This Guide to Electric Boat Batteries from Plugboats has been written to provide a basic understanding of how batteries propel boats and the main things you will want to know about the subject, especially if you are familiar with fossil fuel boats and are thinking of going electric.
It is not meant to cover everything about batteries – the subject is too broad and complex – and does not give information about manufacturers and the specific products they offer, simply because it would be impossible.
There are literally hundreds of battery manufacturers around the world who make thousands of different batteries, and often manufacturers provide solutions that other companies label with their own branding.
For some this guide to electric boat batteries may be too basic. But I figure the basics are a good place to start – and those who get bored can always skip ahead 🙂
So let’s begin.
Table of Contents
1. Physics: ENERGY, FORCE, WORK, POWER
I think it’s helpful to have a quick review of physics because it helps in understanding the electricity aspect of propulsion.
In the end, the distance and speed you will be able to travel in your electric boat and the relationship of the two all come down to the constraints of physics: how much energy and power (which are two different things, as you will find out) it takes to move the weight of your boat against the resistance and force of the water.
Every day we all throw around phrases like ‘I have a lot of energy’ … ‘does the government have the power to do that’ … but when it comes to figuring out what you need for your boat, physics has very specific definitions for those terms.
ENERGY: Energy is the ability of something to do work.
FORCE: Is a push or pull upon an object resulting from an interaction with another object.
WORK: Work is done when energy is transferred to an object that causes movement of the object.
POWER: Power is the rate at which the work is done.
The equations are:
Work = Force X Distance and Power = Work ÷ Time.
Work is measured in joules, and so is energy (we will get to that in a minute). Force is measured in newtons and is related to the weight of the thing that is being pushed or pulled. Distance is measured in metres. Power is measured in Watts (which we will also get to) and Time is measured in seconds.
The reason energy and work are both measured in joules is that the work is defined as the thing that is accomplished through the transfer of the energy, so they are measured the same way.
The important concept here is that moving the set weight of your boat a set distance will always require the same amount of energy – the same amount of work (in calm water). If you want to move it the same distance faster, or move a bigger boat the same distance it will require more power: the rate of work. Moving a boat through waves is essentially the same as moving a heavier boat, you’re overcoming the force of the waves so you require more force and power to move at speed.
2: Electricity: VOLTS, AMPS, WATTS
Those examples above are all explanations of mechanical energy, things moving in space. Electrical energy has its own measurements, which are intertwined with these units of mechanical energy. The three basic electrical energy terms you want to know are Volts, Amperes (amps) and Watts.
VOLTS (denoted as V) are the units used to measure electrical energy. To be precise, Volts measure electric potential. Electric potential is how much electric energy is capable of moving from one point to another – that idea of ‘the ability to do work’ applied to electricity. It’s not an exact analogy, but if you think of electricity as water, voltage is the water pressure.
AMPS (denoted as A) are the units used to measure the flow of the energy. If voltage is the water pressure, imagine a meter on the water pipe that measures how much water, how many water molecules, are passing by a point on the pipe at any one time. With electricity the meter is measuring the individual electric charges that are going through a wire, and the flow, the current, is measured in Amps.
WATTS (denoted as W) used to measure the electric power.
Watts are the units used to measure both electrical power and mechanical power. This comes in handy when trying to figure out how fast and far the electrical energy from a battery can move a boat.
In terms of your boat, the energy is stored in your battery, that electrical energy is transferred to mechanical energy – the motor and drive train and propeller – which put force against the water to do the work of moving your boat. As we said, higher speed and/or bigger weight require more power transferred from electrical energy to mechanical energy – all measured in the same watts.
We know the formula for mechanical power is Power = Work ÷ Time. There is also a specific formula for calculating electrical power: Watts = Volts x Amps.
To be clear, a watt is a watt is a watt, it doesn’t matter if it is measuring the rate of work of electrical power or mechanical power or if it is being exerted by an electric motor or fossil fuel motor, steam motor, or hand cranked.
With electric boats, since the power of the batteries and the power of motors are both measured in watts, it is fairly simple to match a battery with your motor and also figure out how much work it will be able to do for what amount of time.
Many people are used to the term horsepower when talking about boats. However, if you are going to be looking at or using an electric boat, I think you would be well advised to forget about horsepower and think only in terms of watts, or even better, kiloWatts.
