Outboard Battery Cables

The most efficient method of connecting the batteries in series is to construct a lead-wire cable that is long enough to route the cable with each battery, from an end post on the chassis of the vehicle, to one or more engine room access holes or through-hulls. It can be routed inside the existing fuel lines and also between cells of adjacent batteries (as close as 3/4 inch apart). The cables should be thick enough so as not to flex excessively when making turns. They should also be able to carry high current loads without melting due to heat buildup. In applications other than powerboats, small gauge wire may do; while in a boat application
it needs to be much larger.


Batteries with posts are connected using high conductivity solder. The battery terminals should be clean and free of oxidized corrosion buildup. Note that the positive (+) and negative (-) terminal designations apply to the battery, as well as when connecting the cable wires to them.

The connection of one end of each battery cable to a chassis rail or other common grounding point is mandatory in order for all batteries to have zero volts potential between them (equal voltage). All lead-wire cables must also connect across from one another at both ends. This is accomplished by soldering each cable’s negative post (at the engine room side) to its opposite end’s positive post (on the vehicle side), with heavy gauge wire, which will not melt under large charging currents. This connection is necessary to form a POSITIVE GROUND in the engine room, with respect to the chassis. All remaining end connections to other items such as lights and instruments are then soldered using wire of an appropriate gauge. If there are multiple cables or many devices attached in parallel (as in a golf cart) remember that all cables must also be connected together at their ends, e.g., positive post from cable 1 to negative post from cable 2, etc., on each device or the chassis rail.

All marine lead-wire battery cables must be sealed at any point where they pass through bulkheads, floors and decks by wrapping them together with waterproof tape prior to passing them through small holes cut for this purpose in these mediums. Waterproof tape is a very effective way to prevent moisture intrusion, as well as protect against sharp edges cutting the insulation and causing wires to spark.

In multi-battery set-ups it is important that all cables carry equal current loads. The most efficient method for balancing currents in medium or large size battery banks using lead-wire cables is to connect each engine room cable (positive post) from one battery, through a fuse link(s), across to one end of the next battery in series. A double-pole link can also be used with two fuses connected in parallel but this requires more space on the vehicle chassis, however this may be desirable if there are corrosion problems with positive posts which will impede soldering. From the next battery’s negative post, a wire is connected to each battery end. The total current loading then divides evenly between both cables of the parallel batteries. A balancer must be used at each engine room access point, which will divide up any excess current that may come into the boat from external sources like solar or wind power panels, not strictly powered by the engine(s) alternators.

The power cables need to be constructed with high conductivity tinned copper cable wire and fuse links should be made from silver where possible due to its superior electrical conductivity (one-thousand times better than copper!) in order for them to carry full starting currents without overheating and melting. Since silver is becoming increasingly expensive, good quality aluminum or copper alloy alternatives may be used instead. Un-plated copper wire of large diameter is sold for this purpose and is very cheap, but its conductivity degrades quickly when in contact with chemicals like salt water. Silver coated conductivity plating lasts ten times longer than copper or aluminum before it wears away.

The alternator voltage reguator/dynamo should have an internal regulator or field plate output (which creates a filtered DC source) to charge the engine starting battery(s). It should also have a reverse polarity protection diode(s), which can handle the full charging current of each installed lead-acid battery without ignition switch/battery isolating relay shutdown during heavy loads above the normal battery charger load output range due to voltage regulator field plate(s) diode failure or excessive voltage, caused during charging by more than one alternator or non-regulated current drawn into the system.

If an ignition protecting relay is used (for safety reasons), its contacts (contacts are rated in amps and must handle full charging currents of each installed battery without overheating and melting down) should be mounted directly to the engine room’s positive battery cable. If a selector switch is used, its contacts may fail from the large currents passing through them while it cycles through several hundred cycle/second pressure pumping strokes per minute as with marine diesels when running at fast idle RPMs. This will cause it to become very hot and literally burn up! Be sure to use heavy duty relays which are designed for marine applications, and isolate the ignition key from high currents, which should otherwise be passed directly to the battery(s). It is also important that they not be too large in diameter as it may cause them to stick or slow down during heavy charging motor loads. If you are using separate alternating engine/alternator-fed charging sources on a deep cycle storage battery bank (which is being charged by solar panels at other times) then an isolating relay should still be used between each alternator when charging and both must have reverse polarity protection diodes connected across their positive input leads from the batteries.

