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Most cars don’t fuse the starter motor feed. What are people’s thoughts on this? Good practice to fuse it of not?

A number of vehicles have a core feed wire from + terminal on the bat to starter, and then a core feed + wire to main fuse block. Some have main fuse block fed from bat directly. Again any pros & cons against this, or is it just a way to keep cable costs down? 

VSR load rating. The max current draw of the range rover classic VSR is 40A, yet you can have it feeding front heated screen (2x 25A), rear screen 30A, and optionally split charge when fitted. Assuming that LR didn’t just ignore the max operating limit, is there something I’m missing here?

Similarly, the existing ignition relay doesn’t seem to be heavily rated. I’m planning to use a Bussmann RFRM as my main relay and fuse block. It has two 100A busses. If I used an Albright SPST isolator instead, I’m assuming I’ll be ok for this to be kept fully open all the time ignition is on, as it’s rated - Thermal Current Rating 100% 250A

https://www.firstfour.co.uk/albright-su280-isolator-250a.html

cheers, Steve 

Edited by SteveG

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Any fuse has to be high enough not to blow under peak current which is going to be a big fuse. I usually base a fuse on stopping it melting the wiring and starting a fire rather than protecting the device at the end unless there is a specific need too. I think a starter fuse could be big enough to allow that to happen without blowing?

Some cars put fusable links in on the higher current sizes. Iv'e had suzuki with this in but not land rover although I'm sure some of them did???

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You tend to fuse at around 80% of the rating of the wire. As already stated you're protecting the wiring, not the device. The wires are typically sized to cause less than a 10% voltage drop at full load and to within the manufacturer's specified limits. A fuse will fail more-or-less instantly at double it's rated current. Run at it's rated current and you're in the zone where it may blow at any time, or never!

I will be trying to fuse it on my loom but accept I may need to bridge the fuse if I get nusance trips. Seem to recall coming up with an expected current of 150-200A for my V8i starter.

You've got to also be aware that there is a maximum current the batteries can dish out. If for instance you picked a 250A fuse with appropriate wiring your batteries would need to push 500A through the fuse for a guaranteed fuse blow. Less than that and the fuse is somewhere between a sound safety addition and a useless voltage drop and potential failure point.

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Thanks Al and Wes, useful info.

cheers

Steve

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If you need to use a fuse of 200A, chances are if something goes wrong, the damage will be done before the fuse blows. So not much point. Adding a fuse could also cause a voltage drop, something you certainly don't want in your starter circuit.

As for running 12V+ wiring via the starter, I'm not a fan. It can be useful to simplify and reduce cable length. But a different cable directly to the fuse box is 'cleaner' in my book, and makes troubleshooting easier. A lot of cars have the alternator connected via the starter. That does make sense as they're usually relatively close so you don't need 2 full lengths of thick cable.

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If you have sized cabling to handle at least 250A and you batteries can kick out more than 500cca or whatever the more conservative value is rather than CCA then the fuse will protect the wiring as intended.

Arc damage at whatever is shorting the lead is almost definate, but it should blow before your wiring ignites, or the batteries overheat.

Putting a 200A fuse in an OEM loom is probably no help as they tend to undersize the cables on the basis that the starter is rarely used for more than 20 seconds or so.

Undersizing with regards to voltage drop on LR looms is an issue as demonstrated by the benefit a headlight relay can give. This however is likely exacerbated by poor components.

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Posted (edited)

Thanks for the replies. I’m planning to use 50mm2 from battery to starter, starter to bulkhead pass through and then this to SafetyHub 150 inside the vehicle. A bit overkill, but I already have a couple of metres of both red and black from previous winch installs. 

The consideration of fitting a fuse was more from a safety point of view, rather than protecting devices. Albeit unlikely, I’d personally rather have a fuse stop a short in the starter wire/starter and theoretically prevent the risk of fire than not.

To go with the 50mm2 I ordered a 300A fuse. Reading through is this now too high? 

Edited by SteveG

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There's good info on wire and I think fuse selection on the 12v planet site.
Some would argue I've gone over kill too. Got a lovely chunk of a bus bar ready for my loom. Big enough to happily take a feed to a winch. They pull daft currents. Serious enough for me to be thinking hydrualic is the way to go on my truck.