A kiloWatt is one thousand Watts
A watt is the amount of work one joule of energy can perform in one second, which is not much. Individual joules and watts are small. To give you an idea of how small a joule is, a common unit of energy we all use is the food calorie, and a joule is about 1/4000th of a calorie – 4,184 joules in a calorie. You burn about 2,000 calories a day, so you burn about 8,368,000 joules a day (congratulations!)
Because a watt is so small, we often use kiloWatts to refer to any sizeable amount of power. A KILOWATT (denoted as kW) is the work that one thousand joules can do in one second. If you insist on a comparison to horsepower, a kiloWatt is roughly 1/3 bigger than a horsepower, 1 kW ≈ 1.3 hp. Put another way, a horsepower is about 3/4 of a kiloWatt.
3. Battery power for your electric boat motor
Working with Watts = Volts X Amps
Let’s put this common measurement to use.
Because you are transferring electric power to mechanical power to move your boat and they are measured in the same units, that means to get all of the benefits of a 10kW electric motor you will need to have a battery with a voltage and amperage that can supply at least 10 kW – 10,000 watts.
In theory, it could be any combination: a 50V battery sending a current of 200A would do it, as would a 100V battery sending 100A. In real life, though, there are constraints, which you can see by taking it to the ridiculous – a 2V volt battery sending 5,000A would be like trying to run a boat off a cellphone battery.
To give you some real life examples, an electric trolling motor of 1kW might be perfect for fishing from a small boat, so a 12V battery and a current of 50A could achieve that.
At the other end, there are now electric outboard motors of +150kW and inboards well beyond that. To get high power, you need high voltage, high current, or medium both. In terms of amps, you’re getting up there when you’re talking about 300 amps, and electric car motors generally have voltages of 360 – 400V and up to 800V. If you had an 800V / 300A motor it could put out up to 240,000 watts of power – 240 kW, while a 600V/200A motor could produce 120kW.
In the middle range of electric boat motors, the voltage is usually somewhere between 48V and 144V and the amps could go up to 250A or 300A.
When we’re talking about this equation of Watts = Volts x Amps and theory versus reality, there are some other things to be aware of. First is that motors have an input power and an output power. If you put in 10kW of power, some of it is used up in spinning the motor and some energy is lost as heat, and so the output power will be smaller than 10kW. Not a lot smaller, electric motors are very efficient. But smaller.
Motors may also have two power ratings. Peak power measures the maximum amount of power the motor has available, but only for short periods of time. Continuous power measures the power that can be put out at a constant rate for long periods of time. Some motor manufacturers only tell you one of the ratings, many tell you both.
Similarly, your battery may have a couple of ratings for the volts like nominal voltage, voltage range, discharge voltage and others. We’ll get to that a bit later on in “Reading battery specifications“. First…
4. Power and range – kiloWatts, kiloWatt hours, Amp hours
One of the nicest things about working in kiloWatts as the common measurement of both electric and mechanical energy is that it makes it easy to figure out much energy is required to keep your boat moving and how far it can go at what speed without recharging. With an understanding of kiloWatt hours and Amp hours, you can easily calculate that range.
kiloWatts, kiloWatt hours, Amp hours
The thing you want to know about your battery to make the range calculation is how many kiloWatt hours it has. NOTE CAREFULLY: kiloWatt hours, NOT kiloWatts. KiloWatt hours are denoted as kWh.
The difference between kiloWatts and kiloWatt hours is very important. A kiloWatt is a measurement of power, as we know. But a kiloWatt HOUR is a measure of energy.
How can that be? Let’s return to that other measurement of energy, the joule, for a minute.
We learned that a kiloWatt is the work that one thousand joules can do in one second, and it follows that a kiloWatt HOUR is how much work 1,000 joules per second can do in 1 hour – 3,600 seconds.
It is a measurement of energy because it tells us the number of units of energy being transferred in one hour to do the work: 1,000 joules per second X 3,600 seconds in an hour = 3,600,000 joules of energy converted to work in an hour.
This makes it easy to know how long the energy in your battery can supply an accompanying motor. Simply divide the battery’s kiloWatthour capacity by the motor’s kiloWatt power rating. A 10 kiloWatt motor – running flat out the whole time – needs 10 kiloWatts of electrical power each second and will use 10 kiloWatt hours of energy in 1 hour. In other words, it will draw 3.6M joules of electrical energy out of the battery in an hour, when running at ‘wide open throttle’, to use a fossil fuel term.
