Batteries For Electric Trolling Motors: What You Need to Know Before You Buy

To ensure your electric trolling motor continues to work optimally, it is essential that you carefully research the optimum type of battery for your motor and the conditions under which you will use it.

At a Glance:
1️⃣ Battery selection: Choose between AGM, Gel, lithium-ion, or other battery types based on your trolling motor’s requirements and usage conditions.

2️⃣ Battery characteristics: High energy density batteries are recommended for trolling motors as they provide a consistent current flow for extended periods.

3️⃣ Size and capacity: Determine the appropriate battery group size to fit your battery compartment, and select a battery with sufficient amp hour rating for your desired range and usage time.

The first decision you must make is to select what technology battery suits you (AGM batteries, Gel, lithium-ion, or one of the others).

After you have decided on the battery chemistry, you need to consider the unit’s physical size and whether it will fit in the battery compartment (group size).

Finally, you will need to choose the voltage (12v,24v,36v) required for your motor and the amp hour rating to ensure that it gives you sufficient range and that you are not constantly having to check whether there is enough power to carry on.

🔋 Battery Types For Electric Trolling Motors

Batteries are designed to produce either higher density power or high energy power. 

Battery Type	Description	Best For
High Power Batteries	Designed to produce bursts of power for a short period; suitable for high-power-density applications.	Applications like a car’s starter motor.
High Energy Batteries	Provide a regular, even current flow for long periods without the surges of high-power batteries.	Sustained power delivery in various devices.

Lead-Acid Batteries 

Lead-acid batteries are the oldest type and have stood the test of time in an almost unchanged form.

• These batteries consist of a series of cells laid next to each other.
  
• Each cell consists of identical, tightly packed, dense sheets of lead (anode) and lead oxide (cathode), which are placed in alternating layers.
  
• The cells are placed in a common pool of sulphuric acid.
  
• When the battery functions, the following basic electrochemical reaction takes place.
  
• A current flows from the lead oxide cathode to the lead anode, and when this happens, the lead gives up electrons (units of electricity) which the lead oxide accepts.
  
• When this exchange takes place, it turns both plates into lead sulfate.
  
• Each cell produces 2 volts and, when linked in series, produces 12 volts.

These batteries are originally designed to be used as high-power density devices. They can release high amperages (over 440 amps) used as a short burst to turn a vehicle’s starter motor.

Once the motor comes to life, the alternator takes over the duties of powering all the electrical equipment and also recharges the battery.

With modifications, manufacturers have modified lead acid batteries into deep-cycle units. The changes are as follows:

• The cells are spaced further apart
• The electrodes are thicker
• They are lifted above the base of the battery to give room for waste sulfate to drop

Lithium-Ion Batteries

Lithium-ion batteries are energy-dense, deep-cycle batteries designed for constant discharge and recharge cycles.

Like lead acid batteries, a lithium-ion (Li-ion) battery is an electrochemical cell that consists of a cathode and anode.

That is where the similarity ends. A lithium battery is charged in the following sequence:

1. The cathode (+) is a piece of aluminum foil coated with a lithium compound (Lithium-ion phosphate).
   
2. The anode (-) is made of copper foil coated with graphite.
   
3. Separating the cathode and anode is a thin plastic sheet.
   
4. The structure contains an electrolyte full of lithium ions (Li+).
   
5. When the battery is charged, electrons are passed from the cathode (+) to the anode (-).
   
6. As the electrons move from the cathode, the cathode changes from lithium-ion phosphate to Lithium-ion (Li+) + the electron (e-) plus ion-phosphate (FePO4).
   
7. The Lithium-ion’s (Li+) migrate through the plastic separator and recombine with the electron that has passed over from the cathode (+) to the anode (-).
   
8. When the battery discharges, the reverse reaction happens, and the electrons move from the anode (-) to the cathode (+) to power the electrical consumers.

