Question on fusing a car battery

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.

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.

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.

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

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.

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

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.

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.

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.

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

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

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.

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

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.

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.

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.

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:

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:

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

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 (+).

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.

And how do you wish to deal with starter currents?