Like any boat motor, though, a 10kW motor does not need to go at full speed and use all the peak power all the time. At half speed it needs 5kW, at one quarter speed it needs 2.5kW, etc.
That’s the good news about the calculation. The bad news is that not every battery manufacturer provides a specification for kiloWatt hours. Some do, some don’t. However, almost all battery manufacturers provide a specification for Amp hours, designated as Ah.
You can still easily figure out the energy storage using amp hours, though. Just adapt the Volts x Amps = Watts equation to say Volts X Amp HOURS = Watt HOURS. (Be careful when calculating – the answer is in watt hours, not kiloWatt hours.)
Since amps are a measurement of how much electricity is flowing past a single point in the wire at any one time, an amp hour is how much flows past in an hour, how much energy is coming out of the battery.
Using our same 10 kiloWatt motor, it is going to use 10,000 watts in an hour (flat out), so with a 48V battery we will need a specification of about 200 amp hours: 48V x 200Ah = 9,600 watt hours, which is just less than 10 kiloWatt hours. We can go at top speed for 1 hour, half speed for 2 hours, and quarter speed for 4 hours.
5. Battery specifications
When you look at your battery specifications to find out what the amp hours and other specifications are you may find a few references that could be confusing – different kinds of voltage, maximums and minimums for amperages, and other unfamiliar terms.
Nominal voltage, for all intents and purposes, is the voltage of your battery. Nominal means ‘named’, so it is the named voltage of your battery even though the actual measured voltage might be higher or lower. A 48V battery might actually be a 51.2V battery when analyzed, but it is easier for everyone to name it (and similar batteries) a 48V battery.
Going back to the electricity as water analogy for a minute, when you draw electricity out of a battery, the ‘pressure’ fluctuates a bit – when you want to increase the power of your motor for instance – and overall it gradually goes down as more electricity is used. At a certain point there is not enough ‘pressure’ to send the current to the motor.
So the voltage range will give you the maximum voltage you can expect and the minimum voltage. For example, a battery with 48V nominal might have a voltage range of 58.4V to 40V.
Minimum discharge voltage
This is the minimum voltage when there is not enough voltage to get the current flowing.
This is the voltage the charger will use. If you are buying a battery with a compatible charger you won’t need to worry about it. Almost all electric boat batteries can be charged using a household current (Level 1 charging) and many are designed to be compatible with standard Level 2 electric vehicle fast chargers.
Continuous amps will tell you the electric current that can flow out of the battery on a continual basis, the ‘cruise’ amperage.
Maximum amps will tell you the maximum electric current that can flow out of the battery. When you need more power to get through waves or go faster, you will need more amps: Watts = Volts x Amps.
Charge amps or charge current is similar to the charge voltage, it is the current that will flow back into the battery to recharge it, but you will not need to worry about it with a compatible charger.
Amp hours we have already covered, it is the energy storage capacity of the battery.
Some batteries may specify the kiloWatts,the power rating. Similar to peak power or maximum amps, it is the power the battery can deliver at any single point in time.
kiloWatt hours may be specified as the storage capacity on some batteries but as noted earlier, Ah is the more common designation and can be easily converted to kWh through Volts x Ah = kWh. Be careful on this – sometimes even manufacturers (or the people in their labelling department) will confuse kW with kWh.
6. How batteries work
This seems as good a time as any to look into how batteries work. Basically they all work the same. And a note to all experts out there, this is exTREMEly simplified!
A battery has one kind of material with atoms that want to shed their electrons, another type of material with atoms that want to gather electrons, and in between them a material that facilitates a chemical reaction that sets the electrons free so they can go and collect on the other material.
As a kid you may have made a battery made out of a lemon, a nail and a copper penny. The zinc in the galvanized nail wants to get rid of electrons, the copper in the penny wants to gather them, and the lemon juice is the material in between.
The nail and the penny are called the electrodes of the battery. The nail is the negative electrode – the anode – the penny is the positive electrode – the cathode – and the lemon juice is called the electrolyte.
In a rechargeable battery, like the kind you want in your boat, the chemicals are a lot more complicated and there is also a one-way barrier. Hooking up a motor between electrodes creates a chemical reaction with the electrolyte that makes the electrons flow through the motor to get from the anode to the cathode (this is called electricity) and the motor spins. When you plug it into a recharger, the one way barrier lets the electrons go directly back through the barrier to the other electrode.
