Boat batteries. What could be simpler? Pick them up at the marine dealer, strap them into the trays, connect the leads and head for the lake. Obviously, it can be that easy, but if you take the time to assess your power requirements before slapping cash on the counter, you can ensure yourself lasting power without having to buy more battery than you need.Deep-Cycle Difference
The most important thing you can do is to check the battery's rating, comparing it to your boat's appetite for amperage and the demands you'll make during a hard day’s fishing.
There are a couple of ways manufacturers rate batteries. The one that most boat owners recognize is Marine Cranking Amps (MCA), or simply Cranking Amps (CA). Batteries for recreational fishing-type outboards range from about 460 MCA for engines up to 60 horsepower and 525 MCA for those up to 150 horsepower. Many of today's large outboards require starting batteries of 1,000 MCA. In fact, some retailers report that it’s the No. 1 seller.
But what does MCA really mean? The test to determine a battery's MCA rating is a variation of the Battery Council International’s standard test for Cold Cranking Amps (CCA) for auto batteries. The difference is that the CCA test is done when the battery is at 0 degrees Fahrenheit, while the MCA test is performed, for obvious reasons, at 32 degrees.
Manufacturers commonly give deep-cycle batteries an MCA rating, but you should be more concerned with its capacity-described in either amp hours (115 amp hours, for example) based on a 20-hour discharge time, or reserve capacity (the number of minutes needed to completely discharge a fully charged battery with a 25 amp draw at 80 degrees). To turn reserve capacity into approximate amp hours, simply multiply by 0.6. For example, 120 minutes reserve capacity x 0.6 = 72 amp hours.
To make these numbers meaningful, an angler must determine the number of amps being pulled per hour from the battery, as well as estimate the length of a fishing day. Since amp draw from sonar, GPS and other accessories is minimal, the focus is on amp-hungry electric trolling motors.
Say your bow-mount motor draws 35 amps per hour at full power. A fully charged 115 amp hour battery will power the motor (at its highest speed setting) for approximately three hours.
A battery with 120 minutes reserve capacity (72 amp hours) will power the motor at full tilt for about two hours.
Since the relationship between speed setting and amp draw is nearly linear, you can estimate the same motor will draw approximately 17.5 amps per hour at half speed; about 8.75 amps per hour at one-quarter speed. That extends the 115 amp hour battery's charge to about six and a half and 13 hours, respectively, if the motor runs continuously. The 120 minute reserve capacity battery will power the motor for about four and eight hours, at the lower speed settings.
If the motor is in momentary mode—turned on only when the button on the foot control is pushed—the charge in each battery will last much longer.Calculating Demand
These are the factors you need to consider to make a buying decision, but how do you put them together? Start with the motor's maximum amp draw (35 in this case), then divide by the power setting you use most (one-quarter speed = 4), and finally multiply by the number of hours you put into a hard day of fishing (say 9). It looks like this: 35 / 4 x 9 = 78.75 amp hours required.
Now add about a 50 percent cushion (39.37) in case the outboard breaks down and you have to use the bow-mount motor to get back to the launch ramp. You end up with 118.12 amp hours. Therefore, a 115 amp hour battery should be just about right.
A two-battery, 24-volt system doesn't offer extra capacity, just more voltage. Wired in series, as they must be for a 24-volt electric motor, the total capacity of two 115 amp hour batteries is...115 amp hours. The calculation is the same; you just need to buy two batteries.
Only when two batteries are wired in parallel to power a 12-volt motor does the capacity double. Thus, a pair of 80 amp hour batteries will provide a 12-volt motor with 160 amp hours of power.
Doing some homework before choosing marine batteries only makes sense. On one hand, you ensure yourself enough power to keep fishing all day long. On the other, you may find that you can downsize to a smaller, less expensive model.