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ContinuousWave: Small Boat Electrical
Battery Connections, Wiring
|Author||Topic: Battery Connections, Wiring|
posted 06-23-2008 11:00 AM ET (US)
For boaters that have larger outboard motors which require a battery to start and run, the boat's battery and the primary battery distribution wiring are crucial components for proper operation. I have mentioned this several times in the past, and will repeat it here: don't rely on wing nuts for fastening terminals to a threaded post on your battery. Outboard engine manufacturers often specifically warn against the use of wing nuts. Wing nuts are difficult to tighten with enough torque to maintain a positive connection. When mechanical connections to the battery become loose, the electrical connection often fails or become intermittent. Arcing contacts in the battery distribution can cause many problems.
Because the starter motor circuit is an inductive load, during engine cranking if the battery becomes momentarily disconnected due to an open connection in the primary distribution wiring, strong voltage transients can be generated by the collapsing magnetic field in the starter motor. Normally these transients are mainly absorbed by the very low shunt impedance of the battery, but since it is not connected, the voltage transients can rise to much higher levels. High-voltage transients can cause damage to other components connected to the electrical system, including the battery charger circuitry, other engine electronics, attached instruments and gauges, and other vessel electronic devices connected to the circuit (such as SONAR and GPS devices).
Most 150-HP and larger outboard motors are typically not intended to be started manually by a pull starter, and many provide no facility at all for pull starting. The electric starter motor is the only reasonably means to start them. The electric motor engine start circuit requires very high current. Even the smallest amount of resistance in the circuit will cause a significant voltage drop, preventing engine starting even with batteries in perfect condition.
Low voltage electrical circuits can have the flow of current interrupted by very small amounts of insulating material. Unlike higher voltage circuits which can punch through a tiny insulating surface, low voltage current will be stopped by a layer of insulation that is all but invisible. To prevent problems with insulating layers, be certain that all terminals and connectors are clean and free from oxidation or other insulating material. Terminal connectors should be burnished with a medium non-metallic abrasive like 3M-Scotch Brite abrasive hand pads.
Low voltage electrical connections are often enhanced when a star-washer is used. The sharp points of the star washer are forced into the material of the adjoining surfaces, promoting piercing of any surface layer of oxidation. Star washers are often recommended when bonding to an aluminum chassis or surface. Star-washers made from copper are often included in OEM connection kits. Don't lose these copper star washers as they are difficult to source from retail suppliers. The star washer should be placed onto the terminal post first, then the terminal connector added above it. The star washers also act as a lock washer and help prevent fasteners from loosening.
Use elastic stop nuts (also called nylon lock nuts) to fasten terminal connectors to battery threaded binding posts. When tightening these hex nuts, be careful that your wrench does not come in contact with other conductors. A wrap of Scotch 33+ Vinyl Electrical Tape will provide good insulation. The tape can be easily applied to and removed from the wrench as necessary.
To protect a good connection and prevent corrosion, apply a protective spray such as CRC Battery Terminal Protector to clean terminals:
As a general practice in electrical installation, attaching multiple connectors to a single terminal post should be avoided. Stacking three or more connectors on a single terminal post is a poor practice. The negative terminal of one battery often serves as a common point for the negative bus, and this is accomplished by placing several terminals over the battery post. This violates the recommendation above.
The best practice is to have a battery negative bus bar and to connect all the battery negative terminals to it, each with its own screw fastener. And other leads which need to connect to the negative bus can also be bonded there. You can buy such devices at modest cost.
Because the battery negative connection affects all circuits, it is especially important to maintain this connection.
In a boat with an open transom and the batteries located in the transom splash well area, it may not be practical to install a battery negative bus bar. In that case, try to minimize the number of connections at the battery negative terminal. If there are dual batteries, the negative terminals must be bonded together. Share connections between the two terminals so as to keep the total at any one terminal to a minimum. Move lower current connections to a terminal bus and bring a single connection from the bus to the negative battery terminal.
Battery wiring should be clearly marked with polarity. A wrap of vinyl electrical tape of the appropriate color is a good technique for marking positive (red) and negative (yellow) conductors and terminals. Conductors whose function is not immediately obvious should be clearly labeled.
posted 06-23-2008 11:40 AM ET (US)
Is there a torque specification for battery terminals? Too much can damage the battery, too little will impact the electrical connection. If these extremes are necessary for a modern motor, tightening the connection should not be left to arbitrary feel.
posted 06-23-2008 12:10 PM ET (US)
My understanding is that the torque specification for a threaded battery terminal post fastener is specific to the battery. The battery manufacturer would be the appropriate source of a value for torque.
I don't have the torque specification for my battery, nor could I find it in a cursory search of the manufacturer's website. I tighten my elastic stop nut battery terminal fasteners with a small wrench until they are very snug. I would judge the torque to be less than 10-ft-lbs.
The elastic stop nut and the star washer help to prevent the fastener from becoming loose, more so than just being tightened to the limit.
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