The alternator output voltage regulator/dynamo’s internal field plate circuit (for filtered DC current creating low AC ripple current charging) should be bypassed if using an external regulation or voltage sensing relay (for constant regulated voltage charging) to charge the engine starting battery(s). This is because when the field plate diode fails and allows high charging current into the ignition switch’s contacts, it will cause them to melt down due to overheating. Boat alternators that I have tested in the past which had internal field plates on their regulators were found to put out more than ten times higher AC ripple voltage during heavy motor loads than those without them.

The power cables are run under carpets and upholstery then connected with alligator type clips between battery banks after internally connecting their negative posts together. If they look good when installed, this means none of the insulation has been damaged, which is important in preventing corrosion and unwanted electrical shorts. If they do not look good or if you have any doubts about their insulation integrity or crimps when installed, remove and replace them. Where the cable ends are bare at the connections make sure that there is a sealant between positive (red) and negative (black) posts and cover with heat shrink wrap before installation. One of the greatest causes of corrosion damage on boats is poor quality alligator clips used to connect battery bank cables together. They use only two small metal spring fingers to clamp onto the wire ends, which will fatigue themselves quickly through repeated high current usage as well as contact corrosion due to electrolysis while they are being heated by arcing due to bad insulation or improper crimping/connections. The best alligator clips are solid copper or bronze with heavy duty spring fingers (which can be used with soldering irons) and they will not fatigue due to repeated usage and the use of passivated stainless steel pins for connections, which prevent electrolytic corrosion damage as well as cracking when heat is applied for soldering.

If a large gauge wire is used on the positive (+) charging cable, it may cause problems due to voltage drop if you have two alternators (or even one!) since both must share its total charge current capacity and their combined output together will easily overload large battery cables which are sharing them. This problem can be minimized by using higher amp alternators with built-in voltage regulators each. If using a large gauge positive wire to charge the engine starting battery and it is run under the cabin sole, then it should be separated from any negative charging cable and run through a separate conduit. This way if there is ever a fire in the cabin under the sole or one of the cables becomes electrified due to an insulation breakdown or pinhole, each will burn separately.

Due to corrosion of boat hulls around their holes through which water has entered during years of use, I prefer not to install any large gauge wires directly between two battery banks. Instead I route them separately inside conduits along side each bank’s negative dock connector lead(s) with at least 6 inches separation between them and connected together near where they enter/exit both ends of the boat. This way if one cable becomes electrified, it will not cause damage to the other bank which may be full of gasoline and have a charged battery fully connected. When installing any gauge wire inside a conduit on an aluminum boat hull I make sure to install more than one ground strap between its negative post(s) and the metal conduit each time it goes near these holes or out in the water away from the hull, since this is where most catastrophic shorts originate due to poor connection design (and corrosion weakening over time). The positive (+) heavy duty charge cable should use at least 3/0 AWG copper wires with an internal buffer tube for strength when going up vertical surfaces with sharp bends in them since they must support own their weight plus any possible load it may have from being in contact with large heavy batteries. This cable should never be smaller than 4 gauge wires for its total length, and the negative dock connector lead(s) on each boat bank can use 2/0 AWG copper wires or larger.

Use 10 AWG wire on all battery cables to an engine starting battery which is used in conjunction with a high output alternator having built-in voltage regulator to prevent overcharging and damage. When installing wire into conduit, make sure it does not contact sharp edges inside the conduit since this will cut through them very quickly causing electrical shorts and wiring problems due to loose connections where corrosion has eaten away at insulation conduits’ wall thicknesses in spots making them too thin to hold a soldered connection after time.

Alternators usually have some built-in resistance in their wiring, and this can cause voltage drop during heavy charging if the wire size is too small (a problem which will occur with smaller gauge positive wires on two alternators installed together). This is why it is best for all accessory circuits on dual engine boats to each have separate high output alternators as opposed to using both when doing 50% of the total work load. Single boat banks are okay since they tend not to overlap much when running at any given speed. Running two large deep cycle batteries in parallel may solve this problem since their combined internal resistance will be much less than what either would produce individually making them produce more current due to having fewer electrons to pass through during charging. However, this may not be a good thing in cold weather since their electron impedance will increase from having less electrolytes available when the outside temperature gets lower, causing them to charge more slowly than they otherwise would if warm (as any effect caused by colder temperatures on batteries is directly proportional due to exponential thermal resistance increasing with lower outside temperatures).

