posted 07-20-2002 12:16 AM ET (US)         

"A battery does not put out a magentic field, only an electrical one. You would
have to worry if you were around an alternator, but not a batter, especially a
DC batterty. A home computer usually has its power supply right next to the
disk drive and that is 120V AC for comparison."

posted 07-20-2002 10:03 AM ET (US)            

Current flowing in a wire creates an eletro-magnetic field, but the field strength is proportional to:
--number of turns of wire
--amount of current
--distance from wire.

Thus a modest current of an ampere or two flowing in a single wire at a typical distance of six inches is not likely to produce much of a magnetic field, at least not enough to affect a laptop computer.

The field from a wire might affect a nearby compass. To minimize this effect it is suggested that wires carrying current be run in twisted pairs, one conductor carrying current to the load and the other carrying it away from the load. This will tend to cause the fields created by the wires to cancel each other, and the resultant field from the wires will be minimized.

This same technique--twisted pair circuits--was employed in the wiring of the telephone system and allowed thousands of twisted pair circuits to be carried in a single cable with very little crosstalk among them.

I was quite surprised when I read this thread that the question was to batteries and computers. I think there are much more important issues about battery location on a boat and possible dangers.

In terms of location, the stern has traditionally been the place for a battery, but increased weight of outboards has led to a trend to move the battery more forward in the hull. This gets some weight out of the stern, but it puts the battery farther from its primary load, the starting motor.

The eletrical implication of this is the need for longer and larger cables to connect the engine to the battery.

Distributing more weight forward in a boat is acceptable, but only if the weight is kept close to the center of gravity of the boat and the center of pitching resistance.

Imagine two identical boats. One has all of its heavy components, the engine, the battery, etc., tightly grouped and at the point of the boat's center of gravity and close to the boat's center of pitching resistance).

The other boat has the same weight, but the heavy items are located either at the extreme bow or extreme stern. This boat will have a much different motion in a seaway than the first boat.

When a planing boat is on plane there is only a small portion of the hull in the water, perhaps only the rear 25% of the boat. Any weight that is located forward of this area is going to have a considerable moment and will tend to create more pitching motion.

The battery can be a producer of gases. During charging a battery can emit hydrogen gas. If the case is damaged and cracks, the battery can release acid. These dangers are generally mitigated by locating the battery in a well ventilated area and enclosing it in a secondary container ("battery box").

posted 07-20-2002 07:17 PM ET (US)         

I realize that in heavy seas, things will bounce around less in the stern of the boat, and we definitely do not want the battery bouncing around, but it is strapped down. So I assumed that it would be okay under the console. And there is wood under it (embedded in the fiberglass). I increased the gauge of the wire (from 4 gauge to 2) to compensate for the increased length of the run.

posted 07-20-2002 11:12 PM ET (US)         

Since I have only owned the boat for a month, I'm going to take out the battery and put it in my spare car (which needs a battery) and install a brand new marine battery on the boat. Since Jim mentioned the danger of spilled acid, I may replace the old battery box as well.

posted 07-20-2002 11:56 PM ET (US)            

Someone also mentioned the 'tinning' of the cable - but the term tinning only applies to the connections at each end - and not the entire cable.

And a bit further on the subject of battery cables - when you make your connections to the end of the cables, smear some vasoline or grease around the cable at the end of the insulation - prevents corrosion problems in the cable. ------ Jerry/Idaho

posted 07-21-2002 09:13 AM ET (US)            

Marine wiring requires some special considerations. Because of the vibration and motion likely to be experienced, the wire should be flexible. This comes from having many strands of small diameter wire. You can find large gauge stranded wire which has only a few strands; marine wire should have hundred and hundreds of tiny strands cabled into large gauge conductors.

Marine wire is also "tinned" copper. This protects the bare copper wire from corrosion. It also makes soldering easier, but it is often recommended NOT to solder large gauge wires in marine applications. This probably stems from the idea that soldering stiffens the wire which may lead to fatigue failure from vibration. If properly done, I personally think soldering improves connections, but you have to be careful not to let the solder wick up into the wire and cause it to stiffen.

The insulation used on marine wire is important, too. Sunlight (UV radiation) causes many wire insulation platics to alter their characteristics, so marine wire is made with insulation that is resistant to UV degradation. Of course the insulation should also resist water and oil, too.

All of these factors make marine wire more expensive than common insulated wire.

Obtaining good marine grade wire in small lengths and in different color insulating jackets is difficult other than at the boat store, so sometimes you just have to pay the high-dollar price.

When buying wire remember this collarary to Murphy's Law which is used in the electrical industry: