Electrics on Andante – Understanding Onboard Power Systems

The electric system on a narrowboat is the most important thing. This article is about power systems on a narrowboat in general, the considerations, potential issues, best practices and troubleshooting of power problems.

The focus will be on the all-important “need to know” practical issues rather than deep technical understanding of how it all works. This article tries to give info for everyone who want to hire our boat, from the more experienced ones, but still want to understand more, to those complete novices that ought to know how the ‘power’ works on the boat.

Interestingly, the challenges of living with ‘on-grid’ power and particularly ‘off-grid; power are the same regardless of the situation. First, it is important to categorise the different boat “situations”:

  • Marina based boat – the boat is moored up in a marina that only “goes out” for the odd weekend and/or a couple of weeks once or twice a year. It is plugged into 230V with a decent charging system (more on chargers later). This type of boat doesn’t really have to worry too much about power. This situation is pretty much a power setting as you would have at home, with some limitations normally set by the marina.

Andante is part of this category.


  • A (live aboard) continuous cruiser boat – a boat who spends from one night up to two weeks, sat in one location, before moving on for a day or two. This type of boat is fine while moving as batteries are kept charged, but then has constant battles with keeping power up and running whilst sat enjoying time in one place.
  • A (live aboard) on a permanent mooring without power. The hardest type of boat to power manage as it doesn’t need to travel with the benefit of charging batteries as it moves.



Boat Power – Best practices and source

A boat setup according to accepted best practice will have two power systems.

  1. 12V DC low voltage for all “essential” systems. An essential system is one that in theory you do not want to do without and needs to be available at any time. Things that fall into this category would be: lighting, pumps for water and drainage, toilet systems and most importantly refrigeration.
  2. A 230V “mains” system that has standard 13A plugs and to the untrained eye works the same as in a house. This however is power for “luxury” systems – that is to say things you can do without and only use occasionally, such as washing machines, vacuum cleaners and entertainment systems. Why have these at 230V instead of 12? In an ideal world it would be great if you could buy 12V domestic appliances at an affordable price with the same range of choice we are used to in Currys. However the market does not exist for 12v luxury appliances so for many of these items 230V is the only way to go. Where it really IS worth spending extra money on a 12V unit is the fridge or freezer as these are on 24 hours a day and if you are not plugged in and where is that power going to come from? An inverter, I hear the more informed of you cry! Yes, but see later for the big power cost….

Andante has both power systems.



Source of Power

For boats in the first category, boats moored in a marina, the boat is plugged into mains power – this means all 230v sockets on the boat are running straight from the mains power as they would be in a house. Most boats will also have a 12V charging system fitted. This will not only keep the batteries topped up it will also act as a mains powered “battery” and run all the boats 12V services directly from the mains – so in this scenario life is a doddle!

Once offshore power however, as for boats in other categories are, then everything changes. The source of ALL power is the humble 12V battery. Without doubt, the hardest part of living afloat without being plugged in, is POWER MANAGEMENT. In this scenario essential systems described above run straight from the batteries, luxury 230V systems get their power from an inverter – a device that converts 12V DC power to 230V AC power as if by magic BUT in doing so generally consumes HUGE amounts of power (more on this later).

Andante has a 3Kw automatic charger/inverter that automatically switches the power from the batteries to the mains (when plugged into the mains power of the marina) and vice versa, back to batteries power when unplugged from the mains.



Understanding Batteries

A battery is like a water tank, but full of volts. (12 of them). Imagine a pipe from the battery instead of a wire and a tap on the end. Open the tap and amps flow down the pipe and out of the tap (opening the tap is, say, turning on the TV). The more the tap is opened, the more amps are flowing. As the amps flow, the volts in the battery drop until it’s empty. Charging the battery refills the battery with volts by passing current in amps back into the battery. This is done using some sort of charging system (the most well-known method is, like a car, by running the engine).


