The Ultimate 12-Volt Power Guide

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Portable Chargers and Jump-Starters
Smaller boats may not have a convenient spot for a permanent charger, and sometimes you just need a backup. Here are examples of charging products we’ve found handy on our boats.

Minn Kota MK 110P
Works By: Monitoring battery status and delivering charge in a dual-stage profile that brings dead batteries back quickly and then maintains them over long periods. One- and two-battery models are available.

Charge Power/Profile: Flooded cell at 10 amps only

Why We Like It: Handy and quick to set up; LEDs indicate charge status or report bad battery if battery has failed or has no detectable current.

Best Choice When: Batteries are carefully maintained and never lose all current. Also handy for mower and off-road toys.

Stanley 500-Amp Jump-Starter
Works By: Combining a small but potent cranking battery in a unit with its own charger. Stanley has added a 120 psi air pump, 12-volt and USB outlets and a trouble light in a compact unit that fits in the trunk and many will stow in a compartment in a boat.

Charge Power/Profile: This device can be charged by plugging it into a wall socket, but its duty is to jump-start a dead vehicle or boat to allow the alternator to take over.

Why We Like It: It provides emergency starting power in a pinch and even roadside assistance for a flat or for other work while en route.

Best Choice When: Anybody needs emergency power for a boat or related electronics.

Xtreme Charge Marine
Works By: Electronically monitoring each battery to determine type (flooded- or gel-cell or AGM) and power needs. It additionally determines if battery is recoverable and then uses electronics to monitor and meet charging needs while sending periodic pulses to remove sulfates from battery plates.

Charge Power/Profile: Automatically determines battery type and provides 10 amps of current as needed, dropping power as battery approaches full charge.

Why We Like It: While AGM and gel-cell batteries are less susceptible to sulfation, batteries can recover from sulfation damage. LED readouts indicate viability of battery and charging status. We’ve recovered batteries thought to be extinct with this one.

Best Choice When: It can be permanently mounted in a dry, onboard area or used as a portable. Charger will not charge a completely dead battery, so regular maintenance is required.

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Effects of Sulfating
During discharge, lead can dissolve and form into hard crystals on the battery plates, impeding electrical flow and performance. Fully recharging returns most sulfates back to solution. But here’s the rub: Batteries, especially flooded cells, are often not fully recharged since, as a battery reaches full charge, its resistance increases, decreasing the “charge acceptance rate.” The battery refuses to accept more juice. Current from an almost-charged battery is often not enough to prevent the sulfates from crystallizing, especially deep inside the battery on the inner plates — except in gel and AGM batteries, which are made to combat that effect. But there is a solution, with high-tech chargers offering de-sulfating features as described above.

Tip: Over time batteries lose a percentage of power naturally — even when disconnected. Also, wiring issues, like ground faults, can drain batteries. The two forces can leave your battery dead after just a week or two. Solar chargers that fasten to a hardtop or mooring cover can compensate for that power loss and save the need for a jump-start battery to get your day going.

Do the Math
How much battery capacity do you need? Electrical appliances are rated in either watts or amps, as found in their manuals or on labels affixed to the devices. Add up the total amps required and you can easily size battery needs.

1. Convert the power requirement of all electrical appliances and devices into amps using amps = watts/volts and watts = amps x volts. For instance, a 30-watt microwave uses 2.5 amps, since 30/12 = 2.5.

2. Add up all the devices, estimating the usage time per day for each, and you will have the total “ampacity” — in amp-hours — required of your house battery or batteries (battery bank).

3. It’s then a simple matter to purchase a battery or batteries to suit, since deep-cycle batteries are rated in amp-hours.

4. Add a cushion of 30 percent to the calculation to account for imperfect charging and reduced battery performance with age.

Note: Once you have the daily amp-hour requirement, you should also use it to size the charging system. That is, if you require an estimated 100 Ah use per day, you need to be able to replace that daily need as well. As a practical matter, for us powerboaters, unless we are spending days on the hook, never getting under way, our amp-hour need is less because the engine will be running while many of the appliances are in use.