Batteries for Trolling Motors – Knowledge Matters, Part 1
David HuttonPalmetto Fly n Fish, March 2021
Introduction
A lot of information goes around about batteries for trolling motors, and most boaters have at least a passing familiarity with them. But alot of what passes for knowledge is anecdote or opinion.
Some is just repeated out of simple ignorance of the facts, and some is pure hokum.
Some is just repeated out of simple ignorance of the facts, and some is pure hokum.
I've spent my entire working life around electronics in some form or another. I'm no Rhodes scholar, but I understand the concepts behind basic electricity.
And batteries and motors are, if nothing else, pretty basic.
I've also learned that most folks know very little about the electricity they get from batteries, and this especially applies to small boats and trolling motors, in particular.
Fortunately, there are countless resources to help you learn. Batteries have changed over the years and new technology has been applied to them, so it pays to do a little research.
However, it seems that almost no one does.
Someone says something on a facebook page, and without knowing anything else, the would-be battery buyer runs out and plunks down his hard earned money.
This situation gives us a great opportunity to educate. It will be a challenge, but it may save frustration, time, and money... and that's worth it.
So, lets get into it.
Battery types
There are different types of batteries available, but the main types of batteries you might use for a trolling motor fall into a few categories:
1. Traditional flooded, lead-acid, or “wet” batteries
2. Absorbent Glass Mat, or AGM
3. Lithium iron phosphate (LiFePO4), or lithium-ion
The Old Standby ... Lead -Acid
And batteries and motors are, if nothing else, pretty basic.
I've also learned that most folks know very little about the electricity they get from batteries, and this especially applies to small boats and trolling motors, in particular.
Fortunately, there are countless resources to help you learn. Batteries have changed over the years and new technology has been applied to them, so it pays to do a little research.
However, it seems that almost no one does.
Someone says something on a facebook page, and without knowing anything else, the would-be battery buyer runs out and plunks down his hard earned money.
This situation gives us a great opportunity to educate. It will be a challenge, but it may save frustration, time, and money... and that's worth it.
So, lets get into it.
Battery types
There are different types of batteries available, but the main types of batteries you might use for a trolling motor fall into a few categories:
1. Traditional flooded, lead-acid, or “wet” batteries
2. Absorbent Glass Mat, or AGM
3. Lithium iron phosphate (LiFePO4), or lithium-ion
The Old Standby ... Lead -Acid
Traditionally, batteries rely on a chemical reaction between lead, and a solution of water and sulphuric acid, called "electrolyte," to produce electrical current. This process within the lead-acid battery hasn’t changed much since Frenchman Gaston Planté invented it in 1859.
In truth, it hasn't needed to change.
It’s still thin lead plates in a bath of sulfuric acid and water, inside a durable, polypropylene case. Improvements have been added over the years, but Monsieur Planté would probably still recognize his invention. The problem is, the DEMANDS placed on batteries have changed.
Lead-in-electrolyte batteries are just fine, as long as you just want a quick, big jolt of current to turn over an engine’s starter. And fortunately, they are relatively inexpensive to make.
However, they can’t run much of anything over a drawn out period of time. So for trolling motors, they're not the best choice. They will work, but they won't hold up for long.
However the next generation of batteries can.
The Absorbed Glass Mat battery, and the Car
An AGM battery is a battery designed for two jobs: delivering powerful bursts of current to a DC motor, and being able to also deliver lower current over a long time.
The bonus is, they also tend to last longer than a regular flooded battery.
“Absorbent glass mat,” means what it sounds like – woven fiberglass mats sandwiched between ultra-thin lead plates, acting as squishy sponges. Because the plates are very thin, more glass mats and lead can be shoehorned tightly into the battery's case. And more lead equals more power.
AGM batteries also have internal gas valves that control the amount of hydrogen and oxygen gas allowed to escape during charging. These two gases are a result of the chemical reaction that occurs when recharging a battery, and the valves bleed off the pressure they develop without harm.
The term you'll see to describe batteries with these valves is, “valve-regulated,” or VRLA (valve regulated lead acid). It is an important safety feature of these devices.
AGM's were developed in the 1970's to serve as backup power for telephone systems and computer banks. Today, they've blossomed in popularity, and we see them in motorcycles, cars, military vehicles and aircraft, and as power storage for everything from emergency lights to office buildings.
So what's the big deal over standard, flooded batteries?
Actually, the “big deal” is quite amazing, especially with the car-sized batteries we boaters are interested in.
AGM advantages
To understand the advantages of AGM's for trolling motors, we have to start by talking about cars for a little bit.
Up until the last couple decades, AGM's weren't much seen in automobiles. In 2014, you could count the number of vehicle models that needed an AGM battery on your two hands.
