Lithium batteries are an excellent match for trolling motors, replacing traditional deep-cycle marine batteries.
They are lighter, offer better performance, longer run time, and last much longer than most comparable lead-acid batteries.
This article discusses the advantages of lithium batteries, the most popular battery sizes, and what you should know before buying one.
Let’s get started!
Advantages of Lithium Batteries
Lithium batteries offer huge performance advantages when compared to deep-cycle lead-acid batteries.
Let’s take a closer look at the main advantages:
Longer Lasting Power
A lithium battery can keep your trolling motor at the same speed for almost twice as long as lead-acid batteries of the same rated capacity.
A lead-acid battery should only be discharged to roughly half of its rated capacity (Ah), which basically means you need to get a battery double the capacity you actually want to use.
If you regularly discharge your battery below 50%, it will deteriorate faster, and you will have to replace it sooner.
Most lithium batteries can be discharged down to 10-20% SoC (State of Charge).
For example, you can use 80Ah out of a 100Ah lithium battery. This would normally compare with a lead-acid battery that is rated at 160Ah.
Lithium Batteries Don’t Suffer From Peukert’s Law
Lead-acid batteries have significantly lower capacity when discharged fast, known as Peukert’s Law. For example, a battery rated for 100Ah can deliver this capacity only when discharged slowly, over a 20-hour period, which means the current of 5 Amps. If you want to increase the current to 20 Amps, the same battery will give you only 80Ah, and the available capacity rapidly diminishes as you draw more current.
The same isn’t true for lithium batteries. You get the same rated capacity even when discharging them very fast.
Longer Life Expectancy
Much longer. That’s the reason why the initial costly investment is worth it.
Good lithium batteries can handle 3000-5000 charge/discharge cycles.
Most manufacturers will admit that after 3000 cycles, the rated capacity will go down to 75-80% capacity.
3000 cycles equal 3000 fishing days. Incredible, isn’t it?
As a comparison, good lead-acid batteries last only 200-300 cycles if discharged only to 50% of their rated capacity. However, if you regularly take them down to 20% of capacity, you will quickly damage the battery, which will have perhaps only around 30-50 cycles available with available capacity diminishing as well.
No More Battery Sulfation
Sulfation is a buildup of lead sulfate crystals on lead-acid battery plates. It occurs whenever a battery is left without a full charge, and it is the main reason lead-acid batteries lose performance and fail. Sulfation is responsible for longer charging times, shorter run times, and battery life.
Lithium batteries don’t suffer from sulfation, which occurs in most lead-acid batteries.
Lithium batteries typically weigh 60% less than marine lead-acid batteries and take less valuable space.
A lighter boat is faster and more maneuverable.
This weight reduction makes the batteries much easier to carry. You will appreciate this, especially if you need to remove your boat’s batteries after each trip to charge them.
Flat Voltage Curve
Traditional marine batteries suffer from constant voltage drop as you use them. This means gradually less thrust will be available. And if the voltage is too low, your trolling motor may stop altogether, even though there is still some power left in your battery.
On the other hand, lithium batteries will only have around a 3% voltage drop as they are discharged.
A lower voltage means the motor will draw more amps to provide the needed power.
That is why it’s recommended to oversize your battery wires when you use lead-acid batteries.
Faster Charging Times
Recommended charging current for lead-acid batteries is 10-30% of the rated capacity. For example, you shouldn’t fast charge a 100Ah lead-acid battery with more than 30 Amps.
Lithium batteries can be charged with as much current as 100% of their Ah capacity, which means 3-5 times faster than lead-acid batteries.
This probably isn’t hugely important if you charge your trolling motor battery overnight. However, fast charging brings about the possibility of recharging your batteries during the day or using a powerful outboard alternator or generator to top up your lithium battery.
The main downside to lithium batteries is that they are much more expensive than lead-acid batteries with a similar rated capacity.
However, since they can be discharged much deeper than lead-acid batteries, it also means you need a smaller capacity battery for your needs.
You should also consider how long they last.
Because lithium batteries have a lifespan of 3000 cycles and more, you won’t have to worry about replacing them anytime soon.
