Question on fusing a car battery

[alan]

Hello,

I hope this question is not beneath the level of the EE's here, but...

Recently I have been working on a project to relocate my car's battery
to the trunk for better weight distribution. This basically involves
running a long cable thru the car up to the former battery location for
the + and a short cable from the - to some nearby chassis bolt in the
trunk. It is generally suggested to run a fuse on the + line, close to
the battery, because a short on the cable won't set the car on fire.

At some point, I asked if it was OK to put this main fuse on the - line,
between teh battery and nearby chassis bolt. The reason being the same
as why you disconnect the - terminal of the battery when you work on the
car - so that if you put a wrench between the + terminal and case,
nothing happens. I cannot believe what a sh*tstorm this raised in the
car audio forum I posted in. Several people thought that this wouldn't
work, because it was on the "wrong" cable, or that it doesn't protect
components because it is at the "end" of the circuit.

Anyway, I'm mainly asking if there is any reason that putting the fuse
on the negative side of the battery is worse than on the positive. IOW,
looking for potential failure modes. I thought of one reason why it's
better to put the fuse on the - side, as mentioned above.

 

[John Tserkezis]

At some point, I asked if it was OK to put this main fuse on the - line,
between the battery and nearby chassis bolt. The reason being the same
as why you disconnect the - terminal of the battery when you work on the
car - so that if you put a wrench between the + terminal and case,
nothing happens. I cannot believe what a sh*tstorm this raised in the
car audio forum I posted in. Several people thought that this wouldn't
work, because it was on the "wrong" cable, or that it doesn't protect
components because it is at the "end" of the circuit.

Anyway, I'm mainly asking if there is any reason that putting the fuse
on the negative side of the battery is worse than on the positive. IOW,
looking for potential failure modes. I thought of one reason why it's
better to put the fuse on the - side, as mentioned above.

No, it's a good idea actually.

This depends highly on the application, but as you said, under certain
circumstances, it's quite possible to bypass the (+) fuse and have the full
battery current available to go through your spanner or such.

I did the same thing with a piece of equipment where I used to work. It's a
low current device, but uses a large SLA for capacity. The (+) line close to
the battery is fused, and the negative has relatively thick cable all the way
through the electronics and the chassis.
If a certain auxiliary cable was attached to the relevant port, and the
right bit shorted to the + battery terminal, it would cook a fair bit of the
wiring.
A good hour's work to strip everything down and rewire.

The easy fix for us was to replace the short wire that grounds the negative
terminal to the metal chassis with some very light duty wire. (effectively a
fuse on the - side). It doesn't carry much current anyway, so was quite
viable. This is in addition to the existing + side fuse of course.

Should this small wire link fuse, it's several minutes work instead of an
hour, and cheaper from a manufacturing perspective than a fuse, for an event
that was rare to start with.


I wouldn't pay too much attention to the comments of the car audio people,
I'm sure some are quite cluey, but most appear to know just "enough". Just
"enough" to be dangerous.

 

[PeterD]

Hello,

I hope this question is not beneath the level of the EE's here, but...

Recently I have been working on a project to relocate my car's battery
to the trunk for better weight distribution. This basically involves
running a long cable thru the car up to the former battery location for
the + and a short cable from the - to some nearby chassis bolt in the
trunk. It is generally suggested to run a fuse on the + line, close to
the battery, because a short on the cable won't set the car on fire.

At some point, I asked if it was OK to put this main fuse on the - line,
between teh battery and nearby chassis bolt. The reason being the same
as why you disconnect the - terminal of the battery when you work on the
car - so that if you put a wrench between the + terminal and case,
nothing happens. I cannot believe what a sh*tstorm this raised in the
car audio forum I posted in. Several people thought that this wouldn't
work, because it was on the "wrong" cable, or that it doesn't protect
components because it is at the "end" of the circuit.