However, hybrid electric and the new, start-stop vehicles need an advanced battery to keep the air conditioning running, even if the engine turns off.
The same can be said of all the futuristic technology: lane-keeping assistance, pre-collision warnings, remote engine starting, GPS locating systems, keyless entry and ignition...
Finally, more and more motor vehicles are employing this “always-on tech.”
That means they need batteries that can deliver power over long periods. And that means AGM batteries are here to stay - and we benefit.
AGM batteries give you...
- More starts per battery
- Faster recharging
- More durable construction
- Safer handling
- Special valves protecting the battery's lifespan
AGMs recharge faster, too. Once a car is started, the alternator takes over and replaces the current taken out to start the vehicle. Fast charging is a benefit, at that point.
Because of the cushioning effects of the absorbed mats, AGM's withstand shaking and vibration better than typical batteries. Seems ideal for knocking around in boats, wouldn't you say?
They’re also listed as spill-proof, meaning they are safer to handle all around.
It probably sounds too good to be true, but it isn't. It's fact.
How AGM Batteries Work
These benefits result from two special changes to the original wet battery, and a few extra design changes that fundamentally expand what batteries can do.
Big Deal #1 - The Valves
First, an embedded gas valve prevents evaporated water from leaving the battery case. Its little more than a one-way check valve, but is the secret to an AGM’s long life.
The AGM's fundamental chemistry is still based on lead, sulfuric acid and water. When you draw current from the battery, the acid molecules move to the lead plates, leaving water and lead sulfate behind. You are briefly removing the sulfuric acid from the liquid solution inside.
This enables a chemical reaction between the paste on the plates which results in an electrical current.
The process is then reversed when you recharge the battery, and the sulfuric acid is mixed back into the solution.
However, some molecular water loss will always occur when an electric current runs through water, because this splits it into hydrogen, and oxygen, gases.
Losing those gaseous water molecules through evaporation means the electrolyte becomes more acidic over time. This changes the chemical reaction on the plates, and ultimately shortens the life span of the battery.
This is also the reason you had to replenish the water in the old, wet-cell, lead acid batteries
But the valves in the AGM battery stops those water gases from leaving.
And if you overcharge the battery, the valves save your butt again.
Using a charger that doesn't reduce the current supply as the battery nears full charge is a problem... an overcharging problem.
Just pushing full current the whole time means the current must push through, regardless... and it keeps breaking up water molecules, and builds gas pressure inside.
That’s when another safety valve mechanism kicks in, again, this time to release excess gas vapor and equalize pressure inside the battery.
Big Deal #2: Fiberglass Mats
The next big improvement feature is the “GM” part of AGM: the fiberglass mats.
Ultra-thin glass fibers are woven into a spongy mat that soaks up all the electrolyte (water and sulfuric acid) into thin pillows that cushion the lead plates. Instead of a free-flowing, liquid acid sloshing around inside the battery, the AGM carries its charge in soaked, fiberglass sponges between the lead plates. Neat, huh?
The glass mats’ complete coverage makes it easier to summon more power from an AGM battery — and they make it easier to recharge.
That’s when another safety valve mechanism kicks in, again, this time to release excess gas vapor and equalize pressure inside the battery.
Big Deal #2: Fiberglass Mats
The next big improvement feature is the “GM” part of AGM: the fiberglass mats.
Ultra-thin glass fibers are woven into a spongy mat that soaks up all the electrolyte (water and sulfuric acid) into thin pillows that cushion the lead plates. Instead of a free-flowing, liquid acid sloshing around inside the battery, the AGM carries its charge in soaked, fiberglass sponges between the lead plates. Neat, huh?
The glass mats’ complete coverage makes it easier to summon more power from an AGM battery — and they make it easier to recharge.
In power, speed, long life and durability, the AGM battery has standard batteries beat by a mile.
Lithium Batteries
Now, let’s discuss lithium batteries, the new darlings of the battery world,… the new kid everyone wants to know.
And in fact, there are a ton of advantages to lithium batteries, and lots of people are jumping on the lithium bandwagon.
But like nearly all new tech, they also have at least one HUGE drawback that is usually glossed over, especially by their makers or those that have sunk money into them.
But before we get to that, lets see what they bring to the table.
The first benefit to portable lithium batteries, technically referred to as lithium iron phosphate (LiFePO4), is that they don’t require any specific battery charger. They can be charged with most any current regulated charger, because they are outfitted with an on board battery management system (BMS) which controls the charging. Most makers want to sell you a special “smart” charger, of course, but the battery is self-regulating.
Next, lithium batteries have longer-lasting power. A standard lithium-ion battery with a BMS has a long charge-cycle lifespan – up to 2000 discharge/recharge cycles, for example.
