Battery fuese/trips

[StarCruiser]

The problem will vehicle fires is the battery is quite happy to keep throwing out hundreds of amps through shorted cables, once cable insulation has melted, an extinguished fire is almost certainly going to reignite, so shutting off the current at the battery or batteries is the first priority, this being my justification for fuses at the actual terminals.

Often by the time the bonnet has been opened (assuming you still can) and the batteries disconnected, the damage is already done and your vehicle may be a write off, fuses are quick and decisive IMO.

Whilst on the subject, given engines and gearboxes and so forth are isolated from the body to offer insulation from vibration ect, the main path for earth is the heavy cable from engine to body or battery. The importance of this cable must never be underestimated, without it the earth return is going to be via a throttle or bonnet release cable.

Regards

Dave

Totally agree with all of that.

One thing, if you get a short on something smaller than your main starter cables it’s fused or fitted with a fusible link, so we can make reasonable assumptions that the fuse or fusible link will blow, yes?
What about if a 10A skinny earth wire going to an earth point or even the negative terminal of the battery, gets shorted across to a heavier fused cable?

Also, (ok, two things ) how do you know that fuse you’ve fitted will actually blow in the event the starter cabling is damaged?

 

[MarkW]

Bonnet release cable failure in vehicle fire is very very common. Thats why the fire brigade carry the necessary tool to open up the engine bay when needed. A quick google search reveals a fit of discussion on relevant forums and website about how to tackle this problem from the professionals.

If you are building an overlander or going on a long trip a secondary release is a wise choice along with some way of disconnecting the battery. Particularly given the normal routine of lots of electrical modifications to the vehicles, each of which introduces a potential failure point in the system.

 

[StarCruiser]

This is exactly my thinking Mark.

 

[Dave2000]

Totally agree with all of that.

One thing, if you get a short on something smaller than your main starter cables it’s fused or fitted with a fusible link, so we can make reasonable assumptions that the fuse or fusible link will blow, yes?
What about if a 10A skinny earth wire going to an earth point or even the negative terminal of the battery, gets shorted across to a heavier fused cable?

Also, (ok, two things ) how do you know that fuse you’ve fitted will actually blow in the event the starter cabling is damaged?

Yes all minor circuits are fused in the normal way, I am not quite sure what you mean about a skinny earth cable? In general a skinny earthed cable would have to be in close proximity to a heavy cable to short out, that in itself is poor wiring practice, if someone has done that then they should not be touching vehicle wiring, if you had no choice then you could fit a fuse to the earth cable.

Fuse capacity decision is based on current it must carry and cable size, if you have for example a 2.5kw starter, it will pull a little over 200 amps, so to allow for a stalled starter I have a 250 amp fuse, connected via a 300 amp cable.

In the event of the starter cable getting shorted, which given the heavy insulation, plus an additional outer sheath is unlikely, then the fuse would fail, the objective being the fuse fails before the cable heats enough to burn the insulation.

As mentioned earlier, my starter became fused as a consequence of my two batteries being linked via a cable running under the slam panel, I was protecting against cables shorting if I rear end someone or hit something hard enough to deform the front that the cables get shorted.

Regards

Dave

 

[StarCruiser]

All spot on in theory but what current will the battery deliver at its worst state, say when pretty old after a startup? Will it actually deliver enough current to blow the fuse and at what current does the fuse blow? There should be time current graphs for the fuse and probably the same for the battery.

As an example, 200A fuse won’t blow immediately a 210A load is placed on it, it will take a certain time and during this time the battery capacity is diminishing so at some point they may meet and the fuse may blow but if your battery is depleted first, which is how it is designed, then the fuse will not blow.

The fuse will also have an instantaneous blowing current for a dead short. As long as your battery is strong enough to deliver this current the fuse will blow.

Hope this makes sense.

 

[Chris]

Quote "Bonnet release cable failure in vehicle fire is very very common" maybe but that's like saying that issues on spacecraft during manned missions to the moon are common. You have to set this against the millions of miles travelled, even if only on overland journeys, where vehicles don't catch fire. It's all about context. Focus on not causing the fire rather than what to do in the very unlikely event that it happens. Preparation and execution controls beat recovery controls every time.

What if your emergency bonnet release doesn't work? Will you have a secondary back up emergency bonnet release? You might as well just fit rally style bonnet pins in the first place and do away with the release handle.

Oh no, someone might steal your engine!

