What Size Battery Do I Need for my Trolling Motor?

OPTIMA Batteries
Harris Chain, Florida

Even though we originally posted this in February, OPTIMA-sponsored Edwin Evers has already won his first MLF Bass Pro Tour stop tour of the year & by that point and people were already buying trolling motor batteries for their boats. With those purchases, we begin to see people choosing the wrong batteries for their trolling motors. Is it possible to select the wrong trolling motor battery for your boat? Absolutely! When we did a quick google search of "trolling motor battery" we saw two OPTIMA batteries in the results that are probably two of the last choices we'd offer to folks looking to buy a trolling motor battery.

One of the batteries that showed up in the search results was OPTIMA's 6-volt REDTOP battery. First and foremost, all REDTOP batteries are designed and warrantied for SLI (starting/lighting/ignition) use. Essentially, that means they are designed & intended for starting engines, not for long-duration, deep-cycle use, which is exactly how a trolling motor battery is used. This is important to note, because the OPTIMA Group 34M BLUETOP is essentially the marine version of our REDTOP starting battery and SHOULD NOT BE USED AS A TROLLING MOTOR BATTERY.

Instead, we would recommend using either the D34M, D27M or D31M BLUETOP batteries for trolling motor use. All three of those batteries are designed & warrantied for both starting and deep-cycle (trolling motor) use and are identifiable by their blue tops and light-gray cases, while the 34M BLUETOP has a black case color that it shares with REDTOP starting batteries. Now that we've identified the correct type of battery for trolling motor use, we need to look at which of those three batteries might be the best for your application.

There's a lot of information in here to remember, so here's the simplest formula we can give you for selecting a trolling motor battery: Choose the biggest battery (most reserve capacity) that will safely fit in your boat

Some trolling motor companies might suggest a minimum reserve capacity for a battery or a specific number of batteries for a certain number of pounds of thrust from your trolling motor, but the reality is, you first need to look at the space you have available for your trolling motor batteries in your boat. If you're trying to put a trolling motor rated at 105 pounds of thrust on the back of your canoe or skiff, you may need to use three batteries and you need to make sure you have enough space for three batteries, yourself and all your gear. You may find putting 180 pounds of batteries and a 180-pound fisherman in the same end of your boat is not a viable option.

Once you've identified the space you have available for trolling motor batteries, consider how much you're going to use your trolling motor and where you're going to use it. Are you on a "no wake" body of water, that will make your trolling also your primary means of moving around on the lake? Are you going onto a river or other body of water with a strong current that may require a lot of trolling motor use? Will you be going into windy conditions that may also require a lot of trolling motor use? If you answered "yes" to any of those questions, then you may want to look for the largest trolling motor batteries you can fit in your boat.

When looking at battery ratings, there's often a default to go straight to the "Cold Cranking Amps" rating when looking for the biggest, baddest or best battery. In fishing applications, cold cranking amps (which are measured at 0°F) doesn't really matter as much as a battery's reserve capacity. For example, our 34M and D34M BLUETOP batteries share identical exterior specifications in terms of length, width and height. However, that D34M, which is designed & warrantied for deep-cycle use, is rated at 55AH and has a reserve capacity of 120 minutes at 25 amps, while the 34M is rated at 50Ah and has a reserve capacity of just 100 minutes at 25 amps.

A lot of tournament bass anglers prefer OPTIMA D31M BLUETOP batteries, because they have two important attributes- slightly less weight than a traditional Group 31 battery and the ability to operate well beyond their rated capacity. Less weight matters to pro anglers like Edwin Evers, because it allows his boat to move across the water faster to the best fishing spots and it allows him to move into shallower water if his boat isn't sitting lower in the water.

The cast straps that connect the cells in an OPTIMA battery also allow them to accept and deliver current much better than a traditional trolling motor battery, that uses "tombstone welds" to connect cells. While batteries are typically considered to be at 0% state of charge when they get discharged down to 10.5 volts and their high internal resistance may not allow them to continue deliver power, it's not uncommon for OPTIMA anglers to come off a windy day on the water with their batteries discharged down well into the single digits.

The flipside of having those thick, cast straps connecting the cells of an OPTIMA battery, is that they can be recharged much faster than a traditional flooded battery. This matters a lot for a tournament angler, who may not get a chance to start charging his trolling motor batteries until 7 or 8PM at night, but needs to have it back at the launch before sunrise the next morning. If he's staying at a hotel with other anglers and they're all trying to recharge their batteries from the same outlets and they're using long, thin gauge extension cords, some of those batteries may not be fully-charged by the next morning. Those cast straps and faster recharge rate may even matter for a weekend angler, who wants to get out on back to back mornings.

When looking at the size of trolling motor battery you are considering, you should also look at the size of the brand behind the battery. Many trolling motor batteries are being sold out of private homes, rented PO boxes or as side hustles from other businesses. We've watched such brands come and go for more than a decade now and when they go, they leave their customers with nothing more than expensive paperweights. Take the time to look up the physical address of the battery brand you are considering. If you can't find a commercial building with a sign out front, there's a good chance you'll have a hard time finding anyone from that brand, if (and probably when) you need warranty service. You can see the world headquarters of OPTIMA's parent corporation, Clarios right here.

One last point we want to mention is the choice you make for your other battery (or batteries) that don't run your trolling motor. It wasn't that long ago, that the non-trolling motor battery just needed to start the engine. With each passing year, that is less and less likely. The starting batteries in modern fishing boats have turned into the starting/livewell/power poles/fish finder battery. In other words, they are no longer just a starting battery, but likely to also be used also as a deep-cycle battery. If that's the case in your boat, we would suggest you focus your "starting" battery choices on batteries designed & warrantied for both starting and deep-cycle use, which would include our D34M, D27M or D31M BLUETOP batteries.