CONTINUOUSWAVE

I'm powering a small boat with a trolling motor that is powered two small 12-volt AGM batteries. I have used Attwood automotive-style circuit breakers. I'm using two plain-old 90-degree corner-braces to connect the vertical battery terminals to the circuit breakers that rest on top of the batteries. These corner braces and connecting bolts, washers, and nuts are regular, hardware-store, zinc-plated pieces-- not copper, aluminum or anything usually used for electrical connections.

I know there are items specifically made for this purpose, but due to time constraints I used what I had on hand and it seems to work. The voltage reads the same from the terminals to the load end of the circuit breakers. The trolling motor runs fine and the batteries are holding up well. Rust or corrosion isn't a concern as this boat is for occasional fresh-water use only.

Although this works, however jury-rigged it may be, am I doing something fundamentally wrong by not using copper or aluminum? This almost seems too simple and easy.

Thanks for any insights. I apologize for my ignorance. I'm new to this.

I assume the "corner braces" are made of steel. Steel is an electrical conductor. The conductivity of steel, aluminum, zinc, and copper are compared below, with copper the standard at a metric of 100. A higher rating means more electrically conductive, or less resistance to the flow of electical current:

COPPER = 100
ALUMINUM = 63
ZINC = 27
STEEL = 3 to 10

From: https://www.ihiconnectors.com/IACS-cond ... alloys.htm

As you can see, copper is a much better conductor. We can say copper is 10 to 30-times more conductive than steel. This means 1-inch of steel as a conductor has the same resistance as 10 to 30-inches of copper of the same size. By using, say, 5-inch inches of steel in a "corner brace" as an electrical conductor, the resistance will be the same as if you used copper of the same dimension that was 50 to 150-inches long.

Galvanized steel has a coating of zinc. If the "corner braces" are galvanized steel, the zinc may improve the conductivity.

When you measure a voltage drop, you must measure the voltage drop when the load current is flowing. Voltage drop is proportional to current times resistance. The more current flowing, the more voltage drop. Measuring voltage drop when no load current is flowing is meaningless. There won't be any voltage drop with the tiny current flow created by the Ohmmeter to measure resistance.

As general rule, no electrical component should be held in place mechanically only by the electrical conductors attached to it. In the case of circuit breakers, the circuit breaker should be mechanically fixed in position by some means of fastening to a surface, and the electrical conductors attached to the terminals.

Using automotive electrical circuit breakers on a boat is not a good idea. Many boats have gasoline fuel tanks with vents to the atmosphere, creating the possibility of gasoline vapor. Electrical devices should be rated for marine use. However, if the boat has a completely open deck, accumulation of gasoline vapor to an explosive concentration is unlikely due to diffusion into the air of the vapor by movement of the and wind.

Thanks for the response. The circuit breakers are marine-rated with manual resets, and are the small, two-post style I've used in cars. They're held on to the top of the batteries with industrial-strength Velcro. There is no gasoline on board. Each battery is in a separate battery box and is an independent system. Splitting the weight of one Group 27 battery into two smaller ones allows me to safely lower and lift them in and out of the boat from high docks. I switch the trolling motor from the used battery to the fresh one midway through my trip, usually when I've tied off at a beach, but sometimes while on the water.

I did a voltage drop test, described below.


Batteries: AGM 12 volt

Inverter: 500 watt pure sine wave

Load: 200 watts. Dual prong pedestal lamp, with two 100 watt incandescent bulbs (the trolling motor usually runs in a range of 80 – 200 watts).


The routing:

Positive battery terminal >>> 90-degree corner brace fastened with bolt, washers, and nuts

90-degree corner brace >>> “Batt” terminal of circuit breaker

“Load” terminal of circuit breaker >>> 8 AWG 18” cable, secured to the circuit breaker with a wing nut

8 AWG 18” cable >>> external, waterproof ON/OFF switch

Waterproof ON/OFF switch >>> Inverter 8 gauge 22” cable/alligator clamp

Inverter 8 gauge, 22” cable/alligator clamp >>> Load

The negative battery terminal is set up similarly with a 90-degree corner brace bolted to the terminal, a bolt fixed vertically to the horizontal end of the brace, and a wing nut on that bolt securing the negative cable of the trolling motor. In this test, the inverter’s negative 8 gauge 22” cable/alligator clamp was attached to the wing nut.

Before starting, I tested all contact points and intervals mentioned below for continuity.


These are the voltage drops under load:

- From the pos battery terminal to the bolts, washers, and nuts securing the corner brace: 0.00 volts

- From the pos battery terminal to all exposed parts of the corner brace: 0.00 volts

- From the pos battery terminal to the “load” terminal of the circuit breaker: 0.02 volts

- From the pos battery terminal to the “load” terminal of the ON/OFF switch: 0.06 volts

- From the pos battery terminal to the positive terminal of the load (inverter): 0.14 volts


- From the neg battery terminal to the bolts, washers, and nuts securing the corner brace: 0.00 volts

- From the neg battery terminal to all exposed parts of the corner brace: 0.00 volts

- From the neg battery terminal to the negative terminal of the load (inverter): 0.07 volts



While under load, the reading of the pos battery terminal to the neg battery terminal: 12.05 volts

While under load, the reading of the pos inverter terminal to the neg inverter terminal: 11.79 volts


From what I’ve read, the above readings are acceptable. Please point out anything incorrect or incomplete.


Thanks for any insights.

Wilco71 wrote:While under load, the reading of the pos battery terminal to the neg battery terminal: 12.05 volts...While under load, the reading of the pos inverter terminal to the neg inverter terminal: 11.79 volts

The total voltage drop in the power distribution between the battery and the load is 12.05 - 11.79 = 0.26-Volts. The percentage of voltage drop is then 0.26/12.05 = 2.1-percent, which is below the usual design criterion of not more than three-percent voltage drop in power distribution.

The load current is unknown. Assuming the inverter was delivering 200-Watts to the incandescent lamps, and the inverter efficiency is 90-percent, then the inverter load would be 222-watts. At 11.79-Volts the current flow would be 18.8-Amperes.

The 18.8-Amperes sounds about right. A load of about 200 watts is the high end of what I run the trolling motor. I usually cruise around between 85 and 120-Watts or 7 to 10-Amperes.

My next project is to clean up [the electrical power distribution] by connecting the positive battery terminal to the circuit breaker with a shortened 8-AWG cable, and the negative terminal in the same way to a junction post to which I can attach the negative cable of the trolling motor. That will remove the corner braces from the set up.

It seems as though everything works well. Thanks for the input.

An ideal configuration for using terminal posts has only one ring terminal under a fastener on a terminal post. However, that can be impossible to achieve, so just keeping the number of ring terminals under a fastener on a terminal post to two is a practical goal. I confess that I think there are three ring terminals on one post in my boat power distribution wiring.

Some general guidelines are described in

Boat Electrical Circuits and Wiring Practices
http://continuouswave.com/whaler/reference/boatWiring.html