Lithium-ion batteries are considered deep-cycle batteries, which can undergo multiple charge/discharge cycles.

85% of the battery capacity is available for use (depth of discharge). Discover the undeniable advantages of lithium trolling motor batteries and if they’re worth it.

Each Type Of Battery Compared

The list below summarizes the advantages and disadvantages of each type of battery.

	Lead-Acid Batteries	Lithium-Ion Batteries
Depth Of Discharge	85%	50%
High power vs. high energy	High Power	High Energy
Deep Cycle wet-cell battery – Suitable for constant discharge/ recharge cycles	No	Yes
Time to discharge	Shorter run time and quicker discharge	Run for longer periods
Time to recharge (using the correct recharger	Slow	Much faster
Life expectancy (longevity)	3-4 years	+10 years
Withstand rough use	Yes	No
Power supply per battery	Measured in 100’s watts	Measured in Kilowatts
Weight	Heavy	Lighter
Cost	Low	High

Lithium-ion batteries cost more than lead acid units; however, the lifetime costs are generally lower for the following reasons:

1. Lithium-ion batteries produce more power meaning fewer are required.
   
2. The life expectancy of a well-maintained lithium-ion battery is conservatively 5 times longer than an equivalent lead acid battery.

✅ Choosing The Right Size Battery For Your Trolling Motor

Once you have decided on the battery technology which is best for you, it is important to calculate the size battery that is optimal for your needs.

How To Determine The Size You Need

There are several factors to consider with size.

The Battery Group Size

The first factor to consider is the battery’s physical dimensions and what will fit into the battery storage compartment. This is called the “group size” of the battery. The group size is a single value relating to specific battery dimensions.

Battery Group Size	24	27	31
Length	10.25 in (15.04 cm)	12.06 in (20.53 cm)	13.00 in (33.02 cm)
Weight	6.81 in (17.40 cm)	6.81 in (17.40 cm)	6.72 in (17.07 cm)
Height	8.87 in (22.53 cm)	8.75 in (22.53 cm)	9.44 in (23.98 cm)

Cranking Amp Rating

Cranking Amp Rating relates to power-dense units, as describes the battery’s ability to provide short periods of high amps to turn a starter motor.

This is only relevant to gas-powered boat motors with a starter motor and is of no concern to an electric motor used in a boat.

The Amp Hour Rating Of The Battery

Amps measure how fast the electrons flow in an electrical circuit. Another word used to describe this is the electrical circuit’s “current.”

Amp hours measure the maximum capacity (in amps) that will be produced each hour.

Importance Of Selecting A Battery With Sufficient Amp Hours

Knowing this value is important because the amp hour rating is the maximum amps produced by the battery for one hour.

Amp Hours (Ah) = Current (in amps) x Time (in hours)

If the battery is rated at 200-amp hours and the motor runs at a power rating that draws 20 amps, the battery will last 10 hours before needing to be recharged. The formula is “200-amp hour battery / 20 amps drawn = 10-hour run time”. 

If it’s a 100 amp hour battery and draws 20 amps, the battery will last 5 hours.

This is not strictly a true rating of the battery capacity, as no batteries can be safely discharged to zero without damage being caused.

• A lead acid battery can only be discharged to 50% and, therefore, could only be run for 5 hours in this scenario.
  
• A Lithium-ion battery can be discharged to 15% and, therefore, can be run for 8 hours and thirty minutes in this scenario.

A general rule is that the higher the group size of the battery, the higher the available amp hours.

If the amp hour rating is not shown on the battery, using the group size value is a reliable indicator of the amp hour rating. 

These batteries will also often display a “reserve power” rating. For a trolling motor, a reserve power of at least 180 minutes should be selected. 

The following lists the estimated amp hour rating for different 12-volt batteries in each group size.