A high-quality AGM lead-acid battery may only be good for 500-600 cycles. That's still good, but it doesn't hold a candle to the lithium's cycle life.
Non-lithium batteries lose voltage as they are drained, too. As the current goes out, the voltage across the plates diminishes. When these batteries reach about 50% voltage charge, they may lose their ability to run the trolling motor.
Now, let’s discuss lithium batteries, the new darlings of the battery world,… the new kid everyone wants to know.
And in fact, there are a ton of advantages to lithium batteries, and lots of people are jumping on the lithium bandwagon.
But like nearly all new tech, they also have at least one HUGE drawback that is usually glossed over, especially by their makers or those that have sunk money into them.
But before we get to that, lets see what they bring to the table.
The first benefit to portable lithium batteries, technically referred to as lithium iron phosphate (LiFePO4), is that they don’t require any specific battery charger. They can be charged with most any current regulated charger, because they are outfitted with an on board battery management system (BMS) which controls the charging. Most makers want to sell you a special “smart” charger, of course, but the battery is self-regulating.
Next, lithium batteries have longer-lasting power. A standard lithium-ion battery with a BMS has a long charge-cycle lifespan – up to 2000 discharge/recharge cycles, for example.
A high-quality AGM lead-acid battery may only be good for 500-600 cycles. That's still good, but it doesn't hold a candle to the lithium's cycle life.
Non-lithium batteries lose voltage as they are drained, too. As the current goes out, the voltage across the plates diminishes. When these batteries reach about 50% voltage charge, they may lose their ability to run the trolling motor.
Lithium batteries don't suffer this voltage drop while discharging.
To be fair, they will also just quit on you with no warning, once their current capacity is depleted. You can at least monitor the voltage drop on the other batteries as they discharge, so you know where you stand with them.
So it's not all roses with the lithiums. But, the fact remains that they can provide power, longer, over any other battery type.
Still more advantages accrue to lithium batteries in the form of overcharge and deep discharge protection, built in. This means your batteries can be used for a longer period of time than a comparable lead-acid battery.
They also weigh about 60% less than a conventional lead-acid trolling motor batteries, and they work down to - 20 degrees Fahrenheit (-29 Celsius).
The final advantage is in the wiring needed for lithium batteries. Because the voltage stays more consistent during discharge, you don't need to oversize any of your wires to compensate for voltage loss when you run long lengths of cable.
This saves you money... well, as long as you never use anything else BUT lithium batteries.
And with their built in BMS, you shouldn't need a short circuit disconnect – a circuit breaker, in other words. But you'd be foolish to dispense with the protection they offer, and its still ABYC* code to have them, so we wont call it an advantage.
* - American Boat and Yacht Council
After reading this, you may wonder why other batteries exist at all. Well, like I said earlier, it's not ALL roses with lithium batteries.
There is one huge disadvantage with the lithium batteries, El Numero Uno, the Big Bad Daddy of Drawbacks:
THEIR PRICE!
Even the cheapest ones are 3-5 times more than any comparable AGM battery.
As this was written, a 100-110 amp/hour, deep-cycle AGM like I usually run would cost anywhere between $150 and $250 to purchase.
The same capacity LiFePO4 battery is anywhere between $650-$1000, and up.
Its shocking, really. I spit up my coffee the first time I looked at them.
To be fair, they will also just quit on you with no warning, once their current capacity is depleted. You can at least monitor the voltage drop on the other batteries as they discharge, so you know where you stand with them.
So it's not all roses with the lithiums. But, the fact remains that they can provide power, longer, over any other battery type.
Still more advantages accrue to lithium batteries in the form of overcharge and deep discharge protection, built in. This means your batteries can be used for a longer period of time than a comparable lead-acid battery.
They also weigh about 60% less than a conventional lead-acid trolling motor batteries, and they work down to - 20 degrees Fahrenheit (-29 Celsius).
The final advantage is in the wiring needed for lithium batteries. Because the voltage stays more consistent during discharge, you don't need to oversize any of your wires to compensate for voltage loss when you run long lengths of cable.
This saves you money... well, as long as you never use anything else BUT lithium batteries.
And with their built in BMS, you shouldn't need a short circuit disconnect – a circuit breaker, in other words. But you'd be foolish to dispense with the protection they offer, and its still ABYC* code to have them, so we wont call it an advantage.
* - American Boat and Yacht Council
After reading this, you may wonder why other batteries exist at all. Well, like I said earlier, it's not ALL roses with lithium batteries.
There is one huge disadvantage with the lithium batteries, El Numero Uno, the Big Bad Daddy of Drawbacks:
THEIR PRICE!
Even the cheapest ones are 3-5 times more than any comparable AGM battery.
