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Battery disconnect switches

By Don Amundson, Washington State Safety Team Member
CPPC Member

Battery disconnect switches have been around for a long time. They were popular on boats long before their application was applied to automobiles. On boats, especially on the older shaft drive installations, the disconnect switches from the batteries to the starters were in place so that the holds and motor wells could be aired out before power was sent to the starters. This procedure eliminated a lot of boats from having explosions in the engine compartments due to sparks igniting stagnant gas vapors.

The approved disconnect switch, usually a 30, 40, or 50 amp unit was introduced into the automobile for several reasons. Early on they were used so that the battery would not drain down when the car was parked in the garage when not being used. Usually it was used in connection with a battery tender, a charger of 1 and a half amps that would maintain the charge of the stored battery. Usually one of the two, or maybe both cables would be removed from the affected battery. There are several other types of disconnect switches also in use in the auto hobby. One is placed on the positive side battery post and works by unscrewing a knob that interrupts power to the starter solenoid. These are handy and effective. Others have two copper plates that are activated by a handle, that when disconnected is similar to a circuit breaker in a house wiring system.

The use of a remote battery disconnect switch is an easily installed unit that is both a positive safety feature, as well as a theft deterrent. It doesn't really matter where the battery is located, whether under the floor as is the case with many '20's through '50's cars, under the hood, or in another remote location like the trunk, or on the frame under the bed of pickup truck for example. The ideal place for the disconnect switch is somewhere in the passenger compartment, within easy reach of the driver, should the need to disconnect the power arise from an accident, fire, or other emergency! Locations are left up to the individual owner's imagination. Under the seat, in either kick panel, on the firewall, behind the seat, as in a coupe, etc. The owner determines the location.
No matter where the owner decides to mount the switch on his car, it is imperative that the routing of the positive cable is done in a responsible safe manner. Rubber protected Adel clamps on the frame, grommets that go through frame rails or sheet metal, if not grommets then pieces of 5/8ths heater hose going through metal passageways, etc. Connections must be secure, and the two posts where the cables attach on the switch free from any grounding source around them. With the new coverings of the heavier battery cables, if the cable is securely fastened in its travels from the battery, to the switch, to the solenoid or starter, problems will be avoided.

Recently, on Saturday, June 18th, several of my club members and I participated in the Second Annual Horsepower Car Show at the Emerald Downs horse racing facility in Auburn, Washington. It was raining when we arrived, and rained all day. The four of us parked together, several parking slots away from the feature car that was on the dash placque for this event. It was a customized 1938 Ford pickup that had a tilt bed instead of the pickup box that was on it from the factory. The bed was raised up to its maximum height, looking much like a dump truck would. It was painted several colors of lilac, purple and blue. It had ghost flames to enhance the front end. The door handles had been shaved, with remote openers under the bottom of the cab. The battery was located in the very rear of the frame on the passenger side. Being it was a GM 350 V-8 installation, it made sense to run the Positive cable up the frame rail to the disconnect switch, which was on the floor under the passenger seat, then to the starter lug. The gas tank was a custom stainless steel unit. It was shaped like a box, with welded seams and corners. The vent on the top was less than an inch high. The gauge sending unit was also on the top. It was sealed with some kind of mastic or urethane sealer, and sat towards the front edge of the tank. About a half hour after we arrived on site, we happened to look over at his truck, and blue smoke was pouring out from under it. While we didn't see flames, it was definitely on fire. It was like the gas from the very full tank was either seeping out of weld holes, coming out of the vent, or some other source, and sheeting down the sides of the tank. The source of the ignition was caused by the positive battery cable passing by it on its way to the disconnect switch. Well, my friends and I all carry at least, three pound fire extinguishers, and we drenched the fire with them. That seemed to quell the fire to some extent, but it started up again. The owner wasn't nearby at the onset, but after ten minutes or so, he showed up, and immediately went to turn off his battery disconnect switch. He had trouble getting the passenger door open, but did eventually. But shutting the switch off did nothing to stop the fire, because the power was still traveling up the cable because of the rear mounted battery. My friend who was parked the closest to him, suggested he disconnect the battery, so the guy asked for a half inch wrench. There might be a half inch nut on a truck or other bigger vehicle, but the standard for automobiles is 7/16ths inch. So my friend lent him a 7/16th wrench, and he promptly disconnected the battery cable from the battery. The fire stopped immediately.

On further inspection of the battery cable, we found that whoever had installed it, whether it was the owner or someone else, had used a rubber encased fabric cable. It was only secured in three or four spots to the bottom of the frame rail by 1/8th in wire ties. Near the area where it passed by the tank it wasn't secured, and constant bouncing and movement had caused it to chafe through the covering so that bare copper was showing in several places. First, he was lucky that it didn't short out his electrical system when he was driving along, but secondly he was lucky that it didn't burn the truck up, explode, and cause damage to other vehicles or participants that were parked near him! He took the car home on a commercial flat bed towing vehicle.

Normally, it is best to try to run the ground cable of the battery to a bolt as close to the starter as possible. It that isn't a possibility, then when grounding the cable to the frame, all of the other grounding straps or cable should be equivalent in size to the battery grounding cable. In other words, if your battery cables are #3 or #4, the cable from the frame to the body, frame to the engine, and engine to the body should all be the same size. This equalizes the system and eliminates the resistance that is caused by using smaller wire or small flat ground straps.

Our Washington Safety team has found that there is another issue regarding the attaching of the positive lead from the battery to the starter, or from the solenoid to the starter. When attaching this cable to the starter, first secure the nut securing the lug to the starter. THEN, using a tappet wrench ( a tappet wrench was used on the Chevrolet sixes and other engines that had adjustable tappets. It is a thin wrench that is equivalent to the size of the nut that holds the post secure) hold the nut while you add the battery cable, the lock washer, and the top nut. Too often, whoever is doing this installation doesn't hold that inner nut when tightening the outer nut and they break loose the lug or post inside the starter itself. This causes a heck of a lot of resistance and results in hard starting, fluctuating headlights, and other electrically related problems. You can verify this by reaching down and grabbing the battery cable where it attaches to the starter. If it is secured properly, it won't move. But if it moves easily or freely, then you will experience these problems. If the manifolds are hot, use an oven glove, such as the one your wife uses for hot pans in the kitchen and you won't burn the back of your hand or wrist!