 

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We've developed a theory that fat starter cable kills starters... both myself + TSD have chunky starter cables on our trucks and both of us have killed starters. TSD managed to explode a 200TDi starter gearbox (!) and I managed to burn out a V8 starter. 50mmsq is massive, if nothing else it's a pain because it's too big to route/handle/terminate nicely.

LR use relatively small cable, the theory is over a certain length the cable has enough resistance to protect the starter from abuse and is long enough that the heat dissipation is within what the cable will cope with.

Same with LR's genuine split-charge - an un-fused 70A relay between two batteries, surely madness??? But the relay is mounted about as far away from the batteries as it's possible to get it (top of bulkhead, driver's side) so there's about 5 yards of cable round-trip. Reckon all but the worst possible case the cable would drop enough volts to limit the current + not catch fire.

Sometimes what looks like penny-pinching could just be very subtle and cunning design ;)

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Yeah, great idea for the head lights that isn't it?! Nice single point of failure on that high current switch too. ;-)

Back to your comment though that's interesting. I'd suspect a significant voltage drop due to the resistance and high load does act as a current limiter. Windings on the starter are low or single digit ohms if that.

Essentially they have used under capacity wire under the assumption that the significant heat loss through the wire is manageable due to the low duty the starter sees. Most likely by accident this is protecting the starter as other vehicles using the engines would have  used varying cable lengths. Far better would have been a coil or two more on the starter motor that would have resulted in more torque for cold morning starts.

If money was less of an object I'd seriously consider the geared starter motors. They promise more torque for less current draw. Major draw of mormal starters is for the torque required to intially get the motor spinning over. Higher speed electric motors have got far better in the last couple of decades.

 

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Posted (edited)

Brain fart! Sub single digit ohms as for instance 120A at 12v would only be 0.1 ohm. Guess the figures I've seen may have been measured with equipment that couldn't  record or catch the initial starting current. Doesn't leave much room for cable resistance before you effectively have a significant voltage divider, even before battery voltage drop under load is considered.

Edited by WesBrooks

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1 hour ago, FridgeFreezer said:

We've developed a theory that fat starter cable kills starters... both myself + TSD have chunky starter cables on our trucks and both of us have killed starters. TSD managed to explode a 200TDi starter gearbox (!) and I managed to burn out a V8 starter. 50mmsq is massive, if nothing else it's a pain because it's too big to route/handle/terminate nicely.

 

 

Many years ago I built a regulated 13.8v power supply that could deliver 300A, this was quite a beast and had remote sensing to ensure the load always saw 13.8v.

When completed I tested it using lift motor brakes and had them glowing bright orange at 300A.

I tested it by starting a car, I almost _ _ _ _ myself as the engine twisted round the engine bay stretching the mounts to the limit.

Never again .. and yes, there does need to be a bit of volts drop to the starter.

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2 hours ago, FridgeFreezer said:

We've developed a theory that fat starter cable kills starters... both myself + TSD have chunky starter cables on our trucks and both of us have killed starters. TSD managed to explode a 200TDi starter gearbox (!) and I managed to burn out a V8 starter. 50mmsq is massive, if nothing else it's a pain because it's too big to route/handle/terminate nicely.

LR use relatively small cable, the theory is over a certain length the cable has enough resistance to protect the starter from abuse and is long enough that the heat dissipation is within what the cable will cope with.

Same with LR's genuine split-charge - an un-fused 70A relay between two batteries, surely madness??? But the relay is mounted about as far away from the batteries as it's possible to get it (top of bulkhead, driver's side) so there's about 5 yards of cable round-trip. Reckon all but the worst possible case the cable would drop enough volts to limit the current + not catch fire.

Sometimes what looks like penny-pinching could just be very subtle and cunning design ;)

Thanks John, this is the reason for posting this stuff on here. For example if you go by the voltage drop theory online and use calculators, like those on 12v planet that recommend no more than 3-4%, you end up needing 16mm2 cable for the rear heated window. LR uses what looks like the equivalent of 6mm2.

So from your experience of burning them out, for the starter feed what’s recommended 25mm2? 

cheers, Steve 

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TBH whatever LR use is clearly fine. I can't remember what mine is, maybe 35mmsq, maybe less.

Or to put it another way: How many times has an engine ever failed to start because of the cross-sectional area of the starter cable? :mellow:

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7 minutes ago, FridgeFreezer said:

TBH whatever LR use is clearly fine. I can't remember what mine is, maybe 35mmsq, maybe less.

