From the Complaints You Will Never Hear department: “I fished in the wind all day and still have too darned much power left in my trolling motor batteries!”
You are much more likely to hear a sob story about how great the fishing was until the trolling motor batteries died.
I recommend powering your trolling motor with the highest-capacity deep-cycle batteries that will fit in your boat’s battery compartment for several reasons — the most important being that the deeper their depth of discharge on each fishing day the fewer seasons they last.
It isn’t often possible, but if you could equip your boat with trolling motor batteries so big that they only ran down to 50 percent of a full charge during each fishing day they would last more than twice as long as batteries that were completely discharged.
Deep-cycle batteries lose capacity as they age, and manufacturers consider a battery’s service life over when its capacity drops to 50 percent of what it had when new.
The number of discharge/recharge cycles a battery can complete before reaching that 50-percent point is considered its service life.
I still have an old Interstate Marine and RV battery maintenance guide that spells out the effect that depth of discharge has on service life. A deep-cycle battery that is discharged 100 percent each time might last 325 cycles; the same battery discharged only 75 percent lasts 550 cycles. Discharged 50 percent, the same battery lasts 1,000 cycles — and discharged only 25 percent it lasts 2,200 cycles.
Minn Kota and a few other companies understand all this and have come up with a way to use our engine’s alternator to partially replenish our trolling motor batteries as we run between fishing spots. However, these chargers can’t bring the batteries back to a full charge unless our engine has a monstrous alternator and we run it at high RPMs for as many or more hours than we fish.
I think of them as a way to add a sort of mini charge between fishing spots that mitigates the constant battery draw-down responsible for a deeper depth of discharge over the whole fishing day.
The actual charge provided depends on alternator output at your engine’s cruising RPM and how long you run between fishing stops. Idling around doesn’t count for much because alternators output very little at low RPM. And an angler who races to a spot five or 10 minutes from the ramp, fishes there all day, and then runs the five or 10 minutes back is not going to realize any advantage from one of these chargers because he only runs the engine long enough to put an amp or 2 back into each trolling motor battery.
But a fisherman who runs to 10 spots miles apart and only fishes each one for an hour or less will probably notice that his trolling motor feels just as strong at the last spot as it did at the first. And, minimizing the depth of discharge to each battery means that when he gets home and plugs in his onboard AC charger, it will top off the batteries in a relatively short time.
I’m not a huge fan of the DC chargers that operate as simple battery isolators and link all our cranking and trolling motor batteries together in parallel the instant the engine’s ignition key is turned on: I want the cranking battery to get what it needs first, and that amounts to more charging power than it did in years past.
Our cranking batteries now power more electronics than ever, and usually run livewell pumps and timers, shallow-water anchor systems and other accessories while the boat’s main engine is shut down.
When you start your engine, the Minn Kota DC chargers read the voltage across the starting battery and will not begin charging the others until that cranking battery is above 13.6 volts. It also stops charging your trolling motor batteries if the starting battery voltage drops below 12.8 volts; it then applies all its charging power to the starting battery until it is back above 13.6 volts.
Minn Kota calls its charge-on-the-run products DC Alternator On-Board Chargers, and one of them can be installed right alongside your plug-into-the-wall AC onboard charger. The two charging systems operate at different times using different power sources and will not interfere with each other.
Separate models are available for 12-, 24- and 36-volt trolling motor systems. Wiring one in is simple: You attach one set of positive and negative leads to the cranking battery and one set to each of the trolling motor batteries. You don’t have to remove any 24- or 36-volt jumper wires between the batteries because these chargers are designed to treat each battery independently with separate negative grounds. There is also a purple wire that runs to a positive terminal anywhere in the boat that is only hot when the engine’s ignition switch is turned on.
The Minn Kota chargers are designed to put out 10 amps of charging power to each trolling motor battery, but they can only do that if your engine’s alternator has enough output to supply that much surplus power. The company says this means you need an alternator with an extra output (above what your engine needs) of at least 12 amps for the MK-1-DC 12-volt trolling motor system model, a minimum of 25 extra amps of output for the MK-2-DC 24-volt system charger and a minimum of 35 amps for the MK-3-DC 36-volt system model.
Some of today’s outboard engines have enough electric fuel and fuel injection pumps, powerful-enough electronic ignitions and other necessary accessories to use as much as half of their alternator output just to run themselves at peak load periods.
Still, the fact that 24- and 36-volt motors are more apt to be installed on bigger boats with larger outboard engines having alternators with higher charging outputs makes it extremely likely that the alternator on your boat will be big enough to charge your trolling motor batteries. And, the chargers automatically compensate during those peak load times when less surplus power is available to scale down the amount of power going to the trolling motor batteries. They also automatically reduce their output if the trolling motor batteries don’t need to be charged.
These DC charging systems are also handy for camping fishermen who don’t have an AC power source available for their regular onboard chargers but do have enough gasoline to run their engines long enough to stretch out their trolling motor’s running time.
Keep in mind that all battery types cannot be charged with the same charger. The Minn Kota models are designed to charge flooded, wet-cell batteries of either the full maintenance or maintenance-free type, and they also charge maintenance-free, absorbed glass mat (AGM) batteries.
They are not designed to charge gel batteries, and that requires a bit more explanation. I am regularly surprised to hear otherwise knowledgeable anglers refer to Optima and other AGM batteries as “gel” models. They are not gel models, and chargers designed for wet cell and AGM batteries work just fine on them.
Hitting a true gel battery with a charger set up for wet cell and AGM batteries can literally burn it up.
Visit www.minnkotamotors.com for more information on Minn Kota’s DC and AC chargers.