For dual engine boats where running two alternator(s) are required for normal operation each engine’s starting battery bank should have its own dedicated high output alternator (and built-in regulator), so both charging circuits will produce no voltage drop due to installed wire resistance between each bank and the tanks they are connected to. If doing 50% of total work load, then you could use two alternators, each of which has its own internal regulator. The positive charge cables should have their gauge size increased to 3/0 AWG copper wires with an internal buffer tube for strength where they go near sharp metal edges or out in the water, just in case a short develops between them and large batteries on the other side.

Alternator wiring from a common cable coming off the main battery negative posts needs to be installed with an insulating section separating them at some point (I like to do this every 10 feet), especially on boats having aluminum hulls since it is very easy for these two circuits to develop voltage drop due to built-in resistance somewhere along their run if not separated as I recommend whenever possible. This can also be a problem with wire gauge sizes below 3/0 AWG copper wires. Even marine grade split loom tubing is good for this purpose and can be used inside the engine compartment instead of conduit (unless watertightness or environmental exposure are important, in which case you need to use something like PVC or Lexan). Otherwise use at least 1/0 AWG copper wire on all accessory circuits running from your battery banks regardless if their current draw is light or heavy since its size will never cause voltage drop issues due to wire resistance even under heavy load conditions unlike smaller gauges.

Power-tie straps have an advantage in that they hold wiring tight against sharp edges on metal conduits preventing them from being worn away more quickly than normal by vibration from the engine. Use extra ones at transitions where the wire can get bent over an edge if not supported, and use these near sharp edges in the engine compartment or on outboard motors as well (they don’t have to be marine quality fancy straps for this purpose). If a strap is insulated, then it should be sleeved with some heat shrink tubing to protect cables that run through it from rubbing up against its exposed metal ends which could ground out against them causing voltage drop. I like to use split loom tubing instead of conduit whenever possible since conduits may end up collecting moisture when running outside the boat’s interior due to humidity effects (though many will tell you that waterproofing glyptal paint applied inside enclosures like conduit does water resistance better than anything else).

They are useful for if/when a wire needs to be moved or you need access to something nearby, but they don’t want to cut the old one that is in place. The whole point of this type of installation is because it can be used on any sized wires such as heavy gauge engine ground cables which doesn’t require soldering due to having a crimped splice. If using stranded wiring then use crimp-on terminals for large gauge wire sizes instead at their ends. If using solid core wire then install the proper sized screws with lock washers inside some waterproof marine grade split loom tubing (like 1/4″ outside diameter) so it will not come loose within the slot when torqued down during the splicing process if that is a concern. After the wires are placed into the proper size slot, crimp them down with a tool like large channel style dies on an electrician’s wire crimping tool. You can hit it several times with your thumb as well to make sure they are tight enough against their terminals so they don’t rattle around or come loose over time due to vibration and chafe from movement within the slot during use. Make sure to add heat shrink tubing over any exposed solder connections prior to installation in order to protect them from chafing and shorting out against sharp engine surfaces or other metal parts nearby (it is not needed over screws since they will never wear through insulation due to vibration). Note: these type of power-tie assemblies are not as strong as a soldered connection since they rely on the crimp terminal itself to hold them, and you should consider adding marine grade heat shrink tubing over any connections that may come loose or be pulled apart easier (especially if using stranded wire) so they don’t accidentally short out against metal nearby. These work well for wires less than 1/0 AWG in size though. If your wire gauge sizes exceed this then look at tapping into the terminals via screw type splices instead of these kinds of power ties which just have slots running off their ends without any ability to tap into them later once installed due to lack of space inside smaller diameter split loom tubing being used (see below).

Power-tie style chafe protection is also a great way to run wiring in coolant lines on outboard motors such as the engine ground strap which often gets worn away more quickly due to vibration. It allows you to add a higher quality marine grade power-tie style connector inside this engine compartment space that easily attaches and detaches for inspection or maintenance of its wiring without having to cut any wires or move them around (which may be difficult due to limited space). If you want these extra wire slots that are added into metal bulkheads, then drill holes with an outside diameter the same size as the drill bit used for making power tie type slots into conduit. If using stranded wiring then crimp on some large channel style terminals onto each end before running it through so you don’t have to solder them in place later. For solid core wiring, you can just use screws instead as long as the wire insulation is large enough (i.e.: 1/0 or larger AWG size wires). Note: if your boat uses a rubber grommet for its engine ground lead then you can install this type of power tie inside that space without any drilling needed to protect it from chafe.