Anatomy of a battery

On most boats 12V batteries are used. These batteries come in two main types; starter battery and leisure batteries. The starter battery is used as its name suggests to start the engine only (same as a car starter battery) and is isolated from the leisure batteries that are used to provide 12V power for everything in the cabin described above. Noticed that I have used the term “batteries” plural. This is because to provide the considerable about of power required to keep a boats essential and luxury systems going it is normal to “bank” up batteries (to provide a bigger overall “tank” of power). Although it is possible to buy a very big battery it is not usually practical to do so therefore it is usual to have, say 4 x 12V leisure batteries connected together to form one large battery (just as you would put multiple batteries in say, a torch). Batteries of course have a positive and negative terminal and it is just a case of connecting all of the positives together and all of the negatives together and taking a single supply from each.

Individual batteries – the familiar shoe box + sized black box )that weighs a ton!) are actually made up of 6 x 2V “cells” that together deliver 12V. Starter batteries and leisure batteries differ in that starter batteries are designed to deliver a very high current for a very short time – the time it takes to start the engine – and are never drained (or “deep cycled”). Leisure batteries are designed to deliver a smaller current steadily over a long period of time and are likely to be regularly drained or deep cycled. The difference is actually to do with the structure of the plates inside, however from our purely practical point of view it’s important to note that key difference.

Andante is fitted with a bank of 4 x 110ah AGM leisure batteries and one starter battery.

Leisure batteries are further divided into different technologies, but we will obviously focus on the AGM (absorbed glass mat) type and that is as technical as were going to go!

These use glass fibre matting between plates which makes for a bigger plate contact area and thus a battery that can deliver starter type high current and deep cycle. The AGM batteries are the best way to go for a long reliable life. However they are not tolerant of being over charged so you need to be really sure your charging systems are in good order.


Battery Capacities

Most boats have a bank of 12V lead acid or Gel/AGM batteries and once again, Andante is fitted with a bank of 4 – 110ah AGM batteries.

Individual batteries have their capacity rated in AMP Hours. That is to say how many amps they can deliver over one hour (a 110ah battery can deliver 110 amps over one hour if you are drawing 1 amp). This means if you are drawing say 10 amps with the TV, lighting and other consumers during an evening, then every 6 minutes you will draw 1 amp hour. However – a battery’s usable range is not the full range of the batteries voltage.

Imagine the water tank analogy – the pipe running to the tap is a long way up from the bottom! A fully charged battery will sit at around 12.6 volts assuming you don’t draw anything from it. The usable voltage runs down to about 11.5 volts. Beyond that level if you continue to drop the voltage the battery life will suffer so it’s not advisable to lower the voltage beyond this point. Beyond this 11.5V level the battery will quickly drop down to 10.4 volts which is a truly dead battery – referred to as “voltage end point”.

So, there is this relatively small operating range between 12.6 and 11.5, which means you actually only have access to about 30% of the actual capacity or about 36-ish amp hours. You would expect (through the monitoring tools which will discuss later) around 120 amp hours of actual usable power out of a bank of 4 x 110ah batteries.


Battery Charging

The most widely known form of charging is just like a car, running the engine which turns an alternator that charges the batteries. Batteries need a voltage of 14.4 volts to accept a charge.

Alternators use a very simple system called a taper charge. The alternator outputs a voltage of 14.4 volts and the battery bank at a much lower voltage accepts this and starts reversing the chemical process of discharge and “takes on a charge” as the battery voltage rises, its resistance to the alternator increases and so the alternator current reduces. It’s a bit like an old fashioned balance type weighing scale, eventually the resistance from the charged battery becomes great enough so that the alternator output drops to close to zero. However, at that point the battery is generally only at 80% of its maximum charge. This is not a problem for a starter battery – that is plenty to start the engine BUT if your leisure batteries are only at 80% charge that is another huge drop in usable battery power. It’s just the simple balanced way that alternators work. Alternators range in their output – older boats tend to have alternators of around 50 amps output where as newer engines are often fitted with alternators as high as 175 amps. Clearly the larger the output the quicker those amps get put back into the battery!

Andante is fitted with an 140amp alternator.


