Location of Batteries in Small Boats

Electrical and electronic topics for small boats
jimh
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Location of Batteries in Small Boats

Postby jimh » Fri Oct 23, 2020 12:38 pm

For small boats, the battery is the primary electrical energy source on the boat, and its primary purpose is to start the propulsion engine; this battery is thus known as the engine cranking battery. For this reason, the engine cranking battery should be located as close as possible to the engine so that there is no electrical energy lost in the connecting wiring due to resistance in the wire and connections creating voltage drops.

On small boats the non-engine cranking loads or house loads are very small and consist of only a few Amperes to run electronics, navigation lamps, some cockpit or cabin lamps, and a sump pump. Often the engine cranking battery provides power for the house loads.

Since most outboard engines of more than about 25-HP doe not provide for a manual pull-start, and since many modern outboard engines will not start or run at all if not connected to a battery, having more than one battery aboard that can crank the propulsion engine is a prudent back-up and safety measure.

Because the primary load for one or two batteries is the outboard engine, both should be located close to the outboard engine, which means at the stern of the boat. The stern of the boat also provides the least vertical motion on the boat when underway, so the stern is also the ideal location for a battery to prevent damage to the battery from harsh motion and slamming.

The weight of a large 12-Volt lead-acid battery can be over 50-lbs and perhaps as high as 70-lbs. Boats using outboard engines that are expected to be started by a battery are designed with the intention of the engine cranking battery and a second back-up battery being located at the stern, so the effect on the static trim of the boat of the battery has already by included in the design of the boat.

Moving a battery farther from the stern will required abandoning all existing cables between the battery and the engine and replacing them with much larger cables in order to not increase the resistance of the cables. Such cable of proper conductor size are heavy and expensive, and often boaters neglect to properly size these cables. Long cables composed of conductors of insufficient size create increased voltage drop in the transfer of power from the battery to the engine. The effect is the same as the battery voltage being reduced due to insufficient charge. The inability to start the propulsion engine on a small boat is usually a serious problem.

In some smaller boats, say 17-footers, which have been re-powered with modern engines of a weight much greater than the original engine anticipated by the designer of the boat, may benefit from moving some weight out of the stern. Moving a battery can help, but any extension of the battery cables must include a significant increase in the size of the battery cables to compensate for the longer length.

A separate article gives advice on how to properly choose wire conductor size for longer battery cables.