 

[Dave2000]

All spot on in theory but what current will the battery deliver at its worst state, say when pretty old after a startup? Will it actually deliver enough current to blow the fuse and at what current does the fuse blow? There should be time current graphs for the fuse and probably the same for the battery.

As an example, 200A fuse won’t blow immediately a 210A load is placed on it, it will take a certain time and during this time the battery capacity is diminishing so at some point they may meet and the fuse may blow but if your battery is depleted first, which is how it is designed, then the fuse will not blow.

The fuse will also have an instantaneous blowing current for a dead short. As long as your battery is strong enough to deliver this currentvthe fuse will blow.

Hope this makes sense.

I see what your getting at, and yes the fuses will 'hold' for a short while but, you are creating an extremely unlikely event, certainly more unlikely of a short in the first place.

For example, we need a scenario of a low battery that just managed to start the engine, so it must have had at least enough amps avaiable to do that.

By coincidence and immediately after the engine started there is a short circuit, but the already virtually flat battery is being depleted further, so the amps are no longer there, moreover the voltage would fall to nothing, I agree certainly not enough to blow the fuse, but also unable to maintain sufficient voltage to push enough amperage for long enough to heat up a 300 amp cable to the point it burns through two layers of insulation.

The 80 can create this scenario by failure of the starter contacts, you crank the engine and it starts, you release the key and find the contacts have welded together, I would argue this more likely than a sudden short of the starter cable. Naturally you switch off but the starter will continue to crank the engine, this will continue until you disconnect the battery but, what if the battery was low in the first place? The starter would stall, the battery fast becoming depleted and simply dies, but the cable does not burn through.

I am no expert and certainly happy to have my thinking taken apart and thrown in the bin however, no one can dispute the fuses being there are at least some defense against a fire cause by a short circuit.

Regards

Dave

 

[StarCruiser]

Your thinking is far from bin worthy Dave, it’s spot on.

Every day in my work I have to check every circuit I install has no more resistance in it than the maximum value above which the fuse will not blow. I have to take into account the lowest voltage and effects of temperature too so I have to aim for no more than 0.8 of the figure given for the fuse or circuit breaker. All I’m saying really is if the fuse is there, will it actually do the job when it comes to the crunch so to speak. With a battery as the power source rather than the mains it’s vastly different as the stored energy will be depleted rather than be continuously fed into a fault.

The general design is that starter cables don’t need to be fused because the cable won’t burn through the insulation before the battery becomes depleted. The fuse might blow. There may be enough energy there to blow it but, as you pointed out, if not then the cables will withstand the short until the battery is depleted which isn’t very long. I’m curious therefore as to the value of putting the fuse there in general. I’m wondering what calculations have been made (by the fuse manufacturers or the designer) to say this size fuse for this size battery, cable size and length.

In short, I’m saying don’t rely on this fuse to be the ultimate protection. For this I would personally like to see how it performed under various scenarios. Or more simply, I would say that fitting a robust isolator that could be operated remotely if needed, would have more value in most scenarios.

Aside from that Dave, I’m curious as to how they are assembled. Mainly how the ring terminal is insulated from the metal post that is cast onto the battery clamp so that it connects to the fuse and not to the post. Perhaps you could post up some pics?

 

[Richard Turner]

By an insulating nut made of hard plastic. The fuse block is made of ceramic.

When I was MOT testing a stuck bonnet was a daily occourance. You would have one person pulling the handle whilst the other bounced the bonnet. Probably never opened one year to the next.

Causes - bonnet catch not greased and seized, or greased but jammed with dirt, meaning the cable would become stretched or frayed, or worn through the nylon inner sleeve and jammed, or the handle had broken right off!

Or, improperly adjusted after an accident repair, or the cable had become pinched.

On my truck a couple of weeks ago I did some work on the bonnet catch as it was stiff, it had been adjusted to maximum adjustment as there was wear on the latch pawl, I re welded it and filed it back, and cleaned the latch and re oiled it. Now it opens with the pressure of a little finger on the lever, so some bonnet latch maintenance is probably just as valuable as a 2nd bonnet cable.

 

[Chris]

Richard that looks a tidy thing, but sorry; what is it? I'm thinking a fusible joint?

 

[StarCruiser]

Interesting. I also wonder about the scenario when overlanding if cranking the truck after a fuel system problem and a fuse like that taking a short crank but blowing or getting very hot at least under prolonged current draw.

Edit
Using the 80 series 24 volt start:-
The starter is 4500 watts

4500/24 = 187.5A so if indeed that fuse can handle 200A (which always seems odd to me knowing what a 200A fuse looks like for mains
https://rover.ebay.com/rover/0/0/0?mpre=https://www.ebay.co.uk/ulk/itm/302432689232) then it should run the starter motor forever and a day.

