Make Your Own Battery & Inverter Cables Richard Perez Distributing low voltage electricity around home power systems has always been a problem. Every element in every circuit, especially in wiring and connectors, is a potential source of voltage loss. And in 12 or 24 Volt systems we need all the voltage we can get. No where is low resistance wiring and connections more important than within the battery pack and the inverter wiring. These circuits may have to transfer over 500 Amperes of current. Even small amounts of resistance in these connections can lead to unacceptable voltage losses at high rates of current. Here's how you can make your own very low loss, long lived, battery and inverter cables. Resistance- our biggest enemy The amount of resistance that a piece of wire has is determined by three factors: 1) the wire's physical material, 2) the wire's cross sectional area [wire gauge or size], and 3) the wire's length. In low voltage systems, the only material to use for wiring is copper. Aluminum has about twice the resistance as copper for the same wire gauge, and is virtually impossible to solder. These factors dictate the use of copper wire exclusively in low voltage systems. Well, the actual physical length of a piece of wire is determined by the job we need it to do. A wire must reach from point A to point B in order to do its job- conducting electricity for A to B. So this factor is a given, and we have no choice to make here. That leaves the cross sectional area or wire gauge up to us as our only choice. The larger the amount of current we wish to transfer through a wire, the larger its gauge must be. For a through explanation of wire gauge and its relationship to resistance see Home Power #2, pages 33 through 35. In the series/parallel wiring that connects individual batteries into packs we must use large gauge cables to minimize resistance. The same is true of the cables that connect an inverter to the battery pack. The large amounts of current moving through these cables can result in excessive voltage loss if careful attention is not paid to the entire cable's resistance. This voltage loss translates into poor inverter performance, and greatly lowers system efficiency. Cables & their Connectors The situation is further complicated by the connectors used on each end of a piece of cable. These connectors are a part of the entire circuit. A connector that has high resistance will cause the same voltage loss as undersized cables or wiring. A series circuit is like a chain-- it is limited by its weakest link. Each and every element within a cable must have low resistance if the entire cable is to have low resistance. Money spent on large diameter cables is wasted if they are not provided with low loss connectors on each end. The method of attaching the connectors to the cable's ends is very important. Connectors that are merely mechanically crimped to the wire's ends are not acceptable. These mechanical connections oxidize over time. Copper oxide is a very poor conductor of electricity. Mechanical connections may have relatively low resistance when they are first made, but after several months their resistance increases as they oxidize. The decay of mechanical connectors is vastly accelerated when the connectors are attached to the poles of lead-acid batteries. Lead-acid batteries always collect a certain amount of sulphuric acid on their surfaces and on their wiring & connectors. This acid rapidly attacks mechanical connections and quickly results in unacceptable voltage losses. So it is not enough for us to use large diameter, low resistance, cables on our batteries and inverters, we must also have low resistance, durable connectors. Soldering the connector to the cable is really the only way to keep the battery's acid electrolyte from attacking our connectors. A soldered connector is permanently sealed, there is no way the acid can destroy the connection between the cable and the connector. Most commercially available connectors are the crimp on type. Solder type commercial connectors are available, but are very difficult to solder with home tools. Cheer up, though, all is not lost. We canÉ Make Our Own Connectors These connectors are made from copper tubing sleeved over the copper cable. Use only clean, thick walled, soft copper tubing. Don't use hardened copper tubing, or oxidized, dirty tubing. The proper copper tubing is available at most any hardware store, and is sold by the foot. Use stranded copper cable with THHN or THW insulation. Use the appropriate gauge copper cable for your application. Consider "0" gauge copper cable as a minimum size for very short (under 6 feet) cables. For cable lengths over 6 feet use 00, 000, or 0000 copper cable depending on length. To figure out exactly what size cable to use in your application see Home Power #2, page 33. Measure the length of cable you require very carefully. These cables are very stiff and all bends require a large (3 inches or better) radius. What follows now are step by step instructions, complete with photos, for attaching soldered connectors to your cable's ends. 1. Strip 1.75 inches of the insulation from each end of the cable. 2. Take the twist out of the individual wires that make up the cable. The wire strands that make up the cable should be fanned out until they are all parallel and not twisted around each other. This makes flattening the finished connector much easier. A set of pliers aids in this process. 3. Cut 2.5 inch lengths of the copper tubing. Use 5/8 inch diameter tubing for 0 gauge cable, 3/4 inch tubing for 00 & 000 cable, and 1 inch tubing for 0000 copper cable. 4. Lightly coat the stripped, untwisted ends of the cable and the interior of the copper tubing with solder flux. I use No-Corrode flux, but use whatever you wish as long it is noncorrosive and not acid based. 5. Slide the copper tubing over the stripped end of the cable. Leave about 0.25 inches of the cable outside of the copper tubing. 6. Flatten the tubing, with the wire inside, in a vise. 7. Pound the entire assembly on a flat surface with a hammer until the connector is flat and even. Note that the copper wire doesn't extend all the way into the tubing. There is about 3/4 of an inch of tubing that has no wire within it. Pound this area flat. 8. Put the portion of the tubing with no wire inside into the vise and roll it back upon itself. This makes a sealed bottom to the connector so that the solder will not run out the bottom during soldering. 9. Chuck the cable into a vise with the cable up and the connector down. Heat the outside of the copper tubing with a propane torch until the flux begins to boil out of the open end of the tubing. 10. Melt solder into the open tubing end gradually, until the tubing is full. When the tubing is full, the solder will overflow the open end of the tubing. Use a good grade of solder, I like Kester 44, 60% tin and 40% lead. 11. Allow the connector to cool before removing it from the vise. 12. Locate the hole to be drilled in the connector with a punch. Then drill the appropriately sized hole. 13. Trim off the end of the connector that doesn't contain any wire. 14. Polish and brighten the connector using a wire brush chucked into an electric drill. 15. The connector is now complete. The result is a soldered connector that actually has less resistance than the cable itself. The strands of wire that make up the cable extend ALL the way through the connector. Compare this with commercial soldered connectors where the wire stops and then the connector begins. These homemade connectors not only have less resistance than the cable, but are actually mechanically stronger than the cable itself. And they a totally sealed, there is not way for the connection to corrode internally. If the connector becomes corroded on the outside, simply remove it from the battery and polish it with the wire brush again until it's bright and clean. It's a lot of work to make these cables, but once you done it, the job is done FOREVER. So dust of the propane torch and make something that really works and lasts.