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Pick a Battery

Updated: Sep 14, 2020

Why Fit a Battery?

I was trained as a system designer. This discipline teaches that, for any project, you establish what you are doing and why you are doing it. Deeply philosophical, but is how I tackle problems. The consequence is that I am going to start right at the beginning with questions like: Do we need a battery? What is it for? and What is the meaning of life? (Well, OK, two out of three ain't bad).

There are many different electric boats, with, at one extreme, cruise liners with huge diesel electric systems that don't have a battery. The diesel generator delivers power that exactly matches the demand of the motor. At the other extreme is the electric trip boat that plugs into the mains at its mooring. It needs a battery big enough to complete a trip and get back to "home" to be recharged.

A narrowboat which is going to cruise for some days away from a charging point lies between these extremes. It is not practical to store the energy required for an indefined period, so we have to generate energy on board. At this point I would like to include a short homage to the Inland Waterways Association's Sustainable Propulsion Working Group, who are pointing the way to the use of fuel cells and hydrogen in the not too distant future. I recommend reading their words of wisdom here:

However, for Perseverance hydrogen or only solar are not yet practical, so it's diesel plus solar for us.

The power needed to propel a narrowboat varies significantly. There is the modest power needed to cruise, zero power to sit in a lock or at a mooring, high power for short periods, such as to push out of a lock or to stop the boat rapidly, and high power for long periods to make headway against a flowing river. The "against the flow" power case is always going to be limiting, and at some point you just have to say we're not going to make it against the current. But this is also the case for diesel powered boats, and there will always be rivers you can't navigate safely, no matter how much power you apply.

The key thing about storing energy in a battery is that it allows us to separate the process of generating energy from the process of using energy. Do we need a battery? Yes. What is it for? To allow us to generate and use energy at different times and at different rates.

Generating Energy

The energy consumed by the motor and domestic consumers has to be created by the generator, solar or ground sources and stored in the battery until needed. To state the obvious, the energy budget has to be in credit all the time. You can't use the energy and generate it later!

Using Energy

The power of the generator no longer has to match the power of the motor, it is the battery which must be able to match the power of the motor, plus any domestic load.

So the battery has to be able to deliver the maximum motor power while the generation systems have to be able to create the energy required over time. The capacity of the battery depends upon how well the energy creation matches the energy usage. After all, if the two match perfectly we don't need a battery at all (the cruise liner case) while if they match badly (a solar powered boat cruising on a cloudy day) we would need a large capacity battery.

Sizing the generator is a balance between a small generator running for a large proportion of the time, or a larger generator running for shorter periods. Also, the larger the contribution from solar power, the less time the generator will run. As a design point, I thought a generator of 6kW would be good, as this is twice the nominal cruising power, so will run for half the time we cruise. Rough and ready, but you have to start somewhere.


Now that we know why we are fitting a battery, and how it relates to the motor and generator, let's get into the detail of Volts, Amps and hours. But first a pointless digression for old school engineers; the joy of a good AVO meter for no other reason than to remember the feel of the bakelite.

I am sad for kids who only ever used digital meters; the understanding gained from watching the movement of a needle on a dial is lost with just numbers jumping about. Hey ho, back to the plot...



The question of battery voltage is easily answered. If you go above 48V the system is considered hazardous, and much more complex protection is required, but if you go below 48V the conductors (which are already huge) become even thicker. Also, I had already picked a 48V motor with the certainty that I would be picking a 48V battery. It's a bit chicken and egg, really.


How big the battery needs to be depends upon how much you want to be able to do before recharging it. Until global warming makes sunshine a certainty in England, it will be necessary to recharge the batteries using a generator. It follows that a key decision is how long the boat should be able to cruise on battery power without running the generator, and this brings me to the key point of this blog: The Pimms Criterion.

The Pimms Criterion

Let's consider a boat designed to cruise all day on battery power, then recharge when moored up. You are at the end of the day's cruise, moored in the countryside with rolling fields on either bank. The evening is still, with not a cloud in the sky. You sit on deck with a cold glass of Pimms and, as the sun dips below the horizon, you listen for birdsong, but all you hear is the generator grinding away in the background. To my mind, this defeats the object of electric cruising. Why glide silently by day, but shatter the evening stillness? The answer is, of course, to make the generator quiet enough so that you can enjoy your sunset Pimms in silence.

But wait. If you can't hear the generator running, why not recharge the batteries while going along? After all, if it is quiet enough not to disturb the still of the evening, or breakfast taken on deck in the calm of morning, why not run it as we are going through a town or passing a factory? Indeed, if it is truly silent what does it matter at all? Charging while cruising has some real advantages, because the battery capacity is no longer defined by the energy required to cruise all day, but by the energy required to cruise for a reasonable period and, of course, the energy required for domestic use.

Cruising Energy Consumption

This was easy to calculate. Take the time to go through Standedge Tunnel, say, 1 hour 30 minutes. Round it up to 2 hours for margin and there's a minimum time to run on battery power alone. In a tunnel we'd expect to use 4kW due to the extra drag between the boat and the tunnel, giving 8kWh as the energy consumed.

Domestic Energy Consumption

Many marinas have a rule that prohibits running a generator at night, and although it may be possible to argue that a quiet generator is not disturbing neighbours, it would be more sociable to comply with the rule. For estimation purposes, we assumed that the rule applies from before we started cooking dinner to after cooking a breakfast. We did the usual thing of totting up all the energy consumers on the boat (fridge, TV, hob, lighting etc.) and came to a "worst case" condition where dinner was a roast turkey and breakfast was a full English. One of the tricky things is to work out the real energy consumption rather than the maximum possible. For example, an oven uses full power to get up to temperature, but then runs at about 25% load to maintain that temperature. Similarly, we will fit an induction hob which can draw a lot of power but only for short periods. In fact, our worst case of cooking Christmas dinner and fried breakfast for six came out at 6.4kWh.

