Part 1d: MEASURING VOLTS and AMPS

by Cameron Clarke

This is the point in the class we get our DVMs (voltmeters) out and actually measure the voltage drop from connection to connection. Try it yourself! Take the positive lead of your voltmeter (set to measure 15 volts or so) and attach it to the positive battery post with a clip. Then put the negative lead on any connection down the wire path of something drawing current. You can measure the drop in each wire and connection as you continue down the path. (Note: you may have to adjust measurement scale. Always start from a higher voltage setting on your meter. Switch down the scale to avoid damage to your meter). Got a lamp on? Try measuring the voltage at the bulb, then at the battery. What is the difference in value? Where did the voltage go? It was dissipated as heat in connections and wire. Ever notice a wire get warm or hot? Ever feel the heat in the battery wires after starting your diesel? Want to make it start easier? Reduce the resistance in the connections. Want to know which connections need repair? Feel them for heat.

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Figure 3, Voltmeter and Ammeter Usage

Refer to figure 3. Volts are measured across the load. Positive test lead to positive, negative lead to negative. Current in amps is measured in series with the load as in diagram above.

What can be done to decrease power loss? It would be hard to have fewer connections. Many installations will have even more connections than this. What we can do is keep the connection resistance low. I suggest crimp and solder all new connection made for items drawing more than 3 amps. In the foregoing bilge pump example, if we reduce the per-connection resistance to 0.01 ohms, the power loss would be reduced from 26% to about 8%. The pump runs faster, pumping more water per minute and reducing the on time, thus conserving battery energy even more. That means you can charge for less time.


When I talk of ring terminals, I like to pass around some examples of wire I removed from boats. Some have crimp terminals that were squeezed by a pair of pliers, not the proper tool. Some are not squeezed enough and are mechanically loose; meaning you can push or pull and move the wire in the connector. Others are squeezed beyond recognition. You have seen them. They are in every boat. Some were in mine too. I have examples of properly crimped connectors on good, tinned multi-strand wire, and on non-tinned copper. I have crimped and soldered terminals. On each of these, I place a tag indicating the resistance in ohms of each. The values range from a low of 0.01 ohms (did you guess the crimped & soldered terminal?) to well over 8 ohms (a loose crimp on untinned copper). Many are in the 0.5 to 2.5 ohms range, as the wire has oxidized inside, after the crimp was made. The poorest connections result from using untinned wire. Next, come crimp connections made without the proper tool. Can you find any terminals so described in your vessel? Invest $10, buy a good quality crimp tool, and replace those loose or improperly crimped terminals with new ones well made. It will save you energy and future trouble shooting time.

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Often a short piece of wire, 1" to 3" in length, with a ring or fork terminal on each end is used to interconnect other wires. This is a poor attempt to establish all interconnected wires at the same voltage potential. It is a series string of connections. For some reason, boats made in Taiwan use tons of these along with barrier strips in place of buss bars. Ever look inside one of those beautiful new yachts at the boat show and open the electrical panel? You will see, row after row of barrier strips (black phenolic strips with screws to attach wires), and many short pieces of wire to interconnect the wires. It may look pretty to the human eye, but it is a source of many future electrical problems. Look hard, as the bundling of wires can make it hard to spot.
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