-cycle flooded batteries

A fairly typical house bank on an older boat, with a pair of deep-cycle flooded batteries

Charging Ahead

Once you’ve decided on a type of battery and the size of your battery bank, you need to think about how you’re going to charge it. It’s no good installing an 800Ah battery bank and expecting a 90A alternator and a single 100W solar panel to fully charge them over a few hours. Even to this day, marine diesel engines are equipped with automotive alternators and regulators, which are designed for topping up thin-plated starter batteries in a few minutes. They are useless for recharging a large capacity, deep-cycle battery bank above 80Ah and quickly overheat if an external regulator is fitted. Another important consideration to keep in mind is that alternator output drops as the alternator temperature rises. Once the alternator is warmed up it will probably only generate half of its “rated” output. If you are going to install lithium batteries, the charging regimen will require changes to the charger and alternator, perhaps even a complete exchange. If you want to keep a wet cell battery for starting, it will also need a different charger and alternator.

Unless you have a dedicated diesel generator or are planning to install one, or you have enough space to fit at least a kilowatt of solar panels, your engine will always be your prime source of power onboard, so it pays to splash out on a high-capacity alternator with an intelligent battery-specific regulator. This will ensure every minute of engine running is used to the full without damaging either the alternator or the batteries. We’ll cover these in detail in a future article.

Other Charging Sources

Depending on the type of cruising you plan to do, you should also consider alternative energy sources. Solar panels have come down in cost dramatically over the last few years, so it makes sense to cover every inch of spare surface with PV panels. If you are planning a longterm cruise in trade-wind areas like the Caribbean, a wind generator is definitely worth having, and if you’re planning on regular long ocean passages a hydrogenerator is a worthwhile investment too.

Finally, if you spend any time at all at a dock you should carry a shorepower AC charger that is powerful enough to completely recharge your batteries from 50 percent capacity in a 12-24 hour period, and preferably one that enables you to “equalize” the batteries every once in a while to stop the plates from sulfating. These will all be covered in future articles.

Battery Isolation


Any older boat will most likely be equipped with “Off-1-2-Both” style rotary battery isolation switches. To eliminate the risk of flattening both your starter battery and house bank by leaving the switch in the “both” position for long periods of time, consider instead installing individual isolator switches for each battery or bank. If you do that, you’ll also need to install either a diode charge splitter or a voltage-sensing relay (VSR) between the different battery banks that will link them all together for charging, but automatically isolate them when the charging source is shut off.

A charge splitter contains several heavy-duty diodes that only allow current to flow in one direction. These are connected so that they allow the charge from a single source (usually the engine alternator) to feed into more than one battery but isolate the batteries once charging has ceased. Some splitters can reduce the charge voltage by 0.5V or more, so make sure you choose a zero-drop (Mosfet type) splitter/isolator that will let sufficient voltage through to fully recharge the batteries.

A VSR is simply a heavy-duty relay that activates between a set of predefined voltage parameters, usually around 12.4V-13.8V. This means that it activates when it sees 13.8V+ on the sensing terminal, linking the start and house batteries together for charging. Once the voltage drops to the lower threshold, it deactivates, isolating the two battery banks. There are variations of VSRs, including dual-sensing (DVSR) and current-limiting (CVSR) devices.

A DVSR allows you to adjust your priorities. Let’s say you have the engine alternator feeding directly to the starter battery, but once that is full you also want it to charge the service bank as soon as the former is nearly full. You might then, say, have a solar array feeding directly to the service bank, and once that’s at a healthy level, you’d like to send some of the charge to the start battery. Connecting a DVSR between the two will ensure they are joined together for charging whenever either source is available.

If you decide to install separate isolator switches you should also add an emergency paralleling switch so that, if necessary, the engine can be started using the house bank.

Once you have decided on the number and type of batteries you want and how you will set up your charging system, you can tackle the next stage of the project—choosing and replacing cables and making connections, and ensuring proper overcurrent protection. We will cover those subjects next month. 

Battery Choices



• Least expensive

• Easy to find worldwide


• Heavy

• Limited cycle life

• Limited discharge capacity

• Likelihood of gassing during bulk charge


Silica gel


• Maintenance-free

• Long life

• Spill-proof


• More expensive than wet batteries

• Require special (slower) charging regime

Absorbed Glass Mat (AGM)


• Totally sealed so can be mounted


• Zero maintenance

• Less self-discharge

• Long life


• More expensive than wet cells

• Can be overcharged



• Very light

• Can be discharged to 20 percent capacity

• Faster to bulk charge


• Very expensive

• Require a smart battery management system

• Easily damaged by overcharging

Cost comparison between deep-cycle lead-acid and lithium batteries:

4 x 110Ah/12V flooded

deep-cycle batteries: $ 800

1 x 40A multistage AC charger: $300

Total: $1,100

4 x 105ah/12V AGM deep-

cycle batteries: $1,600

1 x 40A multistage AC charger: $300

Total: $1,900

4 x 100Ah LiFePO4 batteries

with integral BMS: $4,000

1 x Lithium-capable 40A

charger: $500

Total: $4,500

*Note that although you would get a useable 200Ah out of 4 x 100Ah lead-acid batteries, a bank of three lithium batteries will let you consume 240Ah



BEP Marine 

Blue Sea Systems 

Defender Marine 



Sterling Power 


West Marine 



March 2020



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