Of course, there is no need to add these two together - we're not going to cook Christmas dinner while navigating the Standedge Tunnel!

Battery Size - Energy

As we have established that the voltage is 48V, and if we round up our energy requirement to 10kWh, the battery size comes out at 200Ah of useable storage.

Battery Size - Power

I have got this far without considering the power requirement. That is, the current which the battery has to be able to supply. The motor current load is far higher than the domestic loads, and the 15kW motor I selected (see earlier blog) will draw about 300A. Many batteries will be able to deliver this high current for a short period, but delivering currents of the order of 60-100A continuously can be limiting. In particular, while I was researching the market for suitable batteries, I found one class of batteries that look promising, but which are unsuitable for electric narrowboats. These are the batteries designed for domestic solar power installations, characterized by huge capacity but low power outputs.

While this is the highest drain current we are likely to see for any duration, the charge current also needs to be considered and is, paradoxically, the limiting factor. Charge current relates to generator power, and this is most easily viewed as a balance with the cruising motor load. As hinted at earlier, I used the "finger in the air" technique and said if the boat cruises at 3kW and we want to run the generator for half the time that we are cruising, it needs a 6kW generator. This corresponds to a charge current of 125A. You could fit a bigger generator and charge more quickly (with the commensurate challenge for the battery) or you could fit a smaller generator and accept longer charging sessions. It's a trade off, and I thought the 2:1 ratio felt about right.

We now have the key battery parameters:

  • Voltage = 48V

  • Energy = 10kWh (at 48V = 200Ah)

  • Peak discharge current = 300A

  • Charging current in the range 125A

...and we've pinned down the generator power at about 6kW.


Selecting a Battery

Batteries come in different types and sizes. For each type of battery chemistry, there is a relationship between the capacity of the battery and the current which can be taken. In simple terms, this is the number of hours it would take to charge or drain the battery. For example, if a 30Ah battery had a current capacity of 6A for prolonged use, it would take 5 hours to fill and 5 hours to drain. The number of hours is termed the "C" value, so this would be a C5 battery.

If only life were that simple.

  • Depth of Discharge (DoD) is the proportion of the full capacity which you can usefully and repeatably use. If you fully charge and fully discharge a battery, its life is used up more quickly so there is a balance of DoD against battery life. A lead acid battery is typically operated across only 60% of its full capacity in order to achieve a good service life, or to put it another way, you need a battery almost twice the size you thought you needed to ensure adequate services life.

  • Rate of Charge and Discharge will affect both the life of the battery and the effective capacity. For example, if we discharged our 30Ah battery at 10A (C3), we might only get this for 2.5 hours, making it look like a 25Ah battery. As with using a higher DoD, operating the battery at higher currents will also reduce battery life.

  • Temperature is, for some types of battery, a limiting factor. The "killer" here is that Lithium Ion batteries really don't like being charged when they are cold, and can be damaged beyond recovery.

With the chosen capacity and charge rate, I plan to operate at less than 2 hours to fill or empty the battery (C2). After looking at probably 100 different batteries, and collating the technical details for 40 of these I concluded that a Lithium Ion (Li-Ion) battery was the best option given my needs. I could have used Lead Acid or Lead Carbon batteries but these are best operated at lower charge and discharge rates. For example charging or discharging at 150A from a battery operating at C5 gives a store of 36kWh, which is almost four times more than I need.

To make a comparison, I have taken a typical low cost lead-acid battery, a more modern lead-carbon battery, and the lithium ion battery I have selected. Here are the three batteries side by side, using criteria discussed above.

Battery Comparison Table

I have used an 80A current to set the number of batteries in parallel as, although higher currents can be drawn, this matches the C10 rate for charging and discharging the lead batteries. Also, the price per cycle question is cheating slightly, as the smaller Li-Ion battery will be cycled more often, so the lithium-ion batteries may work out more expensive on balance, but they are being operated well within their specification so may do better than 3,500 cycles while it is easy to operate the lead batteries outside their C10 rating and so reduce their life.



You will have guessed by now that this is the battery I have picked. Two of these in series (with their associated Battery Management System) will fit in a space the size of a 500mm kitchen cupboard.

OK. Blatent sales pitch coming up. These are cheaper than lead carbon to install, cheaper than either lead battery to run, they operate over the same temperature range, take one fifth the volume and about one tenth the weight of the alternatives. They can be charged much faster than other batteries, which means running the generator less time to replace the energy lost while cruising.

It all boils down to whether you are prepared to run the generator during the cruising day. I am, and I am delighted to free up 500 litres of space and pay £3,000 less than Lead Carbon. Who said Li-Ion was expensive?

The one assumption in all this is that we can make the generator silent. But that's a topic for another day...


Postscript: The perceptive reader may ask, "if the generator is silent, why not run it all the time and fit a very small unit?". Imagine we never used more than 36kWh in a day. In that case a 1.5kW generator would be big enough. I go back to the "upstream against the flow" case; with a 6kW generator we can run the motor at 6kW indefinitely, or higher powers for a finite time if we allow the battery to run down. Again, it's a balance between generator size, battery capacity and performance. Oh, and we never will get the generator quiet enough to be inaudible to light sleepers.

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