 

[StarCruiser]

This is the sort of graph I’m referring to. I can so far not find one for the cube fuses.
http://www.cooperindustries.com/con.../product-datasheets-a/BUS_Ele_DS_2049_MAX.pdf

 

[Richard Turner]

Chris, it is a fuse link that can be mounted directly to a battery clamp, in ByronJ's link on page 1 he has a picture of one fitted.

They are used on lorry tail lifts at work, so they must be able to take some punishment as you could be repeatedly lifting 2 tonne loads with the hydraulic pump motor working very hard.

If you cranked a starter motor over for too long it would overheat anyway, on the DAF trucks if you crank it for 10 seconds an inhibitor stops it from working for 1 minute. This would be a likely cause of a starter motor short as well...

What if situations are endless, trouble is finding a good balance!

 

[moggy1968]

My understanding is the fire was probably caused by wiring behind the light, so it was already on a fused cable. Your starter fuse would be rated so high it would offer no protection on the auxiliary circuits, which are by far the most likely to fail. It is likely it was due to an earth short so the fuse wouldn't have operated. Also, the fault current would probably be similar to the rated fuse requirement, so the fuse may well not operate anyway. I would agree with the above, use a battery isolator, mine is mounted on the winch bar for easy access, and then also make sure you have a back up way of opening the bonnet in an emergency. Because I travel to remote places, often as a single vehicle, this was a big concern for me so I am now in the process of going through my vehicle adding mechanical protection to cables I don't think have adequate protection at the moment. Make sure you use good connectors (where possible I use solar ones, rated up to something like 600v and IP rated) and tie cables up so they don't chaffe

 

[StarCruiser]

My understanding is the fire was probably caused by wiring behind the light, so it was already on a fused cable. Your starter fuse would be rated so high it would offer no protection on the auxiliary circuits, which are by far the most likely to fail. It is likely it was due to an earth short so the fuse wouldn't have operated. Also, the fault current would probably be similar to the rated fuse requirement, so the fuse may well not operate anyway. I would agree with the above, use a battery isolator, mine is mounted on the winch bar for easy access, and then also make sure you have a back up way of opening the bonnet in an emergency. Because I travel to remote places, often as a single vehicle, this was a big concern for me so I am now in the process of going through my vehicle adding mechanical protection to cables I don't think have adequate protection at the moment. Make sure you use good connectors (where possible I use solar ones, rated up to something like 600v and IP rated) and tie cables up so they don't chaffe

I agree with all of that except we don’t know what the fusing on Byron’s truck was, it’s only an assumption. Only Byron can tell us if there were any small wires taken from the battery and whether or not they were correctly fused near the battery. I’m not sayingveither way, just that we cannot assume.

Far too often I see many small wires connected in this way with either no fuse or fuses inside under the dash or something. It only takes a chafed through cable to short and start to glow.

I agree, heavy duty oversized battery isolators and a failsafe means to open the bonnet. Both of which need to be resettable and suitably located.

As far as ‘what ifs’ go, I have to design out any likely chance of death, injury or damage to property.

 

[Richard Turner]

Does the isolator have to be on the positive side near the battery or can it be on the negative side?

 

[StarCruiser]

For a battery isolator I would say isolate the negative of the RH battery for sure and the negative on the LH battery.

 

[Richard Turner]

I have a Petrol so only have 1 battery at the moment,

 

[StarCruiser]

In that case omit the second isolator…

 

[moggy1968]

I agree with all of that except we don’t know what the fusing on Byron’s truck was, it’s only an assumption. Only Byron can tell us if there were any small wires taken from the battery and whether or not they were correctly fused near the battery. I’m not sayingveither way, just that we cannot assume.

Far too often I see many small wires connected in this way with either no fuse or fuses inside under the dash or something. It only takes a chafed through cable to short and start to glow.

I agree, heavy duty oversized battery isolators and a failsafe means to open the bonnet. Both of which need to be resettable and suitably located.

As far as ‘what ifs’ go, I have to design out any likely chance of death, injury or damage to property.

Well, the question was about fusing the main battery cables, so debating the assumption isn’t really that relevant, and is splitting hairs. the message is, in order to be safe, correctly fuse your accessories, not the main battery cables!! It’s highly unlikely that a fire would start from a fault on you battery cables for all the reasons above and a fuse on those will not protect you from a fire on cabling for accessories which is more probable.