As you can imagine, the chemical reaction of a lemon, nail and copper coin doesn’t generate a lot of electricity. Surprisingly, neither do the chemical reactions of the latest and most efficient battery chemistries – it’s only about 2 volts to 3.6. volts.
7. Battery cells, modules, packs
The way to get larger voltage is to connect the batteries. The word ‘battery’ is not very precise, we use it to mean everything from something in a hearing aid to something in an electric vehicle. To be more precise, the lemon is a battery cell. Battery cells are connected together to make modules and modules are connected together to make battery packs. Even some of the batteries you use at home, like a 9V battery, are technically battery modules constructed of six individual 1.5 V battery cells.
Here’s a crazy example of how this works – World’s Largest Lemon Battery – that demonstrates how a battery pack is created to increase the voltage.
On a level with a bit more practicality, the battery for a Tesla 3 (or any other electric vehicle) is not much different. It uses lithium-ion cells, each a little bigger in dimensions than the AA batteries you use at home. Each cell can generate 3.7 Volts and there are 2,976 of them arranged in 96 groups of 31 to create the Tesla 3 battery pack of 350 volts. The energy storage capacity of the pack is 80 kWh. You can see the cells, modules and overall pack in the photo below.
The Tesla battery (and all batteries) use two different ways of connecting batteries to achieve both higher voltage and higher kiloWatt hours. If battery cells or modules are connected in series, the voltage is increased, but the energy storage is not increased. If they are connected in parallel, the opposite happens – energy storage capacity is increased but the voltage stays the same. (Photo of Tesla S battery pack by Ted Dillard, Inside EVs)
In the case of a large battery like those used in electric cars and larger electric boat systems, the work is already done for you. For smaller motors, though, you can do this yourself. Two 48V batteries (for instance) can be put together in series to drive a larger motor. Or, they can be put together in parallel to double the range of your boat. IMPORTANT NOTE: This is not true for all electric motors and batteries – do NOT do this without consulting the manufacturers.
Different cells: cylinders, pouches, prismatic
One final thing about how batteries are built. For lithium batteries there are three basic types of cells – cylindrical, pouch, and prismatic. Cylindrical cells are very similar to the AA batteries you use in household items. Prismatic cells are contained in a rectangular can. Pouch cells are exactly that, small pouches.
Each cell type is constructed in a different way and has different advantages and disadvantages for both the manufacturer and user. These are mainly related to efficiency, weight, and of course cost – in both the manufacturing of the cell itself and in connecting them together to make modules and packs.
8. BMS: Battery Management System
We know that a boat has different power needs and draws the appropriate current from the battery pack, but does the current come from one cell at a time or from all the cells at once or somewhere in between?
That’s where the Battery Management System (BMS) comes in. The BMS can rightfully be considered the brains of the operation. It monitors and manages the battery during both discharging and charging, balancing the voltage and current of the cells so that the work of the pack is shared equally and safely.
Overall, its function is to protect the battery – and you – by constantly monitoring every cell in the pack and calculating how much current can safely go in and how much can safely come out. If you think of the minimum and maximum voltages and amps in the battery specifications, they apply not only to the battery pack as a whole, but to each of the individual cells.
Through this monitoring it acts as your ‘fuel gauge’ and calculates the overall State of Charge – how much energy is remaining in the battery pack.
It also monitors temperature and checks for indications of any loose connections, short circuits or insulation problems in the cells, modules and pack.
If the BMS detects anything unsafe in the operation, it will shut down the battery. It will also shut down the battery if it detects activity like excess or deficient voltage that could damage any cells. Aside from safety, this also extends the lifespan of the pack.
9. Battery chemistries
By battery chemistry we mean the materials involved in the chemical reaction that produces the electric charge. There are two base materials used for the rechargeable batteries you will want for an electric boat: Lead and Lithium.
Within each of those two types of battery there are a variety of options.The chemical reaction in lead batteries is between electrodes made of lead (the anode is lead metal and the cathode is lead oxide) and an acid. Hence the name lead acid battery.
Lithium batteries use a wide variety of materials and alloys to achieve the chemical reaction. The specific combination is usually available in the user manuals and brochures, but is often not advertised. The generic Lithium-ion or Li-ion is used.
The one type of lithium battery that usually has the specific chemistry in the name is the LiFePO or LiFePO4 battery, which is not lithium ION but lithium IRON phosphate.