If you expect a decent performance and run time from your lead-acid battery, you will need to replace it every 1-3 years, even if you don’t use them much. That’s because lead-acid batteries deteriorate over time due to aging.
One important thing to know is that you shouldn’t charge lithium batteries in freezing temperatures.
It’s not a big issue for most, as a well-functioning BMS will automatically stop charging the battery when it’s too cold, in case you forget.
Lithium Batteries Compared (LiFePO4)
12V 100Ah Lithium Batteries
100Ah 12-volt lithium batteries have a similar size to the Group 27 traditional leisure battery.
That makes 100Ah lithium batteries the most natural replacement, as they will easily fit on your boat.
They have a maximum discharge current of 100A, which is well above what trolling motors need.
12V 200Ah Lithium Batteries
200Ah batteries are recommended for heavier boats and those fishing in windy conditions. This will make your trolling motor draw more amps and reduce your time on the water.
The extra capacity of 200Ah gives you longer runtime and peace of mind. It also reduces the chance of discharging your lithium battery below 20%, which helps to make it last longer.
12V 50Ah Lightweight Lithium Batteries
These small lithium batteries are great for kayaks, canoes, and inflatables.
Although there are smaller and lighter batteries available with less than 50Ah capacity, they are not always suitable for a trolling motor because of the maximum current a BMS can deliver. Make sure to check the amp draw of your trolling motor.
24V Lithium Batteries
There are 24V lithium batteries available, which are an interesting option for all those running more powerful trolling motors.
A single 24-volt battery will require a dedicated 24V lithium battery charger.
My preferred alternative is, to connect two 12-volt batteries in series. You then have more flexibility in the way to charge them. Two lighter batteries are also easier to carry than a single large one.
36V Lithium Batteries
It’s a similar situation to 24 Volts. Although you can buy a 36-volt lithium battery, there are not many reputable manufacturers to choose from. 36-volt lithium-capable chargers are also harder to find and more expensive.
Our recommendation is to connect three 12-volt lithium batteries in series to create a 36V battery.
Things You Should Know About Lithium Batteries
How Safe Are Lithium Batteries?
You must have heard or read about stories of lithium-ion batteries catching fire or exploding, for example, a smartphone, tablet, or electric scooter.
However, there are many types of lithium-ion batteries, and in this article, we discuss only LiFePo4 (LFP) batteries, which are the easiest and safest to use.
Unlike other types of lithium batteries, LiFePO4 batteries will not catch fire or explode.
There is no lead, acid, or heavy metals involved. They are non-corrosive and non-toxic.
There is no gassing, and no watering is necessary. Maintenance is straightforward, as there usually is a built-in BMS that takes care of your battery’s health.
Calculating Run Time
If your battery has 100Ah capacity and your motor draws 20 amps of current, you can calculate its run time by dividing 100/20 = 5 hours.
Your motor’s current draw depends on the speed you set it to and how heavy your boat is. If you use your trolling motor at lower speeds, you will have a significantly lower current draw and much longer run time.
Weather and water conditions will also affect how long your motor will run. It is harder to push a boat against the current, choppy waters, or into the wind, which means higher current draw and shorter run time.
How Do I Charge a Lithium Battery?
Some battery manufacturers insist on using dedicated lithium battery chargers. This is to ensure the battery is protected from overvoltage and that it’s charged to its maximum capacity.
Other makers, for example, Battle Born, allow using standard lead-acid chargers as long as you can ensure that the charging parameters are as follows:
- Bulk/absorb = 14.4v (or between 14.2 – 14.6v)
- Float = 13.6v or less
- Equalization = None or set to 14.4v
- Temperature compensation = None
Charging Lithium Batteries With an Engine Alternator
You can also charge your lithium battery using your boat’s outboard or inboard engine.
It is then recommended that you install a DC-DC charger between your engine’s starter battery and the lithium battery to make sure it’s correctly charged.
Lithium batteries shouldn’t be stored at a 100% charge, as it reduces their lifespan.
It is much better to keep them charged at 90% and less.
