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. While USA passenger vehicles quickly converted and standardized on 12 volt negative ground systems, many commercial vehicles retained positive grounds. This meant two-way radios had to be built to operated with negative or positive grounds.
In early CB and commercial two-way radios, the negative buss inside fully floated. This included commercial two-way radios like Motrac, Micor, and other expensive high-quality radios.
The floating negative allowed use of radios in either negative or positive ground vehicles.
Radios came with both negative and ground lead fuses; the radio manufacturer had no idea if the final installation would be used with negative or positive ground. The floating negative buss system inside the radio also allowed safe direct connection to the battery posts.
It was 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.
The vehicle could use either negative or positive ground systems.
Negative Ground Only Radios
Over time, vehicles with positive grounds disappeared. As this happened, radio manufacturers stopped using the more expensive, more complicated system of floating all negative power returns.
Some vehicle and radio manufacturers, particularly in the amateur radio market, never wised up. They never re-thought the system, and carried over the good idea in ground independent radio busses to newer radios with grounded negative busses. Not realizing the safety hazard, they continued to fuse both negative and positive radio power leads. They also, despite fire and equipment safety hazards, tell users to connect the negative lead to the battery negative post. This has become common in the amateur radio market.
Quite incorrectly, many assume fusing the negative lead protects internal and external radio wiring, including speaker, microphone, key leads, and antenna cables from an open battery ground connection.
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.
- Anytime the battery negative to chassis ground opens, vehicle accessory and lighting currents, or a portion of vehicle accessory and lighting load current, will flow through the radio
- Anytime the negative fuse opens, the radio will ground itself through the coax or other other external connections
- This also sets the radio up for increased alternator whine, since load currents can superimpose themselves on traces and connections inside the radio.
Grounding to the battery negative post can increase alternator whine. It also places the vehicle's computer and electrical system at higher risk, and is unsafe for 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 as the equipment grounded itself to the vehicle chassis through 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, we totally avoid ground loops through the vehicle's computer system, through the radio, and through anything connected to the radio.
This ground cannot share the bolt that grounds the battery to chassis ground cable. It has to be on its own bolt, preferably good stainless hardware with proper star washers and solid mechanical and electrical connections.
If the radio to vehicle chassis ground lead opens, only radio 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.
We have also eliminated any current path through the radio if the engine block ground 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.
Who Else Agrees with Me?
Mobile radio installation standards agree:
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.