Electricity on canal boats: nerdy detail

After you’ve read this page about the basics of boat electrics, let’s delve further into the technical detail of batteries and all things electrical. If this is the stuff of your nightmares or you’re certain it will be tedious, I would urge you nonetheless not to skip it because believe me you’ll need to have a basic grasp of this as a narrowboater (or in any other type of off-grid accommodation)

Taking Stock

There are three measures you will encounter when discussing boat batteries. These are Volts (V), Amps (A) and Watts (W). Usefully, if you know any two of those, you can calculate the third because Watts = Volts x Amps. Or, shuffling it around, Amps = Watts / Volts and Volts = Watts / Amps.

In simple terms, Volts (voltage) is like a measure of how much pressure is forcing the electricity is through the wires. It’s just like water pressure in a pipe but it’s electrical pressure. So a 12V system has less ‘pressure’ than a 230V system.

Amps measure “current” which is the amount of electricity that’s being fed through an electrical wire at any instant (like how much water is coming through a pipe)

And Watts are the measure of how much energy is coming through the wire, measured as the number of Amps being pushed by a number of Volts of ‘pressure’. Hence why Watts = Amps x Volts.

Now let me introduce you to the notion of Amp Hours (Ah) and Watt Hours (Wh). Definitely not the same as Amps and Watts and electrical experts will roll their eyes and ‘tutt’ at you if you conflate the terms (though many boaters do, and usually you can infer what they mean)

An “Amp Hour” is the number of Amps drawn through any given system in one hour. Equally a “Watt Hour” is the number of Watts used in a system in one hour.

That system can be anything, it doesn’t really matter, the point is to distinguish between Amps – which is current being drawn at a point in time – and Amp Hours which is the total draw after one hour. Likewise Watts and Watt Hours.

I mention this because if we want to talk about how much electricity a battery can store or you want to discuss how much energy is used by an applicance, you clearly need to have some units of measurement.

Volts are no good for measuring storage as they’re just how ‘fast’ the electricity will come out of the battery. And actually, Amps and Watts are useless too because they’re measurements at any single point in time.

But Amp Hours and Watt Hours are amounts used over a set period so they have value as a measure.

Think of it this way (bear with me): if you had a box full of water and made a hole to let some of it out, the instant reading of how much water was coming out at that moment doesn’t tell you anything about the total capacity of the box.

But if you measure the water as it comes out and time how long it takes until the box is empty, you now have a measure of the box’s capacity.

Yes, I know we’re talking electricity here but the idea’s the same. Count how much electricity can come out of your battery over time and you know how much electricity that battery can store.

A crucial yet often overlooked distinction is that you’ll generally see battery capacity quoted in Amp Hours but this is only a valid measure if you know the voltage. 10Ah at 12 Volts is not the same as 10Ah at 24 Volts. Yet many people omit the voltage because on boats and cars and campervans it is just assumed to be 12V.

Here’s an example: a typical 12V boat battery might be quoted as 100Ah capacity. You might also see a 6V battery quoted as 100Ah – but it’s actually only got half the storage of the 12V one. So you see, Amp Hours can be misleading because in terms of actual energy (Watts), the amount available depends on the voltage.

So let’s convert those to Watt Hours, which is a more useful measure. For the first battery, you get 100Ah x 12V = 1,200Wh of capacity (1.2kWh). For the second, it’s 100Ah x 6V = 600Wh (0.6kWh). Clearly the 6V battery has only half the storage of the 12V one, even though both are stated to be 100 Ah batteries!

The upshot is that Watt Hours is a much more reliable measure of capacity. Despite this, battery storage is nearly always quoted in Amp Hours! So be aware that the actual capacity in real terms depends on the voltage[1].

Storage varies

“Phew”, you may think, wiping the sweat from your brow. “I’m glad that’s over”. Sadly, that’s just the start. You see, for traditional battery technology, the amount that’s available to use also depends on how quickly you take it out.

Sounds weird, right? I mean, if a battery has ‘X’ amount of capacity, it shouldn’t matter whether you take it out fast or slow, it’s still ‘X’, isn’t it?

Well, unfortunately no, unless you have the latest Lithium batteries which are frankly much better behaved, easier to use, longer lasting and all round vastly superior yet also vastly more expensive. Lithium capacity is generally as given regardless how you use it but you will pay through the nose for that privilege.

Many boaters therefore still use much cheaper lead-acid type batteries (rather like car batteries albeit designed for slow release of energy rather than the single huge boost needed to start an engine) and these have all manner of annoying properties.

The first of those is that the slower you draw the energy out, the closer to the rated capacity the battery will be. In other words, if you run the microwave and toaster from your lead-acid batteries, you can expect the quoted 100Ah capacity to run out long before you’ve actually had 100Ah of use from it because those appliances draw so much power.

That same battery, fully charged, would run a single lightbulb for much, much closer to the rated 100Ah of use because a light bulb is a very light load.

