Which Electric Trolling Motor Battery should I choose?

There are so many options out there when it comes to electric trolling motor batteries how can you know which one to choose? Well bear with us and we’ll explain everything you need to know so you can buy smart.

Step 1: Checking the power needs of your electric trolling motor

To find the right battery we need to start by checking how much power your trolling motor needs at various speeds. All manufacturers should be able to tell you this but if you’re struggling to tack yours down here are the approximate figures for most models:

Trolling Motor Thrust Max Speed Current Medium Speed Current Minimum Speed Current
30lbs 30amps 15amps 5amps
50lbs 40amps 20amps 5amps
90lbs 50amps 25amps 5amps

Step 2: Identifying deep cycle batteries

For electric trolling motors you want a battery (or batteries) with ‘Ah’ ratings – this means they are designed to deep cycle (regularly discharge over a long period and then recharge). If you see batteries with only CCA ratings avoid these. CCA stands for Cold Cranking Amps which refers to what power a battery can provide for starting diesel or gasoline engines.

CCA rated batteries are cheaper than deep cycle units and they will power your trolling motor but not for long. They are designed to provide large amounts of power over a short period (to start an engine). If you slowly discharge one over long periods you are highly likely to damage internal components shortening the overall lifespan or causing the battery to fail completely at the exact moment you really need it.

So … ‘Ah’ rated batteries only!

Step 3: Understanding the Ah rating of batteries

Next up we need to know what this ‘Ah’ rating actually means. Almost every article you will read on the Internet gets this wrong. They will tell you something like “a 100 Ah battery can power a 5amp appliance for 20 hours, a 10 amp appliance for 10 hours and a 20 amp appliance for 5 hours”.

In other words just take the Ah rating of the battery, divide it by the current the appliance draws and, bingo, you know how long it will last.

This is a common misunderstanding that is repeated over and over all across the web even though it is untrue. However it endures because the real calculations are much more complex.

If you want to dive into the detail it is here, otherwise you are better off using a Peukert’s Law calculator. From this we can tell that a 100Ah battery will last:

Trolling Motor Thrust At Maximum Speed At Medium Speed At Minimum Speed
30lbs 2.8 hours 6 hours 20 hours
50lbs 2 hours 4.4 hours 20 hours
90lbs 1.6 hours 3.4 hours 20 hours

Now beware! Peukert’s Law assumes:

  1. The battery is brand new and in excellent condition – batteries lose their ability to fully recharge gradually as they get older so you will get less run time.
  2. A temperature of 77F (25C) – both hotter and colder temperatures will reduce the performance of a lead acid battery.

Changing either of these can make a big difference as to how long your battery will last so you should always add in a large margin. For example if you think a 75Ah battery will be ‘just enough’ for your trolling needs then purchase a 100Ah battery so you can be confident that in all temperatures, and as it gets older, it will perform.

Step 4: Deciding on AGM or Gel

Deep cycle batteries for electric trolling motors are mostly from the lead acid family. You will see two terms bounded around when you are looking for a replacement. ‘AGM‘ and Gel.

AGM is short for Absorbed Glass Mat and refers to glass based mats which are placed between the lead plates inside the battery. These are soaked with electrolyte (Glass Mats that have Absorbed electrolyte). The mats help reduce buckling in the plates and also ensure there is no liquid that could leak out if the battery is damaged or positioned at an angle.

For almost all trolling needs AGM is what you need.

Opened AGM battery
An absorbed glass mat battery prized open to reveal the plates and acid saturated glass mat separating them. Image: YouTube

Gel batteries are significantly more expensive. Instead of glass mats between the plates they have a silicon electrolyte gel which make the plates even more buckle resistant. However they really only justify the extra expense if your battery is going to be somewhere with a lot of vibration (perhaps in an engine compartment) or regular jolts (possibly caused by larger waves in Saltwater trolling or more often in Jetskis and power boats).

For more on this see: Should I upgrade to a Gel Battery for my trolling motor?

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  1. mark

    Can the BG-121000NB-M start an outboard motor as well as run the trolling motor? I have two batteries in my boat with one battery dedicated to each task. But, they are both charged from one high quality marine battery charger. It is my understanding, and please correct me if I am wrong, that I need to use matched specification batteries when they are both charged with the same charger.

    1. BatteryGuy Editor

      Because trolling motors require a steady, consistent source of electrical power and a starting battery is designed to discharge a large amount of power all at once to start an engine, you’ll need a specialized battery if you want it to handle both. Dual purpose batteries are designed to both crank engines and provide a consistent source of electrical power for trolling motors. The Dual Purpose of this battery type would be to start the boat as well as power the trolling motor (deep cycling). I would use a Dual Purpose battery to achieve what you want or stick with your current set-up.

      Most boats come with specialized marine chargers with multiple charging options for starting batteries, trolling batteries, navigation batteries, etc. They have separate wiring harnesses for different battery types. On the other hand if your batteries are in Series or Parallel, they need to be batteries of the same specifications in this case.

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