Anyway, I'm mainly asking if there is any reason that putting the fuse
on the negative side of the battery is worse than on the positive. IOW,
looking for potential failure modes. I thought of one reason why it's
better to put the fuse on the - side, as mentioned above.

It will work exactly the way you expect it to, the fuse doens't knwo
which lead it is in... That said, I've seen way too many battery
relocations that were flawed because of shorts or potential shorts on
that positive cable.

Also, I'd (personally) run the (a) negative cable to the starter in
addition to the body ground. If you don't run a negative to the
starter, then at least make sure the starter is bonded to the body
with a heavy cable, as most automotive engine to body ground lines are
not sufficiently large enough to handle starting currents.

 

[Eeyore]

Hello,

I hope this question is not beneath the level of the EE's here, but...

Recently I have been working on a project to relocate my car's battery
to the trunk for better weight distribution. This basically involves
running a long cable thru the car up to the former battery location for
the + and a short cable from the - to some nearby chassis bolt in the
trunk. It is generally suggested to run a fuse on the + line, close to
the battery, because a short on the cable won't set the car on fire.

At some point, I asked if it was OK to put this main fuse on the - line,
between teh battery and nearby chassis bolt. The reason being the same
as why you disconnect the - terminal of the battery when you work on the
car - so that if you put a wrench between the + terminal and case,
nothing happens. I cannot believe what a sh*tstorm this raised in the
car audio forum I posted in. Several people thought that this wouldn't
work, because it was on the "wrong" cable, or that it doesn't protect
components because it is at the "end" of the circuit.

Anyway, I'm mainly asking if there is any reason that putting the fuse
on the negative side of the battery is worse than on the positive. IOW,
looking for potential failure modes. I thought of one reason why it's
better to put the fuse on the - side, as mentioned above.

The fuse will work fine in either location.

Graham

 

[Robert Roland]

Anyway, I'm mainly asking if there is any reason that putting the fuse
on the negative side of the battery is worse than on the positive.

In all the cars I have seen, the fat cable for the starter motor is
not fused at all. This is probably because the starter motor pulls so
much current that there is no practical way to implement a fuse.

If you fuse the negative side of the battery, you will have to use a
fuse that is big enough to handle the starter motor current. A fuse
that big may not blow at all if there is a short on one of the smaller
cables, since the small cable itself may provide enough resistance to
limit the current.

If you fuse the positive side, you can leave the cable for the starter
motor unfused, and use a separate, fused, cable for other electrics.

Another reason: A generator without a battery can output voltages way
beyond the nominal 12V. If the negative battery connection is broken,
you will get this overvoltage applied to all your electrics, possibly
causing lots of damage. The generator itself may also be damaged from
running without a battery.

 

[Jim Thompson]

Hello,

I hope this question is not beneath the level of the EE's here, but...

Recently I have been working on a project to relocate my car's battery
to the trunk for better weight distribution. This basically involves
running a long cable thru the car up to the former battery location for
the + and a short cable from the - to some nearby chassis bolt in the
trunk. It is generally suggested to run a fuse on the + line, close to
the battery, because a short on the cable won't set the car on fire.

At some point, I asked if it was OK to put this main fuse on the - line,
between teh battery and nearby chassis bolt. The reason being the same
as why you disconnect the - terminal of the battery when you work on the
car - so that if you put a wrench between the + terminal and case,
nothing happens. I cannot believe what a sh*tstorm this raised in the
car audio forum I posted in. Several people thought that this wouldn't
work, because it was on the "wrong" cable, or that it doesn't protect
components because it is at the "end" of the circuit.

Anyway, I'm mainly asking if there is any reason that putting the fuse
on the negative side of the battery is worse than on the positive. IOW,
looking for potential failure modes. I thought of one reason why it's
better to put the fuse on the - side, as mentioned above.

I had a '61 Renault Dauphine that had the battery in the (front)
trunk, engine in the rear.

The + lead was a copper pipe enclosed in a rubber hose... there was no
fuse.

