How to Choose, Install and Maintain an Onboard Battery Charger

A "bank" is one battery or several batteries wired together that are charged as one. A multi-bank charger has a separate output for each bank, so a 12-volt starting battery and two trolling motor batteries might need a three-bank charger. Each output charges its own battery independently, which keeps one weak battery from dragging down the others.

For 24- or 36-volt trolling motors, most owners use one charger output per 12-volt battery. Check the trolling motor and charger manuals.

BoatUS gives a simple rule of thumb: pick a charger rated at about 10 to 15 percent of the battery or bank's amp-hour rating. A 100 Ah battery would get a 10 to 15 amp output. An undersized charger may never bring the batteries all the way up, which leads to sulfation and short battery life. On multi-bank chargers, check the amps available per bank, not just the total.

Flooded, gel, AGM and lithium batteries each need different charging voltages. Look for a charger with:

• Multi-stage charging (bulk, absorption, float): fills the battery quickly, finishes gently, then just maintains it.
• Selectable battery types for each bank, so you can set flooded, AGM or gel.
• Temperature sensing if possible; BoatUS notes a hot battery accepts charge differently than a cool one.

Do not charge different chemistries together in one bank, or mix old and new batteries in the same bank. One will be overcharged.

Lithium batteries need a charger with a lithium profile that the battery maker approves, and the battery's own management system (BMS) must work with it. Some chargers only support specific lithium brands. Check both manuals, and get expert help for a lithium conversion.

• Waterproof or splash-proof for small open boats where the charger lives in a console or compartment.
• Ignition protected if it will be in an engine space or any compartment with gasoline engines, tanks or fuel fittings. Federal rules require this on boats with gasoline inboard and sterndrive engines (unless the device is properly isolated from fuel sources). Outboard boats are not covered by that rule, but choosing an ignition-protected charger is still a sensible choice anywhere fuel vapor could reach.
• Built-in protection against reverse polarity, short circuits and overheating.

BoatUS advice for a good location:

• Close to the batteries so the DC wires are short, but not directly above them, where battery gases can attack the charger. Distance matters more than it looks: BoatUS expert Don Casey points out that moving a charger from a few feet to about 15 feet away from the batteries can mean wire several sizes heavier, costlier lugs and a special crimper.
• Dry and well ventilated, since chargers make heat.
• At least a couple of feet from the compass, which the charger can deflect.
• Away from fuel lines, tanks and fuel fittings.
• Above the waterline. Minn Kota, for example, says its waterproof chargers should not be run while immersed or mounted below the waterline, and asks for about six inches of open space on every side for cooling; a charger that runs hot cuts its own output.

Mount it with stainless bolts or screws into solid material, in the orientation the manual allows.

With the battery switch off and shore power unplugged:

• Use the wire gauge the charger manual lists for your wire length. Do not substitute thinner wire because it is on hand.
• Keep runs short and tidy, and support them at least every 18 inches with clips or cable ties, as BoatUS recommends.
• Make each connection with a heat-shrink ring terminal and a proper crimp.
• Most chargers connect straight to the battery posts (positive to positive, negative to negative), not through the battery switch, so they charge with the switch off. Follow your charger's wiring diagram exactly.
• Two batteries in parallel on one output can work, but Minn Kota notes its chargers' safety timers may stop before a large parallel bank is full; unplugging and replugging restarts the cycle. Batteries wired in series always need one output each.
• If the boat has an automatic charging relay (ACR), Blue Sea Systems suggests connecting a multi-stage charger to the battery that carries the boat's loads (or using a second charger output), because in float mode the charger's lower voltage may not trigger the ACR to feed the other battery.
• Tape over or cap the ring terminals of any unused output so they cannot short against something.

The charger's internal protection does not protect the wire between the charger and the battery. Put a fuse in each positive lead as close to the battery as you can. BoatUS notes ABYC calls for this to be within about 7 inches of the battery's positive terminal. Size the fuse per the charger manual; BoatUS used the next standard size above the charger's maximum output in their install.

Connect the fuse last, after all wiring is done.

For a boat without shore power, BoatUS shows adding a weatherproof marine AC inlet on the outside of the console so you can plug in a cord without opening hatches. On a boat that already has an AC panel, the charger goes on its own breaker there. Because this is 120-volt AC, have a qualified marine electrician make these connections, including the green safety ground. Do not just leave the charger's plug dangling in a compartment.

Before plugging in, set each bank's battery type (and voltage, if selectable) per the manual. If your charger lets you set the voltage of each stage, use the numbers your battery maker publishes, not a guess. A BoatUS surveyor warns that a charger set too high slowly boils off electrolyte, even from sealed AGM and gel batteries, which vent gas when overcharged; a battery cooked this way can overheat badly. Then plug in at the boat end first, then the dock or wall outlet.

• Check the charger's indicator lights for each bank.
• Measure each battery with a multimeter: the voltage should climb above resting voltage during bulk charging.
• After a few hours, feel each battery and the cables. Warm can be normal during bulk charge; hot is not.
• When the charger shows full or float, each battery should read near the maintenance voltage your charger lists.

After every trip, plug in. Lead-acid batteries last much longer when they are recharged soon after use instead of sitting partly discharged. If your boat lives on a trailer, a marine-rated cord to a GFCI-protected outlet is the safe way to connect. Unplug at the outlet end first when you leave. Avoid extension cords if you can; if you must use one, Minn Kota's guide calls for a cord in good condition with a matching plug and a wire size big enough for the charger's AC rating (its manual has a chart).

Mercury's off-season advice: fully charge all batteries before storage, and if your boat has an onboard charger and power is available where it is stored, leave it connected to keep the batteries maintained. If there is no power, take the batteries out and keep them on a maintainer somewhere that does not freeze. BoatUS notes a fully charged battery resists freezing far better than a partly discharged one.

Check on the boat every few weeks: indicator lights normal, no heat, no smell, cord in good shape.

• Every few months, check that charger wiring and fuse holders are tight and corrosion-free.
• For flooded batteries, check electrolyte levels and top up with distilled water only, as BoatUS recommends.
• Use the "equalize" mode only on flooded batteries that the battery maker says can be equalized, with loads disconnected. It can ruin AGM, gel and lithium batteries.
• If you replace batteries with a different type, change the charger setting the same day.