Battery main fuse

[alan_d]

Would you care to take us through how you chose the fuse?

Why do I suspect this question is an invitation to make a fool of myself?

When I was replacing the damaged cable I discovered that the battery cables were a mixture of 35 and 50 sqmm (which is why the insulation melted on the negative battery cable but not the positive). In the wiring diagram in the owner’s handbook for my engine (Bukh DV20) the starter cable is shown as 50 sqmm, although the 35 sqmm previously fitted seemed to do the job. For the relatively short lengths required, the difference in price between 35 and 50 sqmm tinned cable was not great so I chose to standardise on 50 sqmm. This left me with a considerable margin between the expected maximum current that the starter might draw and the current-carrying capacity of the cable, and I opted for a 200 Amp fuse.

 

[lw395]

Why do I suspect this question is an invitation to make a fool of myself?

When I was replacing the damaged cable I discovered that the battery cables were a mixture of 35 and 50 sqmm (which is why the insulation melted on the negative battery cable but not the positive). In the wiring diagram in the owner’s handbook for my engine (Bukh DV20) the starter cable is shown as 50 sqmm, although the 35 sqmm previously fitted seemed to do the job. For the relatively short lengths required, the difference in price between 35 and 50 sqmm tinned cable was not great so I chose to standardise on 50 sqmm. This left me with a considerable margin between the expected maximum current that the starter might draw and the current-carrying capacity of the cable, and I opted for a 200 Amp fuse.

To blow that fuse in 1 second will take something like 750A.
Your battery probably has a cranking amps rating of something similar.
So it will pretty much take a dead short to blow the fuse quickly.
An accidental short will likely arc and may cause a fire before the fuse blows, or could arc away at a couple of hundred amps for a very long time.
The short circuit current of the battery might be roughly twice the CCA rating. The fuse will pass this for long enough to establish an arc. Thiswill get worse as the battery ages and its short circuit current is reduced.
Anyone who's done a bit of arc welding will tell you a 75A arc is powerful. A 750A arc is ultraviolent.

At the other end of the scale, a cold engine with a slightly tired battery, a few bursts of cranking will preheat the fuse so the next burst blows it. I don't have the data for your starter motor, I suspect you are further from this scenario than the failure to blow scenario.
So I'd suggest carrying a spare fuse, but most of all making sure short circuits cannot happen. Just a bit of common sense and a few naff plastic covers on high current terminals is worth the effort.

 

[alan_d]

..... So I'd suggest carrying a spare fuse, but most of all making sure short circuits cannot happen. Just a bit of common sense and a few naff plastic covers on high current terminals is worth the effort.

The spare fuse I have, and the terminals around the batteries are well protected, but I do worry about the naked terminal on the starter motor - I'll see what I can find to preserve its modesty.

 

[PaulRainbow]

To blow that fuse in 1 second will take something like 750A.

How can you say that, you don't know what fuse he has fitted ?

Your battery probably has a cranking amps rating of something similar.

Another guess !

So it will pretty much take a dead short to blow the fuse quickly.

Based on what ? You don't have any data, just guesses.

An accidental short will likely arc and may cause a fire before the fuse blows, or could arc away at a couple of hundred amps for a very long time.
The short circuit current of the battery might be roughly twice the CCA rating. The fuse will pass this for long enough to establish an arc. This will get worse as the battery ages and its short circuit current is reduced.
Anyone who's done a bit of arc welding will tell you a 75A arc is powerful. A 750A arc is ultraviolent.

At the other end of the scale, a cold engine with a slightly tired battery, a few bursts of cranking will preheat the fuse so the next burst blows it. I don't have the data for your starter motor, I suspect you are further from this scenario than the failure to blow scenario.

As is often the case with your posts, you're just concocting figures and theories to create problems that don't exist. Most boats don't even have battery fuses, so it's hard to see where alan_d has created a problem by fitting one.

 

[lw395]

Paul, I have loads of data for typical 200A fuses.
The Littelfuse Mega range is a pretty good example of a low voltage fuse.
We use them in industry.
Cheaper brands are IME, worse.

As you say, a lot of 12V systems including boats don't have a battery fuse. That's because they can't really be relied on.
A 12V starter circuit is a very tough setting for a fuse.
So they are no substitute for protecting the high current wiring from accidental shorts.

 

[Stemar]

So they are no substitute for protecting the high current wiring from accidental shorts.

What would you suggest, then? Or is it one of those things that is a great idea in theory, but doesn't work out in practice?

 

[pvb]

So they are no substitute for protecting the high current wiring from accidental shorts.

What would you suggest, then? Or is it one of those things that is a great idea in theory, but doesn't work out in practice?

I'm not sure you'll get a sensible answer. If I were to suggest a solution, I'd say, as I have many times, that it's a good idea to fuse the main battery leads, as close as possible to the battery. If you put a 250A MegaFuse in the starter circuit of most small boat engines, it will behave impeccably unless there's a short circuit, in which case it will blow almost immediately and prevent a fire.

 

[lw395]

So they are no substitute for protecting the high current wiring from accidental shorts.