I bought this car while I was a student at MIT. As soon as I arrived
at Motorola Phoenix and became involved with alternator regulators I
installed a 60Amp Ford alternator in it (with remote-sense regulator
;-)

...Jim Thompson

 

[Paul Hovnanian P.E.]

I have a Porsche 928 that has an engine and starter in the front and a
battery in the rear. There is no fuse in either battery lead, including
the (+) cable from the battery to the starter. They took measures to
protect the unfused section from damage and this doesn't seem to be an
issue from what I've heard. The main battery line proceeds fro the
starter to the alternator and then to the main distribution/relay panel.
This panel is where the first fuses in the battery supply appear.

I have added some additional circuits off the battery and I have
installed a DC circuit breaker box adjacent to the battery for these
circuits.

I think the best approach would be to run the line to the starter
unfused, the connection between the battery and the alternator unfused,
or fused such that the alternator will not end up connected to the
system loads after the fuse opens. All other systems should be fed
through at least a battery fusible link.

 

[mpm]

Hello,

I hope this question is not beneath the level of the EE's here, but...

The fuse is to protect the vehicle's wiring from overheating, not your
battery or your car's electronics. Therefore, the fuses should be as
close to the battery as possible, so as to protect as much cable as
practical.

That said, assuming there are no current faults from wiring to
chassis, a fuse can be placed anywhere in series, and interrupt the
current on overcurrent conditions. (Not helpful, in and of itself,
incidentally..)

You must also consider how the vehicle might be "jump started" in case
the battery fails at some point in the future. Will the fuses (placed
where you have placed them), still be effective (i.e., still in the
circuit and protecting the wiring) when jumper cables and an external
voltage source or battery are applied?

Finally, how much voltage drop will there be to the starter, and will
this be a problem?
Are you planning to use gas & oil resistant cables, or even flameproof
insulated cables? And have you considered any electrical noise
problems that might be created by relocating these cables (RFI / EMI
or otherwise?)

Oh, I guess there's one more thing...
Will the battery relocation truly make a material dent in the
redistribution of weight?...

-mpm

 

[Jon Slaughter]

Hello,

I hope this question is not beneath the level of the EE's here, but...

Recently I have been working on a project to relocate my car's battery to
the trunk for better weight distribution. This basically involves running
a long cable thru the car up to the former battery location for the + and
a short cable from the - to some nearby chassis bolt in the trunk. It is
generally suggested to run a fuse on the + line, close to the battery,
because a short on the cable won't set the car on fire.

How does changing moving the battery help at all with the weight
distribution? The battery is insignificant compared to the motor. Do you
plan on putting a bunch of crap in the trunk to offset the weight? (and
hence increasing the overall weight)

I don't see it as being worth the trouble but yes, sticking a fuse inline
would help someone. The main problem is that batteries are used for surge
current and not continuous current(once the car is running it doesn't need
the battery). Therefor a fuse isn't really applicable because if you use to
low of one then the surge can blow it(unless maybe if its slow blow and
rated properly) and if its too high(to handle the surge) then it won't
necessarily solve any shorting issues.

Probably the best bet would be to put a temp sensor on the battery and
monitor it that way or maybe some other device that knows the behavior. I
think a simple fuse won't do much though.

 

[alan]

You must also consider how the vehicle might be "jump started" in case
the battery fails at some point in the future. Will the fuses (placed
where you have placed them), still be effective (i.e., still in the
circuit and protecting the wiring) when jumper cables and an external
voltage source or battery are applied?

Good point. This depends on how much surge current there will be when
you hook a good battery up to a dead one. If the surge is large, maybe
you want to bypass the fuse (+ to + wire, - to batery - terminal)? I'm
not sure which is the right config to use when jump starting.

Finally, how much voltage drop will there be to the starter, and will
this be a problem?

I calculated about 1.4 mOhm due to the extra wire, and whatever else due
to one extra lugged connection. So maybe 0.1-0.2V over the original
setup? My battery has an internal resistance rating of 4 mOhm or so.