Deep Cycle Amp Hour Rating
Run Time	Voltage	Group Size	Amp-Hour
Weaker	12	24	70 – 85
Better	12	27	85 – 110
Best	12	31	95 – 125

🔧 Battery Maintenance And Lifespan

Nothing is more frustrating than getting to the dock, launching the boat, and finding out that the battery is flat or not working.

As with all of your marine equipment, the more thoroughly it is maintained, the longer it will last and continue to operate to specification.

Charging To The Correct Level

Never overcharge or undercharge a battery. Remember that the recharge process is the reverse electrochemical reaction from when it is discharged; therefore, an overcharge can cause physical damage to the battery.

To prevent this, always choose a battery charger designed to provide the charge profile required for the battery technology used in your boat.

An optimal battery charger should have the following features.

• Automatic Multi-Stage Charging, in which the charger controls the profile of the voltage and current to the battery through the charging cycle and is optimal for your battery technology.
  
• The battery charger should automatically stop charging when the battery is full and keep it on a trickle charge as required.

For optimal performance, I’d recommend you learn when to charge your trolling motor battery.

Tips For Extending The Life Of Your Battery 

The following simple tips will help keep the battery in good working order.

Keep The Battery Charged Even When In Storage

There are several rules regarding when a battery should be charged:

1. Always charge the battery after use.
   
2. Never store the battery without first charging it and keeping it on a trickle charge until the next time it is needed.
   
3. Never overcharge the battery.
   
4. Always store the battery in a dry environment at an average temperature and keep the battery on a trickle charge.
   
5. Check the correct fluid levels if the lead acid battery has opening cells.
   
6. Always keep the battery terminals clean and free from rust.

Common Mistakes To Avoid

Several rules must be observed when choosing batteries, as listed below.

Type And Size Of Battery 

• Never mix and match battery technologies (don’t use Lithium-ion or lead acid in the same circuit).
  
• Never mix batteries with different voltages (12 with 24 etc).

Battery Capacity

• Do not select a battery with a too-small amp hour rating; I recommend choosing a battery with a minimum 100 amp/h rating for trolling motors.
  
• Only use group 27 or higher batteries for a trolling motor to ensure that the battery has sufficient amp hours available. If the battery does not have the amp hours displayed, look for one with a minimum reserve power of 180 minutes.

Battery Type

Whether you opt for a Lithium-ion or lead acid battery, never use a battery that is not rated as a deep cycle unit.

❓ FAQs

What Is The Longest Lasting Battery For A Trolling Motor? 

The battery that will provide the longest run time (amps hours) is generally a lithium-ion unit. Although lithium-ion batteries are more expensive, they are generally no more expensive over the longer term.

Whichever battery technology you use, always opt for the biggest amp hour rating, as this will ensure that the battery provides sufficient range for your fishing activities.

Do You Need A Deep Cycle Battery For A Trolling Motor? 

A deep-cycle battery is the only option for a trolling motor (in fact, any electric motor).

Deep cycle technology ensures that marine batteries can be discharged to the maximum permitted level and then recharged repeatedly without excessive damage being caused to the components.

If a deep cycle battery is not selected, it will last for a few charges and cease functioning.

🔑 Key takeaways

To wrap up, here are the critical points to take away from this article:

1️⃣ Carefully research and consider battery technology, physical size, and voltage/amperage rating. Never mix different sized batteries or mix and match different battery types!

2️⃣ Lithium-ion batteries are more expensive but generally have a longer lifespan and produce more power, making them a worthwhile investment for frequent and long-term use.

3️⃣ Battery maintenance includes charging to the correct level, keeping it charged during storage, and avoiding common mistakes.

4️⃣Whichever battery technology is chosen, the battery should always be a deep discharge type. Always choose a battery with sufficient amp hours for your needs.

When you need to purchase a new battery to power a trolling motor, always research the optimal unit for your motor. Most motor manufacturers have extensive documentation on their websites providing input on optimal specifications.

Understanding how electric trolling motors work and their battery requirements will aid your battery selection process.