Small Boat Electrical

The focus of this month’s article is electrical systems on boats under 10 metres.  Often referred to as “small” boats in this market, they pack a lot of electrical punch.  Along with advanced electronic navigation equipment and fishing gear, many boats in this size range also include all of the common conveniences of home including a refrigerator, microwave, heater, stove, stereo, and lights throughout.  The main challenge that these boats face is finding space to mount all the hardware (batteries, inverter, solar panels, fuel cell) to make the boat as comfortable as possible

Starting Batteries vs. House Batteries. In this size range, batteries serve two purposes whether they are flooded lead acid or AGM lead acid.  If choosing flooded lead acid batteries, a boater should choose the right battery for the right application. Starting batteries, optimized for high currents for short periods of time, and deep cycle or house batteries, for lower currents for long periods of time. Some boaters are tempted to get a dual-purpose flooded battery as a way of getting both a starter and deep-cycle profile in one battery. Unfortunately, a dual-purpose battery is neither great at starting or deep cycle applications, if possible it is better to get a dedicated battery for each purpose. In our experience these dual purpose batteries don't really do either well.  We always recommend if your boat is away from shore power and you are have electrical loads while the engine is off then you should have two batteries.  The batteries should be installed so that they can be charged simultaneously but discharged independently.  With this setup, you can use either battery to start the engine using a simple OFF-1-BOTH-2 switch.  There is nothing more frustrating than spending the day on the water, listening to music, relaxing and then pulling up the anchor only to realize that the boat won't start.  By moving the switch to "2" (for your house battery), you do not run the risk of discharging your "1" or starter battery.

Working With Limited Space. You can’t create more space on your boat so you have to figure out ways to use the space you have more effectively.  Since AGMs provide more usable battery capacity for the same battery size, the easiest and fastest way is to swap out your flooded lead acid batteries for AGMs.  Absorbed Glass Mat technology was introduced in the mid-80’s for military aircraft because the batteries were spill-proof, maintenance-free and offered a greater depth of discharge (70-80% for AGMs versus 50% for flooded).  AGMs come in the same sizes as flooded, for instance group 24, 27, 31, even the popular 4D, 8D, and golf cart size. More usable battery capacity with AGMs can translate to increased battery capacity if you keep the same number of batteries or alternatively you can reduce the battery bank size and still have the same amount of usable capacity. For instance, 4 golf cart AGMs will provide about the same amount of usable amp-hours as 6 flooded golf cart flooded batteries. It is up to you to decide how you should benefit from AGM: increase usable capacity and keep the same amount of batteries or keep the usable capacity the same and reduce your batteries by one-third. The only downside is that AGMs are almost twice as expensive. It is worth noting that AGM lead acid batteries can be installed as a starting or deep cycle battery and work well at both.

The most recent entry into the marine battery market is the Firefly Oasis, a carbon foam AGM.  These batteries are a great option for anyone who wants the most usable battery capacity of a lead acid battery. The firefly can easily handle delivering 80% of usable battery capacity (depth of discharge to 20%) and still offer 3 times the battery life of a standard AGM battery. An equally interesting benefit is that the firefly oasis battery can stay in a partial state of discharge i.e. never brought back to 100% indefinitely and still never lose any capacity. Once charged back, the battery will still provide 100% of the stated battery capacity. It is an especially useful battery for anyone who spends long amounts of time away from shorepower. Firefly Oasis batteries have carbon foam imbedded into the lead grid structure found in conventional batteries.  This foam allows the battery to operate or be stored at a partial state of charge for long periods without the risk of sulfation. In other words this battery doesn’t suffer from sulphization the way other batteries do

Once you have increased your battery capacity, it is important to have an efficient means of charging your batteries.  The first thing to consider is your alternator, this is the device that converts your engine’s mechanical energy into DC electrical energy.  The higher your engine’s RPMs, the more energy that is going into your batteries.  If you are idling at anchor, it will take much longer to charge your batteries than if you are motoring to your next destination. If it takes too long to recharge your batteries while underway it is worth considering upgrading your stock alternator to a larger size and also think of swapping your built-in regulator for an external smart regulator. Either one of these upgrades can easily increase your alternator output by 50% to 100%.

The Smart Charger. To make the most of the energy going into your batteries, we recommend a 3-phase smart charger.  Lead acid batteries (including AGMs) follow the bulk, absorption and float charge curve.  In order to maximize the efficiency of your charging while off the dock, it is advisable to stay within the bulk charge.  Which means, you don’t need to charge past 80 to 85 percent unless you have AC shorepower or are underway.  These chargers are safe, easy to use and will not overcharge your batteries.

Once you have configured your battery capacity, maximized your alternator output and charging system, we highly recommend installing a battery monitor.  It is a fuel gauge for your batteries and will offer you more control of your system and extend the life of the batteries.  There is a new Bluesea’s M2 DC SoC battery monitor that allows you to see your voltage, amperage and battery state of charge on one screen.  This is a great improvement from older monitors that you would have to toggle through multiple screens to get each piece of information.

The configurations for boats under 30 feet are endless, and should be based on the way you use your boat.  If you want to leave all the electronic distractions behind and just go sailing, then a basic setup is the way to go.  If you want to anchor for two or three days at a time or dock at an outstation with no power, then adding solar or a fuel cell may be the perfect solution.  The best starting point is to determine how much power you will use on a daily basis and then design a system from there.

Here are some typical examples that we see on a regular basis:



Energy Configuration

Energy System Approximate Equipment Cost

6 metre Inflatable Dinghy

VHF Radio, Chartploter and Sounder

Battery – Group 27:  $125

Charging – Stock Alternator: $0

$ 125

8.5 metre Sailboat


Battery – Group 24 starter: $125

Battery -  Group 24 deep-cycle: $150

Charging – Stock alternator: $0

Charging – 100 Watt Solar Panel: $1000

Charging – 20 amp charger: $300

Monitor –  Victron 702: $275


8.5 metre Trawler

Fridge, Laptop, Cell Phone, Nav Lights, Nav Gear, interior LED lights, furnace


$ 1950

8.5 metre Fishing Machine

Fridge, Laptop, Cell Phone, Nav Lights, Nav Gear, 2 down-riggers, bait tank pump

Battery – Group 31 AGM: $325

Battery – Group 31 AGM: $325

Battery -  2 Golf-Cart AGM: $600

Charging – Stock alternator: $0

Charging – 40 amp charger: $625

Charging – EFOY 80: $3500

Monitor – Victron 702: $275

$ 5,650

If you have a small runabout, consider purchasing a pocket size battery jump starter like the Weego.  It comes with jumper cables, LED flashlight, strobe light, SOS functions and a wall/car charger.

When using a windlass to bring up your anchor, increase the RPMs on your engine to offset the draw.

About the author: Jeff Cote is the owner of Pacific Yacht Systems, a full service shop delivering marine electrical and navigation solutions for recreational boats. Visit their website and blog for info and articles on marine electrical systems, projects and more: