Grounding Negative System



Also see Ground Loops

Safe Battery Installation

This article is mainly concerned with battery ground and ground

For front mounted:

Battery Wiring

For trunk mounted:

Mounted Battery Installation


Any comments appreciated emailed to:

Automotive Electrical System Battery Grounding

A few quick notes:


Automotive systems mostly use the oldest type of rechargeable
battery (invented in 1859), the lead-acid battery. 12 volts is the short common
slang for nominal
battery voltage. It really is not 12 volts.

Lead-acid cells are
2.1 volts per cell at rest, with a full charge. The six-cell 12 volt battery is
12.6 volts at full charge resting, should be over 14 volts while charging, and over 13
volts immediately after charging is removed (without any electrical load). See
Electrical System

Traditional low-voltage (12 volt) electrical systems use a
negative ground system, the “ground” almost always being the vehicle’s entire
chassis. The highest current ground is to the engine block, since that is where
alternator and starter currents flow. That ground has to be a very solid low
resistance connection. It is a direct connection to the battery with batteries
near the engine, and often via the chassis with remote batteries. A ground must always connect from the battery
negative to the chassis with all battery installations. 

The Battery’s Job

A common misunderstanding is the battery supplies normal
running load power. This is not correct, the alternator normally supplies all
electrical energy. Of course the alternator will not support the electrical
system when the engine is off, when starting the engine, and under certain
conditions of extreme electrical loads, especially at slow engine speeds. The
battery supplies power any time the alternator is unable to support the full
electrical demand. The battery kicks-in instantly, even if needed for a tiny fraction of a second,
because the battery directly parallels the alternator’s output.

The battery serves as a giant “electrical
flywheel” to smooth voltage from the alternator. Just as the flywheel on an
engine smoothes piston thrusts and clutch loading, the battery prevents sudden
alternator surges or electrical load changes from radically changing the
electrical voltage.

The battery must be kept in
parallel with the alternator with solid connections. On a running engine, if the battery is disconnected
(accidentally or intentionally) and the electrical load or
engine speed abruptly changes, or if the battery is disconnected from the system
while charging, an alternator can surge to over 100-volts. The surge voltage can
wreak havoc on sensitive electrical parts, including stereo systems, ignition
systems, engine controls, and light bulbs. This is why anything we do to the
electrical system should always be done in the context of having the most
reliable battery connection possible. To avoid damaging voltage surges, the battery to alternator connection must
never be interrupted while the alternator is supplying charging current or
running current!

“Under drive” Pulley Sets

The things commonly called under drive pulleys actually
slow the accessories. All electrical energy comes from the alternator (or whatever
charges the battery if other than an alternator). If the alternator is turning too slow (perhaps through under-drive
pulleys at slow speeds), the electrical system will run from the battery. The
lack or low speed charging depletes the
battery’s charge.
When an electrical system operates from battery charge at slower speeds or idle, the alternator loads the
accessory belt
and crankshaft heavier than normal at higher speeds to replenish the battery’s idle charge loss.

Slowing the alternator below operating charge speed
levels at idle actually increases
alternator drive belt mechanical load at higher RPM. This is because the
alternator has to replenish battery charge lost at idle or low engine RPM.
Under drive pulley systems reduce engine parasitic alternator load at idle and slow speeds, and
increase alternator parasitic loading and waste horsepower at racing speeds.  

Battery Grounding or Negative Battery Supply Connections


The only connection to a battery post
negative should be to another battery negative, the
vehicle chassis, and/or the engine block. There should never be a direct negative post
path to accessory equipment that has any sort of ground path to external

The sole exception to this rule is when the equipment
being connected electrically
isolates or floats the negative terminal power connection from any
external connections. If the negative power feed wire electrically floats from all
exposed conductive cabinet parts or external wiring paths, a fused direct
negative should be safe.

An isolated power ground inside
external equipment is the only condition where a direct battery negative
terminal or post connection is safe.

The starter and alternator are normally the two highest
current devices in a vehicle’s electrical system. The starter can draw hundreds
of amperes, a large alternator can source hundreds of amperes. The starter and alternator ground
carries the same current as the positive voltage hot leads to or from the
starter or alternator. 

and alternators have their negatives common to their metal cases. They mount directly on the engine
block or heavy engine accessory brackets, with the housing 
or case providing the negative terminal or negative
ground connection. The ground path is through mounting hardware to the block. A normal properly fastened mounting system “grounds”
the starter and alternator with extremely low electrical resistance to the engine block
or cylinder head. The mechanical connection makes an almost perfect starter and alternator ground to the timing cover, block, and heads.
Just be sure bolts are not against accidental insulators. Accidental insulators
include anodized aluminum, power coated metal, paint, and even thread locking
chemicals such as Loctite. Loctite will wick down into threads and insulate
threaded joints. Never use Loctite on electrical connections. Dielectric
greases, electrical pastes, or anti-seize compounds behave differently, sealing
air and moisture out to prevent corrosion without harming electrical contact.
They push out of the way.

The ideal alternator and starter
battery ground path is through a heavy block-to-battery cable connection. A very
close second choice would be a timing chain cover grounding boss or stud.
Sometimes, but not always, a cylinder head or bell housing connection can be
used. Generally, the fewer gaskets between the block and battery ground,
and the larger the metal area and thickness at the connection point, the better the ground
connection will be. Never connect directly through Loctited fasteners or against
anodized, rusty, corroded, or painted parts.  The vehicle manufacturer most likely
selected the very best ground points.

Warning! Read this!

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.

Ground Currents and Ground Loops

All of the vehicle’s normal running currents,
which include ignition, radio, lights, wipers, horn, and computer systems, flow from the
alternator through the engine block to the vehicle’s chassis ground, or from the
battery to vehicle chassis when the alternator is below battery voltage. As
alternator voltage falls below 13.8 volts, the battery picks up an increasing
share of load current.

Horns and lights ground to the body
shell, while dashboard electronics typically ground to the firewall or the solidly welded
or bolted dashboard bracing. Critical sensors and pickups normally float from ground everywhere, grounding only to the
computer’s internal negative bus system. The computer negative bus then grounds to the
firewall or body shell. This grounding method prevents
ground loops. Ground loops introduce unwanted
electrical noise and/or sensor voltage errors.

A smaller very short lead from the battery negative terminal,
as well as ground leads from the engine block, go directly to the body shell.
These leads, primarily the heavy short battery ground lead, power all of the
vehicle’s grounded electronics negative terminals. The body shell, not the
battery negative post, is the safest and best common ground point for sensitive

Battery mounted in front engine grounding












The fuses feed all electrical devices, including but not
limited to lights, wiper, heater, horn, radio, and dashboard. Critical devices
often run from fuse links or separated fuse or automatic reset overload limiting
systems. All of these devices return through the vehicle chassis and
the short battery-to-ground wire that is common with the large negative block
connection wire. Alternator and starter currents are through the engine block
and heavy black wire to the battery post.

The unique ground routing for different systems is for very specific reasons. The body
shell serves as a giant low dc resistance and lowest impedance high frequency
and radio frequency ground point. The vehicle
becomes the common point for reducing or eliminating noise in sensitive audio
systems, as well as eliminating noise or voltage errors in sensors and/or trigger
systems. The vehicle chassis is the common point for optimum RFI and noise suppression, not the battery negative post.
The goal is to keep high currents with noise out of the wiring. You can see what
I did to mitigate radio interference
in my Power Stroke diesel here

Rear battery systems are a
little different. Because distance to a rear mounted battery is so long, it
is impossible to have low negative lead impedance through wires. Even though
steel resistivity is several times the resistivity of copper, the
chassis actually becomes a batter ground for signal and starter currents. The
chassis is a superior electrical noise ground, and the chassis is a superior
negative battery conductor for rear mounted batteries, because of cross
sectional area. With a normal wire, conductor cross-sectional area is limited by
the diameter of the conductor. While the chassis is relatively thin, it has a
very wide electrical path. This more than offsets the higher resistivity of the

Example of ground path

Resistance of any uniform conductor is
inversely proportional to cross sectional area and directly proportional
to resistivity and length. In simple words, if we double the cross
sectional area of a conductor we cut resistance (and voltage drop) in
half. If we double the length, we double the resistance and double the
voltage drop.

A number 1 AWG copper wire has an effective diameter of about 0.3
inches. Area of a circle is pi*r squared. This wire would have a cross
sectional area of about pi*.15*.15 = .071 square inches.

Let’s assume a steel body shell is about 16 gauge, or about .06
inches thick. A one foot wide area would have 12* .06 = .72 square
inches of cross sectional area. The physical cross section is about ten
times larger than the copper wire’s cross sectional area.

The resistivity of steel is about 15 ohms per 10-6 cm.  The
resistivity of copper is 1.7 ohms per 10-6 cm. We can reasonably assume
steel has about 15/1.7 = 8.8 times the resistance of copper for the same
length and same cross sectional area. While the body shell has higher
resistivity material, the body also has much greater cross sectional

This means a one foot wide length of steel body shell, if that shell
is only .06 inches thick, has about 10% less resistance than an equal
path length through out copper wire. It’s easy to see why a ground path
through the car body, which likely is several feet wide and much thicker
in many areas, is a small fraction of the resistance of a copper wire.

A four foot wide area of floor pan, just .06 inches in thickness,
would have a cross section of about 2.88 square inches. The equivalent
copper conductor would have to be 2.88/8.8 = .327 square inches, or a
diameter  = 2*  sqrt of A/pi, or .645 inches diameter!
Equaling the resistance of a thin 4-foot wide steel floor pan with a
copper cable requires a cable larger than 4/0 , and we have not even
counted the help from frame rails, rocker panels, or roof paths!  


Since the chassis is lower resistance
and impedance, rear batteries should
generally use the chassis as the negative return for the entire vehicle. The engine block
should be electrically bonded to the chassis. Rear-mounted batteries still use the chassis as
the noise/RFI common point
for filtering or cleaning up electrical interference, but it also becomes a
superior negative high current return. The break-over point where the chassis
becomes superior to an number 1 AWG copper cable is usually about five to eight

The chassis ground, outside of very short runs, is a good system. The system is planned to prevent
unavoidable voltage drops in the ground system from upsetting computer sensor
voltages. It keeps heavy charging and starting currents out of sensitive
electronics, and it ensures a steady supply of clean low-noise direct current to
vehicle electrical devices. It also has consideration of safety in the event
some ground connection or ground conductor fails. If a battery terminal comes
loose, for example, the only damage is a loss of starting or operating voltage.
Electronics typically does not suffer catastrophic damage from poor connections,
while noise typically does not get into stereo and computer systems.    

Aftermarket electronics should be connected in ways that will
not damage existing electrical devices, and will not introduce noise and ground
currents into sensors. The same is true for relocating a battery, or adding a
second battery. We should put as much thought into these systems as a competent
design engineer puts into the original equipment. This includes fusing and how
we route leads, as well as how and where we “ground” or obtain negative supply

Other than very low current systems, like lights or critical
sensors, you will not find very many long negative leads (long ground leads) in
a vehicle. This is for good reason. If we are changing the typical OEM pattern
of minimal length on heavy high-current leads, we are probably doing something
wrong. It isn’t abnormal for a good wiring expert (who are sometimes difficult to find) to pull
50 feet of unnecessary wire out of the rat’s nest made by typical wiring
technicians or hobbyists.

Connecting negative leads to battery posts, and long negative
leads, are almost always a

To most of us, what goes on inside the little boxes we install seems
completely foreign. Most of the world
the heavy black power lead is the negative power, and that all negative power
exclusively flows through that black lead. Only a few understand power negative
is not exclusively via the negative lead in almost equipment, and that anything
metal on the device enclosure, and everything exiting the enclosure like wire
leads or signal terminals, usually share some of the negative supply current.

There are only two conditions where direct power connections to the battery negative
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

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.

Let’s look at why grounding to the battery post is rarely a
good idea!

Here is a diagram of a typical system. The other loads on the
system are represented by one box, and the device we are connecting is tied to
the battery posts. This is typical for radio instructions, amplifier
instructions, and MSD ignition system instructions. Note the “device”, which
could be an amplifier, MSD box, or any add on accessory, connects to the battery
negative post or terminal: 

Ground loop automotive electrical system battery negative MSD


This all looks good on the surface. We assume currents are
like this:


ground loop equivalent circuit




We have 8.9 amperes on each device  (R2)  wire
because we ignored the other ground paths in the shared grounds.




The problem is the device has other grounds on small wires
that connect to the negative power buss. We really have this:


ground loop MSD igntion poor wiring technique



We have unwanted currents in our “device” small-signal grounds.

W1     8.95A

W2     7.16A

W3     1.79A



These unwanted currents are from
Ground loops are caused by improper
wiring, where someone wrongly assumes ground is ground, and the battery negative
post is a good ground or negative power source.


Any resistance from running the long ground lead to the
battery negative, because it creates a ground loop with signal leads, shifts unwanted current into fragile, sensitive, signal leads.

It gets much worse though. If we fuse the negative and it
opens, or if the battery or power source negative lead opens, we have this:


Battery negative ground loop open connection


This results in the following circuit:

battery negative ground accessory devices



W1     8.118A

W2     0A

W3     8.118A




An open black-to-battery, either from an open fuse or a bad
connection, will cause 8 amperes to flow through small signal leads. This can
damage things or be a fire hazard.

While the above illustrates why we should never fuse a negative
lead on a shared buss device, it gets even worse than that!!! What if we have
this case, where a battery ground (W6) opens:


battery ground damage equipment


Now we have this:


battery ground loop MSD igntion



W1    7.667A

W2    45.997A

W3    38.33A


This is devastating to almost any device, and is a major
equipment or fire
hazard. This is why some countries no longer allow fusing negative leads, or
connecting negative accessory leads to battery posts.


The proper connection method is:

correct battery connection ground loop

In a proper system, no add-on device in the system connects to the
battery negative post, battery negative lead, or the ground stud for the
negative lead. The battery negative post and battery negative leads connect
major grounds
, which would normally be the engine block (very heavy lead for
alternator and starter current) and to the vehicle chassis for all other devices! Any
device that has a shared ground buss or a ground lead connected to exposed metal
should NEVER be connected to the battery negative post or wire, and the device
or equipment being powered should never have a negative fuse.

There is only one exception to this rule that will permit a
safe negative fuse or negative to battery post connection.

The only exception to the above rule occurs when the electrical
device is completely ground isolated between the power ground and all signal
grounds or any exposed enclosure or case metallic parts. Negative buss isolation
will break any ground path other than the battery lead.

Spark Return

A final problem using ignition devices. We are all familiar
with the outgoing path to the spark plugs, but we ignore the return path. The
return path has just as much pulse current and energy as the “hot” path. The
system will induce much less noise into unwanted places if the ignition coil
grounds to the engine block, or to a cylinder head, with a short wide lead or
the coil is mounted directly on the engine. Braiding is ideal for the external
ground. This is another reason to use ground straps from the engine to chassis. 

For the best EMI suppression, wide smooth conductors are much
better than woven or stranded conductors. Unfortunately, solid wide conductors
will break when flexed. Life when vibrated or flexed is more important than
lowest impedance, so a shield lay braid is generally the best compromise between
lowest impedance and the best mechanical characteristics. 

Electrical Noise
Electromagnetic interference (EMI)