This is important because as a boater you will rely on your batteries so it’s crucial to know how what you’re powering will affect how much storage you have in practical terms.

Worse still, traditional batteries suffer physical damage inside the cells as you take the power out. The more you empty the ‘box’ of energy, the more damage is done. This is true both for the rate at which you use the power (as just described) and the amount you take out overall.

Thus if you have ‘lead acid’ or ‘AGM’ or ‘gel’ batteries, the manufacturer will almost certainly suggest you discharge them to no more than 50% – yes, half – of their rated capacity, AND they will have a maximum discharge rate.

Mind-blowing, eh? There you were with your so-called 100 Ah battery and in reality you can only use 50 Ah of it before it suffers too much damage[2]

Lithium types don’t suffer this but even they have their own peculiarity; they don’t really like being charged above 90% full and manufacturers generally say they would prefer not to go below 20% left – so even they only have a range of 70% of their stated capacity! They are much less likely to be damaged if you exceed that however.

Battery types

I’ve just touched on this but there are all sorts of different battery chemistries each of which has its own pros and cons.

The traditional lead-acid battery is well-proven and very cheap but you need to keep an eye on the water levels inside (using distilled water to top them up), they give off explosive gasses as they recharge, they’re incredibly heavy because of the lead content and you have to mount them upright so they don’t spill acid all over your engine bay.

Similar but better versions of these are AGM (Advanced Glass Mat) and gel, which don’t use water or liquid acid between the elements of the battery and can therefore be sealed (other than a small vent for the charging gasses) with no need for any maintenance. Plus they can be laid on their side without issue so you won’t get acid spills.

The top-end battery type is Lithium based. It’s relatively lightweight (no lead!), can be charged and discharged pretty much as fast as you want without suffering any damage, has a near-constant voltage as it discharges and is all round a jolly good egg apart from the fantastic cost. That cost is constantly coming down so they’re certainly more accessible now than a few years ago but it’s still a hefty dent in a boater’s wallet. Then again, Lithiums will last much, much longer than the older type so over their lifetime they work out cheaper.

There are several Lithium based battery formulations but boaters tend to go for “LiFEPO4” Lithium Phosphate as these are considered very safe and stable and unlikely to explode or catch fire in the way you may have seen happen to other lithium types in scary YouTube videos. You can drive a nail into a LiFEPO4 battery and it shouldn’t explode or catch fire where other Lithium formulations such as the Lithium Ion used in cars definitely would.

Indeed, many bar-room battery ‘experts’ will try to scare you with tales of how dangerous Lithium batteries are because of well-documented instances of vehicle fires so just be aware your boat batteries are almost certainly a different, safer chemistry.

If you fit Lithium batteries to a boat that had traditional types before, beware manufacturers claiming theirs can just be dropped in as a direct replacement. Your alternator on the engine may burn itself out trying to charge them because Lithiums are so eager to accept current. It’s worth fitting a “DC – DC” charge controller between alternator and battery bank, which will regulate the flow.

A newer variant of the traditional battery is called lead-carbon and is a hybrid that tries to bridge the gap between lead acid and lithium. Only a little more expensive than lead-acid, the use of carbon in the mix is said to give a battery that can give and take charge without complaint so they can be heavily discharged with no damage.

How true this all is, I have no way of knowing though I did install three lead-carbon batteries in my boat and they worked fine. I had no way to scientifically test their stamina, unfortunately and have since sold the boat.

All of these battery types have a limit to the number of times they can be charged and discharged so there will come a point when they have to be disposed of and new ones put in.

For lead-acid batteries, they might be quoted as having just 500 such “charge cycles” (full to empty). Lithiums should typically manage a few thousand. If you consider a charge cycle (full to empty and back to full) as being once a day, you can see that Lithiums should last for several years.

In reality, you’ll be both charging the batteries and discharging them at the same time (for example, going along the canal so the engine is putting charge in but also running a fridge 24/7 so it’s constantly drawing power out. Or, drawing power by using a laptop while the solar panels simultaneously put power back in) so the notion of a formal charge-discharge cycle is a bit unrealistic.

The time scale until the batteries die depends heavily on how gently you treat them. A set of lead-acids that are repeatedly discharged, fast, below 50% capacity can easily be useless within a few months – even weeks. The same set, if used gently, could last 2-3 years.

This is much less a concern with – you guessed it – Lithiums as they can take whatever beating you put them through.


The obvious question then is “how do I know how much power is left in the batteries at any point”?

As you might expect by now, this is more complicated than you might hope and once again it’s different for traditional batteries and lithium ones because of the way they discharge.

The two main methods of working it out are firstly by the voltage of the battery, and secondly by counting how much charge was put in, then counting it as it gets used.

Let’s start by considering voltage and I have to separate out traditional lead/AGM/gel types from lithium here because they work differently.

Starting with the former then, the peculiar thing about so-called 12V leisure batteries is that when fully charged they’re actually at 12.8 V. I have no idea why they’re not called 13 Volt batteries but they aren’t.