Are you planning to use gas & oil resistant cables, or even flameproof
insulated cables?

I used a welding cable because it was soft and could be routed around
stuff. It has "Flex-a-prene" insulation, which claims to be self
extinguishing to flame and oil resistant.

And have you considered any electrical noise

problems that might be created by relocating these cables (RFI / EMI
or otherwise?)

Yes, in the sense that if it causes a problem, I'll move the wire.

Oh, I guess there's one more thing...
Will the battery relocation truly make a material dent in the
redistribution of weight?...

It's like 1.5% or so. Brings down the rear natural frequency by 2%.
Not a lot, but one of the few modifications that both reduces understeer
and ride harshness.

Thanks for bringing up these additional issues.

 

[alan]

In all the cars I have seen, the fat cable for the starter motor is
not fused at all. This is probably because the starter motor pulls so
much current that there is no practical way to implement a fuse.

If you fuse the negative side of the battery, you will have to use a
fuse that is big enough to handle the starter motor current. A fuse
that big may not blow at all if there is a short on one of the smaller
cables, since the small cable itself may provide enough resistance to
limit the current.

I think that is ok. Most of the accessories have their own fuse, on
their positive line. (I think)

If you fuse the positive side, you can leave the cable for the starter
motor unfused, and use a separate, fused, cable for other electrics.

Yes, if you were going to run more than one cable from the battery +
terminal, then you should fuse the + side. But if you are going to have
one big unfused wire running thru the car, why add any additional fuses
at all? Just hook this one big wire to your original battery clamp.

Another reason: A generator without a battery can output voltages way
beyond the nominal 12V. If the negative battery connection is broken,
you will get this overvoltage applied to all your electrics, possibly
causing lots of damage. The generator itself may also be damaged from
running without a battery.

Good point. But this will also be the case if you only have one wire
coming from the battery with a fuse on the + side. (For the two wire
case, I agree to put the fuse on the +)
If the reason that you blew the fuse was a short on the + wire after the
fuse, then this same short will also short the alternator. So I don't
think you will over-volt any of your electronics. But if the fuse blew
due to some other reason, then I am not sure how to protect against
this. Maybe a smaller battery at the original battery location, to
continue to provide filtering? Some kind of zener such that if you
exceed it's voltage, it'll take out the alternator fuse?
I am not sure what is the best way to gaurd against this scenario while
still running a fuse.

Thanks for mentioning these additional failure mechanisms.

 

[[email protected]]

In all the cars I have seen, the fat cable for the starter motor is
not fused at all. This is probably because the starter motor pulls so
much current that there is no practical way to implement a fuse.

If you fuse the negative side of the battery, you will have to use a
fuse that is big enough to handle the starter motor current. A fuse
that big may not blow at all if there is a short on one of the smaller
cables, since the small cable itself may provide enough resistance to
limit the current.

If you fuse the positive side, you can leave the cable for the starter
motor unfused, and use a separate, fused, cable for other electrics.

Another reason: A generator without a battery can output voltages way
beyond the nominal 12V. If the negative battery connection is broken,
you will get this overvoltage applied to all your electrics, possibly
causing lots of damage. The generator itself may also be damaged from
running without a battery.

You are probably correct, but I know on diesel engines they use a
fusible link for the glow plugs. [Maybe 50 amps. I haven't owned a
diesel in a while.] Perhaps it would work in this situation.

 

[neon]

what a question weight distribution? a battery? 10 kilos? gee i think you should also include a pump in the gas tank so when you make a turn it distributes the gas wieght evenly. i cannot sleep at night of all the weight that is in my brain you got the same problem too?

 

[Paul E. Schoen]

Basically there are two issues, the starter current path (minimum fuse
size 1000 A) and the circuits to the rest of the vehicle (fuse size 50 to
100 A). There are some advantages to keeping the hard connections to one
side of the battery; is make disconnecting that side more effective in
preventing maintenance accidents. Think about it.