Another important point to be aware of here is, remember we have a starter battery and a bank of leisure batteries? These are two separate battery banks so that if you drain the domestic batteries, you still have the fully charged starter battery to crank over the engine. How is this possible? Older boats typically only have one alternator so some kind of “split” charging is needed. This means that while the engine is running the charge goes to both the starter and domestic batteries, but once the engine is shut down, the batteries are “separated” so the starter does not get discharged by domestic services.

There are a number of ways of doing this. A split charge relay can be used. This is a simple electro-mechanical “switch” that closes when the starter motor is energised and ensures charge flows to both sets of batteries, then opens when the engine is shut down so both battery banks are isolated. Alternatively, a split charge diode can be used. This is an all-electronic device that performs the same task BUT diodes introduce a voltage drop of a little over 0.5 volts – this hugely reduces the charge.

There are also voltage sensing relays that are very simple to fit that connect both banks together when they sense a charge voltage and split them when the voltage typically drops below 13V. Newer engines also tend to have two alternators, a large output one used to charge the domestic batteries and a smaller one to charge the starter battery – this naturally keeps the batteries separated without the need of a relay or “splitter”.

Andante has 1 starter battery, 1 x 140amp alternator and is fitted with a charge split relay.


Battery Chargers

The second way of charging batteries is to use a built-in mains powered charger. Most new boats are fitted with these as standard and they run while in the marina plugged into a land line. These use a 3-step charge process which is a much higher quality charge than that which you get out of an alternator. This works as follows:

  1. Bulk charge phase – the charger puts in its maximum current at a voltage of 14.4 bringing the battery up to its 80%.
  2. Absorption charge phase – The voltage is held at 14.4 but the current is gradually reduced creating a trickle charge that takes the battery up close to its 100%.
  3. Float charge phase – Voltage is dropped to just below 13.5 so the so the batteries don’t start to gas and current is reduced to very close to 0 amps, but if a consumer demands power from the battery the current is raised to support this.

The float charge stage is very useful as it means while plugged in you simply leave the charger on all the time, it will keep the batteries in tip top condition AND runs all of your 12V services at the same time acting like a mains powered battery in effect. This makes marina living very easy!

Chargers range in power output rather as alternators do typically from 10 amps to 120 amps depending on your budget and the speed with which you want to charge the batteries. In reality somewhere around a 40 amp charger is more than sufficient for a typical 4 x 110ah battery bank. In fact many marina berths have a limited mains power supply and large chargers can cause the trip to go off on the “post” that you connect to.

Andante is fitted with a 3Kw automatic charger/inverter that automatically switches the power from the batteries to the mains (when plugged into the mains power of the marina) and vice versa, switches back to batteries power when unplugged from the mains.


How can I monitor the batteries?

Now we are onto the really useful stuff! Battery monitoring is very important if you are living out on the cut with no shore power land line – it’s the only way you can know what’s going on. However, it’s amazing how many boaters out there don’t have any way of monitoring their batteries. So what can you monitor and what are the options?

Let’s just review battery voltages:

  • 5 – 14.4 volts is a battery under charge. Once the charge stops the battery will quickly drop to 12.6 volts which is where it will sit until you start to draw power. You then have from 12.6 down to 11.5 volts as you use the battery, then it needs charging again to avoid damage.

The simplest “tool” is a voltmeter. An analogue gauge measuring between 0 and 14 volts usually. Many boats have these and whilst they give you a guide, because we are only interested in that small window between 11.5 and 12.6 its actually quite difficult to “see” where your batteries are in that small band on an analogue dial.

Alternatively a digital volt meter is an even better tool. There are various different types all mountable like an analogue gauge. Using a digital meter you can accurately monitor that window between 12.6 and 11.5.

So using this voltmeter you know for sure how long you need to run the engine for and it really is the ONLY way to have a pretty good idea of where you are with your batteries.

Andante has 2 digital voltmeters, 1 in the engine room and the other in the galley.


Battery testing

Various gauges are great for monitoring batteries that are known to be good and in use but what about when you start to have problems? What problems typically occur?

