Fishermen learn the easy way or the hard way that how long a trolling motor battery lasts depends on how competently they charge it. I’d go so far as to say that proper charging can be more important than how well the battery is built and how much you paid for it.

I risk saying this because I know sloppy charging habits can kill even the world’s best battery long before its components wear out.

Back when transformer-based charging technology was the rule, battery charging was difficult enough to give us an excuse for screwing up.

Conventional wisdom said that if you had a dead 100-amp-hour battery and a 10-amp charger you simply left the charger hooked up for 10 hours (10 amps x 10 hours equals 100 amp hours) and the battery was ready to go again.

Unfortunately, conventional wisdom wasn’t any better back then than it is now, and several things limited the success of this seemingly simple process.

Transformer-based chargers didn’t output their advertised charging rate throughout the entire recharging period.

A 10-amp charger only put out 10 amps of charging power to an almost-dead battery; as the battery’s state of charge increased, the charger’s output decreased. So, a 10-amp charger didn’t necessarily put 30 amps back into a battery in three hours or 100 amps in 10 hours.

You gave the amount of charging time your best guess, and then retested the battery’s state of charge to see how close you got.

To make matters worse, a battery’s ability to accept a charge varies according to its age and condition, as well as the ambient temperature.

Once you finally got a 100-percent charge reading, you put up the charger and then checked the battery at regular intervals. You recharged it when it dropped below 75 percent during storage, and you checked it and charged it if necessary the day before your next fishing trip to make sure you started the day with all the trolling motor running time possible.

In today’s world, all you have to do is connect an extension cord to your onboard smart charger’s AC plug and leave it plugged in 24-7 between fishing trips. It does all the tedious calculating and charging automatically.

If you use flooded-cell, full-maintenance batteries, you have to check the water in the cells at least once a month if you are not fishing and once every week or two if you are. I check mine after every second trip in the dead heat of summer.

Keeping the water in the cells covering the plates and doing an annual battery top and terminal cleaning are about it for maintenance.

When you plug an onboard smart charger in after a hard day on the lake, it measures each battery’s state of charge, and then hits it with a beginning bulk charge that replaces most of its amp capacity.

Once a preset point is reached, the charger switches to an absorption mode to finish the charge.

After the charge is complete, some chargers shut off completely and turn on again as necessary, while others stay on and shift to a float charge with just enough output to make up for the normal self-discharge all batteries experience as they sit waiting for your next fishing trip.

Some smart chargers split the charging process into more than three stages, and some include an equalization feature that periodically (or manually on command) delivers a controlled overcharge to the battery to realign the capabilities of all of its cells to accept a charge and produce power.

The best models let you set optimum charging profiles for each type of battery (flooded, AGM, high performance AGM and gel) to give every battery a maximum charge. This usually adds trolling motor running time and extends battery life. 

Smart chargers typically come in one-, two-, three- and four-battery models. They have a separate pair of leads (one positive and one negative) for each battery, and they operate in one of two ways.

Some chargers have independent circuits for each battery, and are essentially several separate battery chargers built into a single case. Other chargers also have a pair of leads for each battery but are smart enough to sense the amount of power left in each battery and then divide up its total charging power according to each battery’s need.

For example, a boat returns to the dock with its two deep-cycle trolling motor batteries run down to 20 percent of a full charge but its cranking battery is only down to 90 percent because the engine’s alternator has kept up with its electrical load.

A 10-amp charger with completely separate charging circuits will treat all three batteries the same: The cranking battery will be fully recharged in minutes, while the trolling motor batteries will take the normal number of hours.

An “on-demand” charger with a 30-amp total capacity might send 13 amps of charging power to each of the deep cycle batteries and just 4 amps to the cranking battery. Its extra efficiency tops off all three batteries in less time.

If you fish once a week, this might not be important to you, but if you are a tournament fisherman (or fish like one) and you only have a short night to get the batteries back up and running, it can mean a lot.

The amount of charging power you need depends on how long you have between trips to get your batteries recharged. If you fish once or twice a week and have 24 to 48 hours of charging time, then a charger that delivers five or six amps to each battery should be all you need.

If you commonly need to put a full charge on severely depleted batteries overnight, then you’ll need a minimum of 10 amps per battery.

I’d hesitate to use a charger with more than a 15-amp-per-battery charging rate because more than one battery manufacturer has recommended that their batteries be charged at a rate no higher than 10 percent of their total capacity.

If you have a 100 amp-hour battery, the recommended ideal rate is no more than 10-amps. I have used chargers that could put out up to 15 amps without seeing any loss of battery performance or service life, but I haven’t gone any higher.

Most onboard smart chargers are safer than the old transformer-based bench chargers. Many are designed not to cause sparks when they are connected the first time or when disconnected for maintenance. They can be left onboard because they are usually waterproof and built to resist damage from vibration and rough-water pounding.

Most are also temperature-compensated so they can output and maintain an optimum charge for the current ambient temperature.

New smart chargers are also unaffected by a low-voltage input. If you were on the end of an AC circuit’s wiring run at a marina or campground and only had 95 to 100 volts available instead of the normal 110 to 120 volts, the old transformer-based chargers would put out less than their rated amount of charging power. Smart chargers compensate for that and still deliver full power.

Any of today’s smart onboard chargers will make charging a whole lot easier than it was in the “good old days.” Pick one with a charging output per battery that suits your fishing trip frequency and check to make sure it can safely charge the type of batteries in your boat.

And, make sure to check the water in flooded-cell batteries at least once a month to make sure it covers the battery plates in every cell.