For lithium batteries there is a six point system that gives ratings on:
- Specific Energy
- Specific Power
- Life Span
We have used the system to give you a basis of comparison for the most common lithium chemistries, and have adapted it slightly for Lead Acid batteries to give you an indication of where they fit in the scheme of things.
The 6 Rating criteria
Specific Energy can also be called gravimetric energy density and refers to the amount of energy that can be stored in the battery by weight: watt hours per per kilograms.
The term energy density is often used, and isn’t entirely accurate because energy density actually refers to how much energy can be stored by volume: kWh per litre. The by volume spec is rarely used in consumer information, so if you see energy density it probably (but not always) means the by weight specific energy specification.
A high specific energy or energy density is better, it translates to a lighter battery. For lithium batteries this number could be anywhere from 100 watt hours per kilogram to 250 watt hours per kilogram, depending on the chemistry.
Two notes here, one very important, one not as important.
Those specific energy ratings for lithium batteries are 100 to 250 WATT hours, NOT kiloWatt hours. The rating in kWh would be .1 to .25 kWh per kilogram of battery material. Lead acid batteries have specific energy of about 35-40 watt hours per kilogram (.03 kWh/kg).
Not as important, those numbers for lithium refer to the energy of an individual cell. The energy density of the battery pack as a whole will be lower because it needs to take into account the weight of all the connections and coolants and glues that go into constructing the pack. The lead acid numbers are for the battery pack.
Specific Power is the amount of power per weight that the chemistry can deliver: kW per kilogram. This is easy to understand now that you know the difference between energy and power. Lithium batteries can range from about 250 watts per kilogram to 350 w/kg. Lead acid is about 180w/kg.
Safety. This specification on the chart is the safety compared to other lithium batteries. When it comes to concerns about battery safety, some people point to news reports of EV fires. The reason there are news reports about EV battery fires is the same reason there are news reports about airplane accidents. They are extremely rare. Nobody reports on the millions of hours of battery use that go on every day with no incident.
There is certainly some risk involved in using lithium ion batteries, just as there is some risk involved in using the highly volatile and explosive liquids that run fossil fuel motors. When used and maintained properly the risk is very low, and are mitigated by thousands of hours of research and testing as well as the BMS monitoring which is going on every second the battery is in use.
Performance in the graphs relates to performance at hot and cold temperatures.
Life span is an indication of the overall life span of the battery and specifically how many charging cycles it can go through.
Every discharge and recharge – called a cycle – results in the battery not being able to get absolutely all the way back to fully charged. and eventually the battery is not useful. The ability to have more cycles is obviously better because it means your battery will last longer.
Evaluating the exact number of cycles a battery can undergo is difficult because the definition of a cycle isn’t absolute in terms of how much energy is discharged and recharged to make up ‘a cycle’. In general, though, lithium-ion batteries in electric vehicles and boats are designed to undergo 1,000 to 2,000 charge cycles.
Cost refers to relative initial cost compared to other lithium batteries. A note on looking at the graphs: the ratings show the desirability of the battery characteristics, so a low cost is higher on the rating scale because it is a good thing in a battery.
Flooded, Valve (VRLA), Gel, Absorbed Glass Mat (AGM)
The type of lead acid batteries used for propelling an electric boat are different from those used in a fossil fuel car as a starter battery. A starter battery is only used to start a motor, which requires a lot of power, but for a short period of time: small voltage and very high maximum amps. It doesn’t need big energy storage. It uses a small discharge to turn the massive weight of the fossil fuel motor, then the alternator (which is just a very small electricity generating turbine) recharges the battery through the spinning of the fossil fuel motor.
For your electric boat you want a battery that has a power output (Volts times Amps) compatible with your motor and lots of available kiloWatt hours for you to fully discharge the battery (up to about 80%) pulling electricity out of it for a long time.
This is called a ‘Deep Cycle’ or sometimes a ‘Deep Discharge’ lead acid battery.
There are a few types of lead acid deep cycle batteries – Flooded batteries and Valve Regulated Lead Acid (VRLA). The VRLAs are further divided into Gel batteries and Absorbed Glass Mat (AGM) types. In the flooded versions the electrolyte is liquid, in gel versions it is a gel and in AGM it is held in a semi solid state by being held in the pores of the glass mat.
Flooded are the heaviest and least expensive…AGM the lightest and most expensive, but still much cheaper than lithium batteries.