Or to put it another way: How many times has an engine ever failed to start because of the cross-sectional area of the starter cable? :mellow:

I’ll look on the donor 91 3.9 and what the factory starter cable is. 

cheers, Steve 

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Posted (edited)
On 08/03/2018 at 12:52 PM, GW8IZR said:

 

Many years ago I built a regulated 13.8v power supply that could deliver 300A, this was quite a beast and had remote sensing to ensure the load always saw 13.8v.

When completed I tested it using lift motor brakes and had them glowing bright orange at 300A.

I tested it by starting a car, I almost _ _ _ _ myself as the engine twisted round the engine bay stretching the mounts to the limit.

Never again .. and yes, there does need to be a bit of volts drop to the starter.

Cranking drops battery voltage to around 9-10 volts. Car battery voltage is around 12.6 while the engine is not running and 13.5-14.5 while the engine is running and alternator is running.

I'm going to assume the starter is a resistive load just to make maths simple. 200A at 10V is 0.05Ohm. Winding voltage upto 13.8V would increase current to 276A. 200A @ 10V = 2000W. 276A @ 13.8 = 3808.8W.

So assuming resistive load your constant voltage test is forcing around 1.9 times the rated power through the starter motor than the design intent? Not surprising it turned over quickly! Yes the system may need voltage drop from 12.6V but doesn't most of this come from the chemistry of the battery and its response to load?

The mentioned 35mmsq has a rated load of 240A and resistance per meter of 0.00056ohm (or at least the extra flexible battery cable from 12V planet does). Guessing a 2m run thats 0.00102ohm. We'll ignore connection losses as they'd be similar for different cable csa. So 0.001Ohm in cabling, and 0.049ohm in starter. Assuming a simple voltage divider between cable and starter your only dropping 0.2V in cabling leaving 9.8V at the starter. Hooking the starter direct to the battery will see the current draw rise to 204A, and the total power output increase by 5%, rather than 90% in the constant voltage test.

Something else a factor here?

 

.

Edited by WesBrooks

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The volts drop is almost entirely in the cell, however the running current is not the peak starting current. Until the motor runs up to its normal speed the current is much greater and hence the i2r losses are greater. The back emf that limits the running current is zero when the motor is stalled or stopped awaiting start.The potential for damage to the motor is greater when the motor is beginning to turn or stalled.

I don't think changing the cable from a 'normal' battery cable to the quoted 35mm cable would cause much difference in terms of long term damage but an already worn out starter turning a cold worn out engine will be allowed to work harder further accelerating wear.

The potential damage to the motor is the energy x the time exposed to it, the big batteries and thick cables do mean when it goes wrong more damage can happen.

I think I use 35mm flexible welding cable on the starting circuit on my 90 as its what I had.

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On 3/7/2018 at 11:53 AM, Escape said:

If you need to use a fuse of 200A, chances are if something goes wrong, the damage will be done before the fuse blows. So not much point. Adding a fuse could also cause a voltage drop, something you certainly don't want in your starter circuit.

No that's not correct. If you size the wire and fuse correctly as stated above, the fuse will blow in the case of a fault.

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On 3/8/2018 at 12:08 PM, FridgeFreezer said:

We've developed a theory that fat starter cable kills starters... both myself + TSD have chunky starter cables on our trucks and both of us have killed starters. TSD managed to explode a 200TDi starter gearbox (!) and I managed to burn out a V8 starter. 50mmsq is massive, if nothing else it's a pain because it's too big to route/handle/terminate nicely.

LR use relatively small cable, the theory is over a certain length the cable has enough resistance to protect the starter from abuse and is long enough that the heat dissipation is within what the cable will cope with.

Same with LR's genuine split-charge - an un-fused 70A relay between two batteries, surely madness??? But the relay is mounted about as far away from the batteries as it's possible to get it (top of bulkhead, driver's side) so there's about 5 yards of cable round-trip. Reckon all but the worst possible case the cable would drop enough volts to limit the current + not catch fire.

Sometimes what looks like penny-pinching could just be very subtle and cunning design ;)

Plausible theory.

I have a mate who works at a local LR dealer, and he offered me a bunch of short lengths of 50mm2 attached to a MegaFuse block with a 400A fuse. He told me that these are temporarily fitted during some phase of the shipping, for some transport requirement. But this begs question, Why do they need extra protection which the end-user doesn't? 

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