I have measured starter current on a vehicle and it was only about 200-300
amps. Here is a waveform of typical starter current, showing an initial
peak of 400 amps and then an average of about 160 amps:

http://www.picotech.com/auto/waveforms/cranking_petrol.html

Since the current is applied for only a few seconds, you can use a time
delay fuse of about 100 amps or a fast blow 200 amp fuse. A starter motor
only needs about two or three HP to crank an engine, which at 12 VDC is 124
to 186 amps.

A diesel engine (with higher compression) requires about twice as much
current:

http://www.picotech.com/auto/waveforms/cranking_diesel.html

One advantage of fusing the negative side is that you could use a bolt-on
fuse directly to the chassis, with an auxiliary copper cable to the engine
block or starter negative. The fuse would not need to be insulated (except
for environmental protection).

Paul

 

[Fred Bloggs]

Recently I have been working on a project to relocate my car's battery
to the trunk for better weight distribution. This basically involves
running a long cable thru the car up to the former battery location for
the + and a short cable from the - to some nearby chassis bolt in the
trunk. It is generally suggested to run a fuse on the + line, close to
the battery, because a short on the cable won't set the car on fire.

At some point, I asked if it was OK to put this main fuse on the - line,
between teh battery and nearby chassis bolt. The reason being the same
as why you disconnect the - terminal of the battery when you work on the
car - so that if you put a wrench between the + terminal and case,
nothing happens. I cannot believe what a sh*tstorm this raised in the
car audio forum I posted in. Several people thought that this wouldn't
work, because it was on the "wrong" cable, or that it doesn't protect
components because it is at the "end" of the circuit.

Anyway, I'm mainly asking if there is any reason that putting the fuse
on the negative side of the battery is worse than on the positive. IOW,
looking for potential failure modes. I thought of one reason why it's
better to put the fuse on the - side, as mentioned above.

There is more to this than just Ohm's law, you need to think fault
conditions. In the event of a collision where the (-) BATT terminal is
crushed against the chassis, the fuse is out of the circuit leaving no
protection for downstream loads, which also have a very good chance of
being faulted, possibly resulting in a fire which would not occur
otherwise. This can't happen with the fuse right on top of the (+).

 

[JosephKK]

The fuse will work fine in either location.

Graham

Just so long as you do not run starter currents through it.

 

[JosephKK]

I have measured starter current on a vehicle and it was only about 200-300
amps. Here is a waveform of typical starter current, showing an initial
peak of 400 amps and then an average of about 160 amps:

http://www.picotech.com/auto/waveforms/cranking_petrol.html

Since the current is applied for only a few seconds, you can use a time
delay fuse of about 100 amps or a fast blow 200 amp fuse. A starter motor
only needs about two or three HP to crank an engine, which at 12 VDC is 124
to 186 amps.

A diesel engine (with higher compression) requires about twice as much
current:

http://www.picotech.com/auto/waveforms/cranking_diesel.html

One advantage of fusing the negative side is that you could use a bolt-on
fuse directly to the chassis, with an auxiliary copper cable to the engine
block or starter negative. The fuse would not need to be insulated (except
for environmental protection).

Paul

While those waveforms are typical for "normal" conditions, they do not
adequate account of extreme temperatures, poor maintenance, degraded
(nearly worn out) starter, degraded engine (harder to turn or start),
or an additional plentitude of other difficulty increasing situations.
Thus the "cold cranking Amperes" ratings of 500 and above. It is
always appropriate to think through abnormal conditions in design.

 

[JosephKK]

There is more to this than just Ohm's law, you need to think fault
conditions. In the event of a collision where the (-) BATT terminal is
crushed against the chassis, the fuse is out of the circuit leaving no
protection for downstream loads, which also have a very good chance of
being faulted, possibly resulting in a fire which would not occur
otherwise. This can't happen with the fuse right on top of the (+).

And how do you wish to deal with starter currents?