Article by Capt. Steve Katz
Call for back-up
When you start your boats engines, do the electronics turn off? Do you need to start one of the engines in order for the second to crank over? Have you run down your engine starting battery after listening to the stereo on the sand bar? If you answered yes to any of these questions they you could benefit from a properly configured dual battery set-up.
There are many options and configurations to accomplish the same goal of having a built-in back-up battery on your boat.
The most common example is a single engine outboard; often these boats will have one battery. This single battery is responsible for starting the engine and running the accessories, such as bilge pumps, lights, electronics, stereo etc. Most of the time this system works well, especially if the captain is aware of the limitations of the system, such as not leaving the stereo on for extended periods of time which can drain the battery and cause the engine not to start (until the battery is recharged). These single battery systems are easy and simple – many boaters with this system may carry a portable jumper pack, so they can jump start their battery if it should go dead while out on the water.
The best way to add a back-up battery to a single engine, single battery boat is with a pre-designed kit, often called an add-a battery kit. This is much more than a battery switch. Years ago a boater would just add a second battery and new battery switch. This new switch would have four positions, one, two, all and off. Seems simple enough, but where should you turn the switch? Position one for ½ a day and two for the other ½ day? both when running? leave it in position one until there is a problem then switch to two? All of the above scenarios may work, but each requires knowledge of the batteries and charge level and requires much user intervention.
Modern methods incorporate a much simpler solution – a special battery switch and separate automatic charge relay (ACR). The special battery switch has two normal positions, on and off – how easy! The switch also has a clearly marked third position to manually combine batteries in an emergency. This switch turns on and off two individual circuits at the same time, usually one engine battery and one house battery. This allows you to have two batteries, one dedicated to the engine alone and the second “house” battery to run the stereo, electronics pumps, lights, etc. While this may seem simple – it is, the charging and discharging is managed by the automatic charge relay or ACR. When properly wired, the second battery will allow the electronics and house loads to remain on when the engine is cranked, since they are now supplied power from independent batteries. The ACR is a relay that AUTOMATICALLY connects the two batteries together (in parallel) ONLY when the engine is running and charging. This system allows the second house battery to be re-charged and be kept full by the engines alternator. This would be similar to the “all” position on a traditional battery switch. Once the engine is stopped, the ACR automatically separates the engine and house batteries, allowing you to run your house/accessories systems from the second battery while preserving the engine starting battery – a great idea for the sandbar or boats that fish with the engine off. This same methodology can be used on any size boat, outboard, inboard etc, though on large boats this type of system is more complex but the theory is the same. Most of these kits include the proper components along with a wiring diagram showing you how to set this up properly and move the house loads to the additional battery.
If you have a twin engine boat, often the boat is equipped with two batteries, one for each engine and two battery switches. These battery switches often have four positions, one, two, all, off. Many mid size boats will not have a house battery; instead they use one of the two batteries as a combination house/engine battery. While this system can provide back-up redundancy in an emergency, proper wiring and labeling is important for it to work properly. The batteries should be labeled on the battery themselves, one and two. Then the battery cables need to be labeled and connected to both battery switches in the same fashion, preferably position one on both switches should go to battery #1, position two wired to battery #2. This way there is no confusion on which battery you are enabling. Wiring this way will then allow you to turn one switch to battery one and the second switch to battery two. This now isolates the batteries and engines, so if one of the batteries were to become drained, you would have a fully charged battery available to start both of your engines.
If you were to put one or both of the battery switches in the “all” position and the house equipment drained a battery, it would drain both batteries at the same time, leaving you “dead in the water”.
While some boaters may want to leave both switches on the same battery, preserving the second battery, you may not know the state of charge of the second battery and when its needed most you find out that it isn’t fully charged. By operating off each battery individually, you can learn if both batteries are functioning properly before there is an emergency.
On some twin engine/dual battery boats the battery switching is an on/off switch for each battery/engine and a separate parallel switch, this is also a good setup and allows you to easily keep the two batteries isolated like in the above scenario, only selecting the parallel mode in an emergency.
Learning about your boats batteries, how they are wired and what the battery switches control can make your boating experience more reliable and therefore more pleasurable.
Captain Steve Katz is the owner of Steve’s Marine Service and holds NMEA, AMEI and NMEA2000 certificates along with ABYC Master Technician certification and factory training from many manufacturers. To reach Steve, call (631) 264-1600.
