Mobile Radio Wiring and Grounding
There is a serious flaw with the suggestion mobile radios be wired across battery terminals. The negative radio lead should actually not have a fuse, and not be wired to the battery post.
History of Mobile Radios
Early vehicles had both positive and negative grounds. USA passenger vehicles evolved, standardizing on "12 volt" negative ground systems. The resting voltage was around 12.6 volts, while running voltage is in the high 13 to low 14 volt range. Many commercial vehicles, however, retained positive grounds. Non-standardization of systems meant two-way radios were designed to operate with either negative or positive grounds. This was accomplished by floating the internal negative supply buss, while grounding all normally accessible external user ports to the case or cabinet.
In most early CB and commercial two-way radios, the negative buss inside the unit fully floated from the cabinet and any external ports. This included commercial two-way radios like the Motorola Motrac, Micor, and other expensive, high-quality, radios. The floating negative was universal across brands including, but not limited to, GE and RCA land mobile equipment.
The floating negative allowed use of radios in either negative or positive ground vehicles. It also solved ground loop issues, allowing direct connection to battery posts without fire or equipment damage hazards.
Equipment manufacturers had no way of knowing if the final installation would be in a negative ground vehicle or a positive ground vehicle. As a result, systems with a floating negative buss were shipped with both negative and positive power line fuses. The floating negative buss system inside the radio allowed safe direct connection to the battery posts, and safe shutdown if either the positive or negative fuse opened.
The floating negative power buss made it impossible for starter or charging currents to flow through antenna cables, microphone, or speaker leads. All exiting connections, as well as the case, were electrically isolated from the negative lead.
Negative Ground Only Radios
Over time, vehicles with positive grounds disappeared. As this happened, radio manufacturers stopped using the more-expensive and more-complicated system of floating all negative power returns.
Most vehicle manufacturers, and virtually radio manufacturers in the amateur radio market, never rethought the system. They carried over the good negative fuse idea used with ground independent power buss systems, misapplying it to radios with internally grounded negative buss systems. Not realizing the safety hazard, they continued to fuse both negative and positive radio power leads. They also, despite ground loop, fire, and equipment safety hazards, advise connection of the negative lead directly to the battery negative post. The bad idea has permeated the amateur and hobby market, and even the audio and racing car markets. For example, MSD Ignition's installation instructions advise direct battery post negative connections, putting the vehicle's distributor and other electronics at risk.
Although absolutely incorrect, a popular assumption is fusing the negative lead protects internal and external equipment wiring, including speaker, microphone, key leads, accessories, and antenna cables from an open battery ground connections.
Normal vehicle wiring is shown and discussed in detail on this page link.
Follow the current in this system....
If the ground or negative wire from the radio to the battery opens, radio current would flow from the radio out through the antenna cable, the speaker jack, the key jack, or any other jack or connector that connects to the radio circuit board grounds to the vehicle chassis.
If the battery to engine block ground opens, or the engine block and battery to chassis grounds open, the battery will ground back through the negative radio connection to the radio, through circuit boards or other connections, and to the vehicle chassis.
While a radio negative lead fuse will protect higher current circuits, it will not protect small traces or components like those connected to insulated jacks or connectors.
For example, when my radio system lost a negative fuse connection, foil traces for the keyer paddle grounds inside my ICOM 751A burned open. I could no longer send CW.
The traces opened even though the negative lead was fused, because the radio's transmitter current of ~20 amperes flowed through small circuit board traces to my CW key. The CW key was grounded by touching a metal bracket, and this melted foil traces on a circuit board inside the radio. With a grounded negative buss and battery post connection:
Connecting the radio or accessory equipment's grounded negative supply does nothing good. Grounding to the battery negative post generally increases alternator whine. It also places the vehicle's computer and electrical system at higher risk, is unsafe for the radio or accessory, and is unsafe for things connected to the radio.
I've had vehicles with higher power radios that have opened a negative fuse, and then melted the coaxial cable shield on RG58/U cables. This is because, once the fuse connection was lost, the equipment grounded itself to the vehicle chassis through the grounded antenna cables.
Correcting the Ground
It is pretty simple to improve the grounding system.
If we ground the radio negative lead to a separate but good chassis ground near the battery ground, ground loops through the vehicle's computer system, through the radio, and through anything connected to the radio or accessory, are completely avoided.
Ideally, this ground should not share the bolt that grounds the battery to chassis ground cable. The negative power ground should be on its own bolt. The fasteners should be good hardware, with proper star washers. The ground must provide solid, reliable, mechanical and electrical connections.
If the radio or accessory device's chassis power ground lead opens, only radio or accessory current will flow through the radio circuit boards. While this may still open a trace, it is much less likely to happen because we have eliminated a needless weak point, the negative fuse, and we have eliminated the direct connection to the battery where battery acid slowly eats away at connections.
A vehicle chassis power ground completely eliminates the current path through the radio or accessory if the engine block ground battery lead opens, and alternator AC ripple no longer can drive the negative lead to the radio. Any connection can fail, and in the very worse case possible we have only radio operating current. In most cases, we will not even have that.
It also helps to have multiple ground points as additional insurance against the negative power lead accidentally floating.
Who Else Agrees with Me?
Published standards specifically tell us to not ground to the battery post, and not to fuse the negative lead!
Motorola manuals for radios without an internal floating negative buss tell us the following:
Motorola warns to not use a negative fuse or battery negative post connection, but rather directly connect the negative lead to the vehicle chassis.
Only the positive lead is fused.
Only the positive lead goes to the battery post.