As this was written, a 100-110 amp/hour, deep-cycle AGM like I usually run would cost anywhere between $150 and $250 to purchase.
The same capacity LiFePO4 battery is anywhere between $650-$1000, and up.
Its shocking, really. I spit up my coffee the first time I looked at them.
I still do.
So, is it worth putting up the money for lithium batteries?
Well, that depends on what you call, “need.”
You literally need food and water to live – but do you NEED a lithium battery for your trolling motor?
There's no definite answer to that question, except in terms of want and personal value choice.
If you want a long-lasting, durable battery - and cost is no object to you - then a lithium battery might be worth consideration.
If you lose your job and have to take what you can get, well, the lithium batteries might not be so sweet.
What About Deep-cycle Batteries?
The next thing you'll hear, over and over with trolling motor batteries, is the term, *deep cycle.*
“Man, you need a deep cycle battery for that thing. Nuttin else will do.”
Well, that depends on what you call, “need.”
You literally need food and water to live – but do you NEED a lithium battery for your trolling motor?
There's no definite answer to that question, except in terms of want and personal value choice.
If you want a long-lasting, durable battery - and cost is no object to you - then a lithium battery might be worth consideration.
If you lose your job and have to take what you can get, well, the lithium batteries might not be so sweet.
What About Deep-cycle Batteries?
The next thing you'll hear, over and over with trolling motor batteries, is the term, *deep cycle.*
“Man, you need a deep cycle battery for that thing. Nuttin else will do.”
Well, here's what it really means.
A deep cycle battery is a lead-acid battery designed to provide sustained power over a long period, and run reliably until it is about 80% level of discharge, at which point it must be recharged.
It is important to note that although most deep cycle batteries can be discharged up to 80%, most manufacturers recommend not discharging below around the 50% level to extend the life of the battery.
It is this level of discharge that is the “deep cycle” people talk about. It is the opposite of conventional battery types that are designed to deliver massive jolts of current in short bursts before they need to be recharged.
To illustrate, a cars starter battery discharges only a tiny percentage of its overall capacity -- usually 5% or so -- each time it is used. But it does it in 600 amp chunks!
Then this relatively small percentage gets replaced by the cars alternator to await the next demand.
Deep-cycle batteries, by contrast, might deliver, say, 30 amps, but they can do it for hours before needing a recharge
For lead-acid, deep-cycle batteries, however, there is bad news. Well, kinda-sorta bad news.
The bad news comes in the form of an inverse relationship between the depth of discharge (DOD) and the number of charge and discharge cycles it can perform.
In other words, the deeper you discharge it each time, the fewer recharge cycles it can handle before failure.
Go too far and you might kill it.
Literally.
This is why lead-acid batteries are recommended to be discharged to an average "depth of discharge" of around 50%... it's a safety margin.
On the other hand, they are still lead-acid batteries, albeit vastly improved ones. This means they bring the undeniable benefit of being way cheaper than the exotic lithium-ion cells.
It is important to note that although most deep cycle batteries can be discharged up to 80%, most manufacturers recommend not discharging below around the 50% level to extend the life of the battery.
It is this level of discharge that is the “deep cycle” people talk about. It is the opposite of conventional battery types that are designed to deliver massive jolts of current in short bursts before they need to be recharged.
To illustrate, a cars starter battery discharges only a tiny percentage of its overall capacity -- usually 5% or so -- each time it is used. But it does it in 600 amp chunks!
Then this relatively small percentage gets replaced by the cars alternator to await the next demand.
Deep-cycle batteries, by contrast, might deliver, say, 30 amps, but they can do it for hours before needing a recharge
For lead-acid, deep-cycle batteries, however, there is bad news. Well, kinda-sorta bad news.
The bad news comes in the form of an inverse relationship between the depth of discharge (DOD) and the number of charge and discharge cycles it can perform.
In other words, the deeper you discharge it each time, the fewer recharge cycles it can handle before failure.
Go too far and you might kill it.
Literally.
This is why lead-acid batteries are recommended to be discharged to an average "depth of discharge" of around 50%... it's a safety margin.
On the other hand, they are still lead-acid batteries, albeit vastly improved ones. This means they bring the undeniable benefit of being way cheaper than the exotic lithium-ion cells.
This is the end of part one. Take a breather and then head to part 2, found here:
Batteries For Trolling Motors, part 2
Then, come visit us on Facebook at:
Palmetto Fly n Fish
Thanks for reading, and Tight Lines,
David
Palmetto Fly n Fish
© All rights reserved, 2021
Then, come visit us on Facebook at:
Palmetto Fly n Fish
Thanks for reading, and Tight Lines,
David
Palmetto Fly n Fish
© All rights reserved, 2021
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