[grobb284]

I have read the MADD discussion for a battery in the trunk of isolating the starter feed wire with a solenoid, so that the starter feed wire is only hot when cranking. There is then a second wire of large size, to charge the battery from the alternator, run clear to the trunk.

I'm thinking that just use just the extremely large wire for the starter feed wire, eliminate the large wire for the charge wire clear to the trunk, instead using current limitation on the starter feed both for the starter and battery charge.

If I would use say a 150 amp fuse at the battery, and a 100 amp fuse at the alternator, I could use the same very heavy cable for starting as for charging. Why couldn't this be done?

The whole idea is to current limit the battery from exploding, when there is a direct short of the cable, such as in an accident. Ths would probably take care of that.

The other concern is fire, due to shorting of the cable. By current limiting the starter cable, would it not be effectively the same as the charge cable of MADD?

The idea of the 150 amp on the battery side is that the on a warm day, the 150 amp fuse is sufficient for the gear drive starter (50 amps) and the ECM/ignition/fuel pump/injectors.

The amps for the starter may appear low to you, but this was the draw from a factory GM gear drive starter in Model Year 2002, to start an LS1. I also understand the inrush is higher, but it is of extremely short duration.

Why wouldn't this work satisfactorily?

[vicrod]

150 amps may be fine when all conditions are perfect for cranking.
But batteries are commonly rated for CCA (cold cranking amps) of 800 amps or more.
Personally I like to have all the CCA I can get whenever needed for cranking. Size the cable from the trunk to the starter at least 1/0 copper.
Install a solenoid at the battery to be energized only while cranking (optional).
Run another fused wire #6 or larger, fused at the battery to handle all other loads and charging.
Optionally this #6 wire could have a disconnect switch at the battery.

vicrod

[grobb284]

That's is right, many batteries have tremendous capacity today.
However, that is what it is, capacity, as no normal starter would draw 600 amps. You did not say that of course; we would both agree that amp draw is whatever the components are drawing, not the battery capacity. That is why many of the LS style motors are started with small capacity Braille batteries.

That being the case, an LS motor with gear drive starter, would either fire immediately or not at all. There is no extended cranking like a carb motor with a choke, unless there is something wrong with the FI LS motor.

We could change the proposal for using the extremely large starter feed as the charge wire by putting a starter solenoid parallel with the proposed 150 amp fuse, that would only engage when starting. Then reduce the 150 amp fuse to 100 amp, same as the one at the alternator.

By having the wire (cable) fused at both ends the same amount, it would not matter if the cable were shorted, both power sources would be protected by 100 amp fuses. Also, by having the battery fuse paralled with a starter solenoid, the cable would essentially be to full capacity when cranking, which would address the problem of a higher current nuisance trip. Thanks for making me think about this a little more, this might be better.

[malc]

I have my battery in the trunk installed entirely with MAD equipment.
Do it as he instructs, trust me he knows.

[ogre]

if you want safe; do as the mad site says.

i've always been told that the starter is the only thing on a car you cannot fuse and that a starter will take everything a battery offers to do a cold start.
the reason mad says to use the separate charge wire and the solenoid on the start wire is to eliminate the 12 foot long un-fused hot wire to your electrical system.
if you install a 150 amp fuse, and you get a short in the battery lead, you will get fire before the fuse blows.

we install remote batteries all the time without the solenoid and charge wire because the customer doesn't want the added expense.

personally: i would use the method that madelectric recommends. i would use the solenoid on the start cable and a fuse or fusible link on the feed to your alternator and fuse block.

[grobb284]

Thanks for the input.

I'm unsure why some don't respond to the option I discussed on post 3. This goes one better than the MADD proposal. I understand how and why the MADD system works.

This eliminates the seperate return wire for charging the battery, fuses it at both ends to act like a #8, and allows full current at start.

What am I missing here?

[malc]

Originally Posted by grobb284

Thanks for the input.

I'm unsure why some don't respond to the option I discussed on post 3. This goes one better than the MADD proposal. I understand how and why the MADD system works.

This eliminates the seperate return wire for charging the battery, fuses it at both ends to act like a #8, and allows full current at start.

What am I missing here?

You have to think about the direction of the current when cranking the motor.
Battery to starter................and alternator.

[grobb284]

The fuses as well as the solenoid in parallel with the battery fuse adress that.

Would a diagram help explain this? I've not heard specifics as to why this isn't viable, other than that's not the way it's done by MADD.

[malc]

From the MAD catalog...........