Compared to litihum batteries, Lead Acid batteries have lower specific energy, lower cycles and lower initial cost. The specific energy is only about 1/5 of the highest density lithium batteries – and the specific power is about 1/2 that of lithium batteries. They generally are very safe, however the flooded versions can release hydrogen gas and explode under certain conditions, especially if the owner has been a bit lax in maintenance.
The initial cost for Lead Acid is about 1/4 the price of a lithium battery, but since they last for only 300-400 cycles, the cost per cycle is higher. They are good for smaller motors, but after about 48 volts, the amount of power needed to move the weight of the battery pack alone makes it an impractical choice.
There are a wide variety of lithium battery chemistries, but some are used for things like laptops and medical devices. Below are the main varieties used for electric boat propulsion. The chemical composition refers to the materials used in the cathodes.
Lithium Nickel Manganese Cobalt Oxide: LiNiMnCoO2: NMC (NCM, CMN, CNM, MNC, MCN
NMC is the most popular chemistry for electric powertrains in EVs, e-bikes and electric boats. The main reason it is so popular is that it has a high specific energy. Different manufacturers will have their own recipes for the nickel, manganese and cobalt proportions so that some will sacrifice a little specific energy for more specific power, and vice versa. Battery manufacturers are trying to use less cobalt because of cost and concerns about ethical mining. More nickel means higher energy density, lower cost, and longer cycle life, but a slightly lower voltage.
Lithium-Iron-Phosphate: LiFePO, LiFePO4 or LFP
Many electric boat owners and companies prefer the LiFePO lithium iron battery chemistry to lithium ion, largely because of two reasons: lower cost and their concerns about the safety of li-ion. They make a trade off in range, though, because LiFePO has a low specific energy. It may not be the best battery choice for a high speed high power electric boat, but is becoming increasingly popular with sailboat owners, where the increased battery weight in relation to the size of the boat is less important and speed under power is a non issue.
Lithium Titanate: LTO or Li-titanate
Lithium Titanate is a very good chemistry for certain large vessels, but is not a popular option for recreational boats. The reasons are that it has a low specific energy – so the batteries are very heavy. On the other hand, they have long life span, are very safe and can be charged very quickly. The other drawback is they are expensive. For a big electric ferry where the weight of the battery is less important but quick charging, and public safety are important, the higher initial cost is offset by the long life span which means the operator is free from the ongoing high operating costs of constantly filling up with fossil fuel.
10. Charging your battery
Finally, we will touch very briefly on charging.
Just as a battery has specifications about the volts, amps and kilowatts that it puts out, it has corresponding specifications in volts, amps and kilowatts about how fast the energy how quickly the electrons can get sent back to their original places. (See the section Battery Specifications).
Almost all batteries for electric boats can be charged through a standard household circuit Level 1), and almost as many have fast charging capabilities (Level 2). In both cases the chargers use the AC electricity coming from the general grid and the technical aspects fall in line with charging for electric vehicles.
Standard existing charging pedestals at marinas can be used to charge electric boat motors and there are also DC high speed charging units being installed in marinas throughout the world. The pace of installation of these high speed units will only increase in the coming years as more people start using electric boats and higher power electric boat motors become more popular.
When you buy your boat and/or motor, the manufacturer will offer or suggest a charging unit, but generally these are compatible across motors. Check with the manufacturer to be sure.
This article is intended to give you a basic understanding of batteries for electric boat propulsion. There are other things that could have been added, but the intent was to try to stay with the basics without drifting off to subtopics.
In an effort to keep it as concise as possible, it also deals with the underlying mathematics of the battery / motor relationship without too many caveats about real life. If you have a 10kW motor and 10kWh battery it may not run full out for exactly an hour. The BMS won’t let the battery discharge 100%, and even if it did the motor may not be exactly 10kW and there lots of other variables. But it will run somewhere around an hour.
Also, there are hundreds of things that could have been covered about the specifics of batteries and motors for different weight boats and different usage. A small day runner obviously has different energy and battery demands than a blue water sailing yacht with needs for food storage and cooking, toilets and power for navigational and entertainment appliances.
If you are looking for more in depth battery knowledge, en excellent resource is the Battery University website. And if you would like to find out more about the electric motors available for your needs, check out the Plugboats Guides to Electric Outboards under 5kW, Electric Outboards over 5kW, Electric Inboards, Electric Saildrives and Pods and Electric Trolling Motors.