What would you suggest, then? Or is it one of those things that is a great idea in theory, but doesn't work out in practice?

What I would suggest is being careful around your +12V lines, making sure they are properly secured, safe from chafe etc etc and covering any vulnerable terminals with plastic covers.
Don't put metal things closer to the +12V than it needs to be.
It's a powerful thing, treat it with respect and it will be fine.

Have a huge fuse if you want, but don't use that as an excuse to be less than careful.

Be aware that extended cranking might blow the fuse, or do the maths to prove it can't.
Have a spare fuse.
Soemtimes fuses just die of old age, fatigue or bloody mindedness.

Where a line doesn't need to carry hundreds of amps, fuse it at a sensible level as close to the battery as you reasonably can.
If 20A is enough for your domestic needs, fuse the cable at say 30A even if it's '100A' cable.

 

[lustyd]

deleted accidental post

 

[Haraka]

I expect you’ve all retired since all the above was discussed, but one question I have concerns the isolator connection domestic battery to engine for emergency use - which is rarely a simple case of an undercharged starter battery, more usually a dead cell or so (as happened to me)

Mostly I see this link connected to either the battery side of both starter and domestic isolators, or to the load side side of both. The former includes the damaged starter battery in the solution (possible short of domestic through starter battery); the latter does too, but also (worse imo) allows spike currents from starting access to domestic equipment.

My preference is to wire the emergency start isolator between the starter isolator load-terminal (allowing connection to the engine etc) and the battery side of the domestic isolator (allowing access to the domestic battery). Then, opening both starter and domestic isolator, before closing the emergency isolator, isolates both the damaged starter battery and the domestic equipment from the engine starting process.

Once the engine is up and running, leave the starter battery isolated till you get back to harbour for a full review, but re-close the domestic isolator to operate equipment etc. Then open the emergency isolator - no longer needed. The alternator output will happily run direct to just the domestic battery (no need to go via any VSR) controlled by the alternator’s interior or exterior regulator.

I’d be very interested to hear any comments on this plan.

 

[Haraka]

PS. Depending how your VSR is wired, you may need to leave the emergency isolator closed as a route from alternator to domestic battery.

 

[William_H]

I expect you’ve all retired since all the above was discussed, but one question I have concerns the isolator connection domestic battery to engine for emergency use - which is rarely a simple case of an undercharged starter battery, more usually a dead cell or so (as happened to me)

Mostly I see this link connected to either the battery side of both starter and domestic isolators, or to the load side side of both. The former includes the damaged starter battery in the solution (possible short of domestic through starter battery); the latter does too, but also (worse imo) allows spike currents from starting access to domestic equipment.

My preference is to wire the emergency start isolator between the starter isolator load-terminal (allowing connection to the engine etc) and the battery side of the domestic isolator (allowing access to the domestic battery). Then, opening both starter and domestic isolator, before closing the emergency isolator, isolates both the damaged starter battery and the domestic equipment from the engine starting process.

Once the engine is up and running, leave the starter battery isolated till you get back to harbour for a full review, but re-close the domestic isolator to operate equipment etc. Then open the emergency isolator - no longer needed. The alternator output will happily run direct to just the domestic battery (no need to go via any VSR) controlled by the alternator’s interior or exterior regulator.

I’d be very interested to hear any comments on this plan.

As you realise it all depends on why the starter battery has failed to start the engine. Not likely just discharged. Paralleling it to domestic battery may pull the domestic battery down but also it may be that starter battery has sufficient volts but is just unable to supply current in which case it is OK to parallel the domestic battery. Having a volt meter on each or just paralleling for a short period before hitting starter switch might be ok. From experience of car batteries and jump starting I would just parallel the batteries not worying about isolating bad battery. ol'will

 

[pyrojames]

[snip]

Very few cars have fuses in the start circuit. Very few cars catch fire.

1. True. 2. Not quite so true in my experience, and the consequences involve the loss of ships....

 

[pyrojames]

Would recreational boat fires be caused by arson?

Yes. Damn, I didn't look at the dates... :/

 

[Haraka]

Thanks Ol’ Will - appreciate your thoughts.
Are you going for battery to battery side connection with starter isolator closed to reach the engine/starter - allowing domestic isolator to remain open to protect equipment from starting by current spikes? Or are you suggesting load to load side with both isolators closed and to hell with the consequences? I agree no more than say 15 seconds between cross-connecting and starting does not give a lot of time for the starter battery to bring down the domestic.

 

[Haraka]

On the other hand, my suggested method is no more difficult to install or operate, so why not?

 

[William_H]

On the other hand, my suggested method is no more difficult to install or operate, so why not?

Yes quite so. Jumping on load side of isolators means you can isolate starter battey. No I would not worry about other services on the same circuit as should onlky be a dive in voltage. ol'will

 

[Haraka]

A final resolution to my quandaries - I find I have an 80A in line fuse adjacent to my domestic battery +ve terminal and I’ve installed a battery monitor which requires a 100A shunt adjacent to the -ve terminal. The former will blow and the latter irreparably damaged (according to manufacturer) if I put a 400A spike through them.
So I’ll live with the risk for now.