Easy question (or at least I think). I've installed a siren in my vehicle which calls to pull power direct from the 12 volt battery. I currently have the circuit completed by connecting to the negative terminal on the battery. I've been told that sometimes it is best to connect to a grounding point on the vehicle like the frame.
Is this necessary for my situation or ever?
Thanks in advanced!
The negative of the battery is connected to the chassis so run your postive to the battery (make sure it is fused) and connect the negative of your siren to the chassis. Look for any bolt nearby, use a ring terminal and fix it down. However do make sure that the chassis surface is bare metal (sand it down), any significant amount of paint will result in a bad connection and cause problems.
You should avoid connecting cables directly to your battery. There's no point at all. The negative terminal of the battery is connected to your car body, so you can connect your siren's negative terminal directly to your closest ground point (car metal body). There's no reason to have multiple cables on your battery's terminal. Cable harness will be more organised.
Let's look at why grounding to the battery post is rarely a good idea!
Battery Grounding or Negative Battery Supply Connections.
Battery path current can be hundreds of amperes during starting, and battery path current is easily 25 amperes or more when charging the battery. Additionally, the alternator supplies all running current for all accessories, with the battery supplying current when an alternator cannot "keep up" with load. With high currents like that, the battery post should be exclusively dedicated to the battery-to-block ground lead, and the battery negative always must have a good solid connection to the vehicle chassis.
Sharing the negative battery lead to engine bolt with anything else or connecting directly to the battery negative post with anything except the block and chassis grounds is a terrible idea. (Connecting electrical devices or hardware directly to a battery negative post is a bad idea (no matter who tells you to do it) unless the negative connection is 100% ground isolated at the electrical device.) When an electrical device is directly connected to the negative post, if the negative post to block or chassis connection opens up or develops excessive resistance, the battery negative post will divert alternator or starter current through whatever is attached to the negative post. This can be hundreds of amperes! Very few devices and wiring will suffer a fault like this without permanent damage. It is also a fire risk.
Grounding directly to the negative post is a fire hazard at worse, and an unnecessary risk to your equipment at best. Battery post connections also increase likelihood of ground loops and ground conducted noise.
On a personal note, I'm not sure why USA and Japanese manufacturers instruct people to connect things to the negative terminal. I suspect it is because they have not thought through the safety problems negative post connections create, and they somehow think a battery post provides a "cleaner" voltage or a more reliable ground because of battery impedance. Accessory or ancillary equipment battery post negative connections are banned in many countries. As a general rule, vehicle manufacturers never make a negative post connection other than block or chassis. Professional or commercial grade accessory manufacturers also do not use negative post connections. The sole exception is when a device has 100% assurance the negative bus can never contact chassis ground in any manner through any path.
The only proper and safe way to connect accessories of any type (this includes ignition and stereo systems) to the negative post is via a path through the vehicle chassis. This is not only the safest path, the chassis is the lowest noise ground path. This is why every vehicle manufacturer has a lead from negative post to chassis, and all devices other than engine block mounted devices obtain negative via the chassis or a designated ground lug referenced to chassis. This is the only safe way to do things, unless the equipment supplier and installer can 100% guarantee there will never be a negative to chassis path through the equipment.
There are only two conditions where direct power connections to the battery negative are acceptable, anything else is risky.
(1)- when the device's internal circuitry fully isolates the negative power lead from the cabinet and all other external ports or leads exiting the device (2)-when the device's external connections completely and reliably float from ground, and any connections leaving the device are "fused" or current limited at a safe level for that lead In all cases where the negative lead has a direct current path through internal circuitry to any external conductors, which would include cabinet screws, enclosures, jacks, connectors, and wire leads, grounding the device negative lead to the negative battery or supply terminal can create hazardous conditions.
Worse yet, these hazardous conditions are not corrected by negative lead fusing. Negative lead fusing actually makes some hazards worse by creating a new problem, and open high current negative while other paths not rated for high current sustain negative current flow.
In a car the amount of iron (even though not as conductive as copper) is so substantial that a short wire to the nearest chassis point is the best (lowest resistance) ground for an accessory in almost all cases.
Running a wire to the battery minus is also fine for low current loads as well.
High current and long earth wire can cause earth potential issues but these seldom matter unless there is a further connection to some other device at the end of the long power cables.