The starter motor cable is the only electrical circuit that cannot be protected by a fuse, circuit breaker, Fusible Link, or other short circuit protection. (Because starters can draw all the current that a battery can put out, so the battery cannot put out enough power to “trip” a protective device with high enough capacity for starter motors. All other circuits can be protected, and Fusible Link wires are best for the heavy-duty circuits.)


The fuses as well as the solenoid in parallel with the battery fuse adress that.

A fuse does not dictate direction of current flow.

[grobb284]

[QUOTE=malc]From the MAD catalog...........

The starter motor cable is the only electrical circuit that cannot be protected by a fuse, circuit breaker, Fusible Link, or other short circuit protection. (Because starters can draw all the current that a battery can put out, so the battery cannot put out enough power to “trip” a protective device with high enough capacity for starter motors. All other circuits can be protected, and Fusible Link wires are best for the heavy-duty circuits.)

I will be pleased to make a drawing of what was mentioned on post #3, to address this, and I think at that point all this will be moot.

Starters do not draw all the current that a battery can put out, otherwise they would be fully discharged at every start. A smaller battery of less CCA can sometimes suffice for a battery that is larger. That would not be true according to what you have copied. They (starters) only draw what they need according to load, as long as the supply is sufficient, and the motor design capacity is not exceded.

Having said that, what I proposed was a starter solenoid around the 100 amp battery fuse, and will make a drawing of such.

[Joe G]

I believe your idea from post #3 about running a mega-fuse parallel with the rear starter solenoid would work to eliminate the need to run a heavy gauge feed/charge wire back to the battery/solenoid.

[75gmck25]

The rationale behind the MADD wiring scheme is to have an unfused 1/0 wire that is only hot when cranking, along with a charge/power wire that is only about 8 gauge, and fused appropriately. The 1/0 wire provides full power to the starter when needed, but the risk of a short is reduced because its only powered when cranking. The 8 gauge wire is large enough to provide fused power for everything else in the vehicle.

There is nothing wrong with your basic wiring scheme, but you will have all power depending on the fuse in the large wire (150 amp was what you mentioned). Any problems with that wire may blow the fuse and you will lose all power to everything. And you don't save that much on wiring (about 20-25 feet of 8 gauge wire).

If you like that wiring scheme, then use it. However, I would replace the 150 amp fuse with a self-reseting circuit breaker. That will restore power if you happen to overload the circuit when cranking.

Bruce

[grobb284]

Looking at this drawing: With the relay engaged, the starter can be engaged, with the relay open the charge circuit works. Assume for discussion both fuses are 100 amp or less, whatever the max. alternator output under system load.

If the relay that bypasses the fuse comes on before the starter engages, and shuts off after the starter completes, then the the large cable can be used for starting.


The two fuses, when the relay is open, can protect the line both for the battery output, and the other fuse the alternator output. Using the very large cable for charging.

This would eliminate the second wire, and would protect it as a charge cable. Also would permit large current to the starter when the relay engaged, should the load require it, for the few seconds to start.

This represents what I spoke of in the third post.

[Joe G]

Originally Posted by Joe G

I believe your idea from post #3 about running a mega-fuse parallel with the rear starter solenoid would work to eliminate the need to run a heavy gauge feed/charge wire back to the battery/solenoid.

^^^ Please disregard my previous post ^^^

After some experimentation, even though your idea seems completely logical, I believe there may be a problem with it...here's what I found:

The problem is resistance. At least in the experiments I did this evening, the resistance across the solenoid contacts is over ten times greater than the resistance across the fuse, and of course electricity likes to follow the path of least resistance, so even with the solenoid activated, the majority of the electrical current will still flow through the fuse.

I set-up the circuit on a bench with a brand-new solenoid, 175A Mega Fuse, a fully charged battery, and equal sized wire all around. For safety, I used a small load - a 6017 sealed beam headlamp with both elements energized. The total load once the bulb was warmed up was just short of 8 amps according to my inductive milliamp probe (which has proven to be very accurate).



When I triggered the solenoid to "bypass" the fuse, the current carried by the fuse went from 8 amps down to 7.3, meaning that the solenoid was only carrying 0.7 amps or about 9 % of the load, and the other 91% of the load was still passing through the fuse.



If you scale those numbers up, a real problem becomes apparent; if the starter drew 165 amps or more, your 150 amp fuse could still blow even with the solenoid bypassing it.

I am not an electrical engineer, and there could be other factors affecting my results that I am not aware of, so take this with a grain of salt. You might want to experiment with this yourself.

Good Luck.....

[grobb284]

Those are some good points, I had neglected the resistance of the contacts. Thanks for both bringing it up, as well as doing some investigation.


My hat is off to you.