Beyond picking a type of battery you need to consider the following sub-factors for a trolling motor battery. These include cell chemical type, power system and battery ratings.
Cell Chemical Type: Flooded vs. Gel vs AGM vs Lithium
First, decide which battery chemistry meets your needs. Each type of chemical makeup inside marine batteries has its own pros and cons. For 2020 I include both AGM and an enhanced flooded battery to help keep costs manageable for wallet conscious buyers.
Absorbed Glass Mat or AGM use glass sheets as separators for the lead plates. The plates and separators soak in electrolyte solution, absorbing and suspending the solution. As a result, the suspended electrolyte always stays in contact with the plates. As a result, an AGM battery produces electricity the same as the older flooded batteries. Likewise, it is spill-proof and the most vibration resistant lead-acid battery available today.
Flooded Enhanced Batteries are an upgraded version of a Wet cell. While less expensive than AGM, they aren’t as popular either.
Next, you should consider the power or voltage of the trolling motor battery. Marine Batteries power ratings are in volts; and are available in 12 volts, 18 volts or 24 volts. This means that if your trolling motor requires 24 volts of power, you will need either two 12-volt batteries or one 24-volt battery. Choosing 24-volt batteries is up to the buyer, but they are expensive and large. Since boating is expensive, we only focus on 12-volt batteries for this article.
Trolling Motor Battery Ratings:
The third factor after you choose a type of trolling motor battery, is to look at the ratings. Like most ratings you see, higher is better, it’s in your best interest to choose the highest ratings within your budget.
Below are the four most important ratings for buying a trolling motor battery:
- Amp Hours (AH)
- Reserve Capacity (RC)
- Group Size
1. Amp Hours (Ah)
This is one of the most important ratings to look for in buying a trolling motor battery. The amp hours rating indicates how much amperage is available when discharged evenly over a 20-hour period. The measurement of amp hours are essentially in units of energy.
In simple terms (For a mathematical explanation click here):
“The number of amp hours indicates the storage capacity of electricity inside battery”.
If you prefer to shop for dual-purpose batteries, you may not see an AH rating. In general, small dual-purpose batteries will have a rating of 25 to 55 Ah. While large group size deep cycle batteries are from 55 to 100 Ah plus.
Choosing a higher amp hour rating provides longer usable power before needing recharging.
2. Reserve Capacity
The reserve capacity (RC) rating is a measurement in units of time (minutes). Calculated by counting the number of minutes a new battery can maintain 25 amps draw until the battery goes from full to flat. The test to find this rating uses a battery in an environment of 80°F (26 °C). The battery is flat when the voltage drops below 10.5 volts.
Consider a Group 27 deep-cycle battery with a reserve capacity rating of 180 minutes. With this 180 RC rating, it can provide a 25-amp load for about three hours.
Like the amp hour ratings, it is best to look for the highest RC rating you can afford. Average deep-cycle and dual-purpose batteries have a 90-minute RC. While satisfactory, try to choose the highest RC rating you can afford.
3. Group Size
Group size refers to a battery’s physical size and every manufacturer builds to a general “group size.” Although, differences occur between manufactures due to specific handles or post height. Therefore, it is important to confirm how much battery space you have on your boat. These are some common group sizes, from small in size to large: 24, 27, 31, 34, 6D, 8D, etc.
For this article, we focus on Group 27-34 since these perform the best for most trolling motors.
While most boaters want to lighten their boat, it is in fact beneficial to use a heavy marine battery. The heavier the marine battery is, the larger the lead plates are and more lead translates to more power.