The batteries shouldn’t be fully discharged or overcharged. They may have 3000 and more cycles in them, but this number will go down if you don’t charge them properly.
How To Pick the Right Lithium Battery
Lithium batteries are expensive, and it can be tempting to go with budget options, for example, batteries shipped from the far east.
My advice is – don’t. If you want to avoid disappointment, you want your lithium battery to come from a reputable source, with a warranty and support.
BMS – Battery Management System
Besides buying your battery from a trusted supplier, make sure it is also equipped with a built-in Battery Management System.
A BMS will preserve the lifespan of a lithium battery and ensure its safety. It will protect your battery from a voltage that is too high or too low. It will stop discharging your battery if the temperature is too high or prevent charging in freezing temperatures.
It also has protection from high current, so make sure the maximum current output is higher than your motor’s maximum amp draw.
A BMS will also look after individual battery cells to make sure they are balanced and have the same voltage.
Choose Your Voltage
Depending on the voltage of your trolling motor, you will need a different number of 12-volt batteries.
A 12V trolling motor requires just one 12V battery; for 24V and 36V systems, you will need to connect 2 or 3 batteries in series. More batteries of the same size mean more capacity and more range available.
As a rule of thumb, trolling motors with 55 lbs of thrust or less operate on 12 volts (single battery), those with 68-100 lbs of thrust run on 24 volts (two batteries), and those above 100 lbs of thrust require 36 volts in total (three batteries).
Pick the Right Capacity
Because lithium batteries can be safely discharged deeper, down to 80% of their DoD (depth of discharge), you can choose to go with a smaller size.
For example, a 50Ah lithium battery will give you 40Ah. That’s equivalent to an 80-100Ah lead-acid battery. Now, if your motor draws 40 Amps at full speed, those 40Ah will power your motor for 1 hour.
Maximum Discharge Rate
When choosing your lithium battery, make sure its maximum continuous output current is higher than your trolling motor’s maximum Amp draw.
Many lithium batteries are rated at 1c, which means the maximum discharge current is equal to its capacity. For example, a 60Ah lithium battery can be discharged at 60 Amps.
In this case, your motor shouldn’t draw more than 60 Amps, preferably closer to 50 Amps.
Warranty and Support
Although some lithium batteries come with a 10-year warranty, you should always read the small print and make your own conclusions.
Some makers may ask you how your batteries have been charged and used, which could make your claim problematic.
I recommend you also read reviews and learn about other users’ experiences with customer support. It’s great if you know you can pick up your phone and get any advice you may need.
Lithium batteries offer significant benefits that are hard to ignore.
They are expensive, but when you consider their much longer lifespan, the cost is comparable to lead-acid batteries, which need to be replaced more often.
If it’s your first trolling motor, and you’re unsure how long you will use it, going with lithium is probably not the best idea.
If, on the other hand, you want a light and high-performing battery that will last you many years, lithium is the way to go.
Trolling Lithium Battery FAQ
How long will a 100Ah lithium battery last on a trolling motor?
It depends on the Amp draw of your motor, which is closely related to the speed you set it to. If your trolling motor draws 25A, you could in theory run it for 4 hours (4h times 25A = 100Ah). However, in practice you will want to use it for 3 hours, to prolong the life of your lithium battery.
Can an outboard charge a lithium battery?
Absolutely, you can charge your trolling motor battery while running your outboard engine. Some drop-in lithium batteries can be charged with any charger designed for lead-acid batteries. If you have a regular lithium battery that can’t be connected directly to your outboard’s alternator, you will need to use a DC to DC charger that can be set up for lithium batteries.
Will using a lithium battery void my warranty?
You will have to check it with your trolling motor manufacturer. Minn Kota motors, for example, are allowed to run on lithium batteries. At the same time, you are warned that if you run your motor at high speed (over 85%) for a long time, it could damage your motor.
Can I use a lithium battery to start my boat’s engine?
Starter batteries are designed to give a short burst of high current, more than 200 Amps at a time. Like other deep-cycle batteries, lithium batteries are designed to provide a steady amount of current over a longer period. While it is technically possible to use a lithium battery as a starter battery, it is not recommended.