A common problem is that batteries that have been under charge for hours do not “appear” to hold the charge for very long. This is usually an indication there is a problem. A common failure of a battery is for one of the 2v cells to “die” leaving you with a 10V battery – this is because metallic substances from the plates drop to the bottom of the cell during discharge, eventually building up to a level where the cell shorts out. But of course your batteries are in a bank of maybe 4 or more so it’s not immediately obvious what is going on but what happens is the good batteries keep draining into the one with the dead cell as soon as the charging stops. The only way to check for this is to disconnect each battery so they are no longer in a bank and run a volt meter over each one in turn until you find the 10V one.

In the short term you can just leave this battery out and reconnect the remaining batteries. Strictly speaking you should then replace the ENTIRE bank of batteries as replacing just one is not a good practice as it will have different charge characteristics to the others. However, if you know the remaining batteries are good then you can get away with it as long as it’s of the same type and size.

One “gotcha” to watch for is that many lead acid batteries have a little window on top of the battery that shows whether it’s fully charged by turning green. Do not rely on this as it’s only looking at ONE of the 2V cells, another cell in the battery might be dead, or need topping up for example.

Other issues can arise with the electrolyte (the acid) not being strong enough any more due to water build up and sulphation of the plates (a build up of material that stops the electrolyte reacting with the plate properly). With lead acid batteries the only way to properly identify if the battery is holding a charge is with a hydrometer. This is a big syringe type device with a float inside it which measures the specific gravity of the acid. You just draw some acid – very carefully I must add – into the hydrometer and see where the float is. This is a sure fire way of telling if a battery is fully charged.

A fully charged battery should measure 1.265 on the hydrometer. You then need to test each cell in turn, taking care not to spill any of that nasty acid anywhere! Cells that have low acid will also output a lower voltage and should be topped up with de-ionised water which can be bought in car accessory shops very cheaply. Lead acid batteries should have their levels checked and be topped up I would say every 3 months.

Another test is to monitor the battery voltage once the batteries have been charged with everything switched off and see if it rapidly drops below 12.6. If it does then chances are the bank needs replacing.

Another classic problem I see often is that without battery monitoring tools a boat owner will run their engine for a couple of hours each day but find that the level of charge seems to get less each day. This is because on day one the batteries get charged to 80% then on day two 80% of 80% and so on, so the actual capacity dwindles. Hence you can see the benefit of the proper management panel (and certainly this is where solar power charging as covered in my last article makes a REAL difference putting a small current back but all day long).

Generators and Inverters

Generators come in different types. The petrol hand pull type, and the more expensive “suitcase” type which are now very quiet. Andante is not fitted with a power generator.


Now this is the important one and a great source of flat batteries if used incorrectly! An inverter is a device that takes your precious 12V power from the battery bank and converts it into 230V mains electricity which is then fed into your boat through a transfer switch (see below). It sounds great doesn’t it? Mains power with no noise coming from nowhere by magic! However, beware – generating mains this way is VERY costly on battery power.

Let’s take an example….

There is an electric kettle on the boat that takes 3 minutes to boil and consumes 3kw of power to do so. The boater has a 3kw inverter on board so that’s all fine – it will work! However if we do the sums 3000W divided by 12V = 250 amps – that’s a massive amount to draw from the batteries and remember leisure batteries DON’T like a high current draw. So we have a general rule of thumb here. Avoid using an Inverter to run things that heat up as they all consume VAST amounts of power.

The inverter should be used for light duty things like entertainment equipment and such like. If you are in a position where you need to run the washing machine and don’t have a generator of any sort but do have a big inverter then the rule is you MUST run the engine at the same time as drawing that load so that you are at least putting the bulk of the current back in AS you draw it out through the inverter.

The other golden rule with inverters is to switch them OFF when you don’t need 230 power. They all consume valuable power (around 2 amps typically) just sitting in idle mode, though some new inverters do have a standby function that reduces this I would still adopt the mantra of if you don’t need it right now TURN IT OFF. That goes for ANY device on board not just inverters.


Remember – if you are not using it SWITCH OFF!

Close Bitnami banner