As the charge gets less and less, so the voltage goes down. You might reasonably think then, that to work out what’s left, you just measure the voltage and bob’s your uncle. But no because the voltage also drops – sometimes quite dramatically – while you’re drawing power from the battery even though when you stop, it goes back up to whatever the battery’s actual charge level is.

In other words, to know what the ‘true’ battery voltage is (and therefore how fully charged it is), you have to be not using any power from the battery at all. This means switching everything off – fridge, lights etc and clearly that’s impractical.

Worse, it takes a while for the battery to recover to its true level so you need to turn everything off not just for a second or two while you take a reading but ideally for a couple of hours to give the battery time to ‘rest’. Only then can you take an accurate voltage reading of the battery to indicate its remaining capacity.

And of course, if you have a solar panel setup, the entire scheme is totally flawed because as long as there’s sunlight and the batteries are connected, the solar charger will be putting power in to the system – which inherently means you’ll see a higher voltage because that’s how charging works[3]. In fact, if you’re charging by any means, it will make a mockery of any attempt to read the battery’s own voltage.

Lithium batteries are even harder to read because although the voltage decreases as you draw power from them, their resting voltage barely decreases until they’re practically empty (when the voltage then dives off a cliff very quickly). And the same caveats about charging making a mockery of any readings still applies.

You can buy gadgets which are designed to read the voltage constantly and, using clever mathematics, work out what the likely state of charge is because they know about all these variables I’ve just described and take them into account but they require accurate calibration from the outset (which means you have to know how charged the batteries are – but hang on, we’ve just established that’s not so easy to do!) and can potentially lose track over time

That’s not least because the older batteries get, and the harder they’re worked, the less charge they can store so over time a ‘100Ah’ battery might actually only store 90Ah when fully charged. So your reading of “100% charged” now refers to a different amount. Sigh.

OK then – what about the other method, counting?

There are devices, called ‘shunts’, which sit between the battery terminals and the rest of the boat’s wiring. They can count any charge going in or out of the battery. They sound ideal, surely? If you have a battery of capacity ‘X’ and you put Y amount of charge in, and take Z out, then you can calculate what’s left.

By and large this is true and many people rely on such shunts. But various factors can affect the reading including the age and condition of the battery; the temperature; the rate of charge; the rate of discharge and so on. Good shunts will attempt to allow for all these factors but even so, they’re an estimate of charge rather than a solid fact.

What’s certainly true is that you’ll know your batteries are shot and need replacing when they simply won’t hold a charge any more. For example, if you did 8 hours of cruising one day (which should generally fill up a battery bank) but by next morning there’s no charge left. That said, it could also mean your alternator is failing, so never assume what’s wrong.

Speaking of charging, that’s a bit of a minefield too. Thankfully it’s also very well known and understood by the technical types who manufacture battery chargers so as long as you set your charger to the type of batteries you have (lead acid / gel / AGM / lithium), the charger will take care of things.

What you do need to know is that while the magical lithium batteries take charge pretty much as fast as you can put it into them and then stop when full, traditional batteries accept less and less charge the fuller they get.

They’re a bit like an old-fashioned vacuum cleaner with a bag. As it gets fuller and fuller, the suction gets less so less dust gets sucked in and so on.

This means they’ll charge up really quickly from empty[4] but the closer you get to full, the slower and slower they accept power so the longer and longer you have to keep charging. If you’re charging by running your engine, you could have to sit there for several hours each day to make sure the batteries are totally full.

If you think “ah, I’ll just stop after a couple of hours when the batteries are at 80% or so”, then don’t because if you only ever charge them to 80% they gradually ‘forget’ that they could ever hold more and suddenly your ‘full’ state is just 80% of what it once was. They need that extra few hours of charging to keep them in good condition.

This is why some boaters will moor up next to you and leave their engine or generator running for hours at a time; it’s not necessarily being anti-social but trying to keep their battery bank in good condition[5].

This is also why solar is so great. You can get the batteries mostly charged from the engine alternator as you chug along the canal but then the solar panels can trickle charge up to 100% once you’ve stopped. With enough solar, you don’t even need to bother with the engine at all and can just moor for days without ever running it.

[1] And making things even more complicated, so-called 12V batteries tend to be 12.8V when they’re fully charged, running down to about 11 when they’re notionally empty so the voltage changes over time! Lithium batteries are much more consistent, thankfully but they tend to sit at 13.2V despite being labelled 12V!

[2] In reality, the damage would only be suffered noticeably over a period of time and charge / discharge cycles. That said, taking a traditional battery down below 20% full will really hurt it and if you empty it out completely you’ll likely have a bit of a job persuading it to be recharged at all.

[3] Typically, batteries are charged at about 14.4 Volts until they’re full, after which they’re maintained at about 13.2 (ish) V to keep them fully topped up.

[4] Batteries, not vacuum cleaners

[5] Or they may be running the engine to heat water for a shower, the engine does many useful things!