What Gage cable Battery to Starter

[ArtGill]

I'm replacing the air starter with a 12 vdc starter on my NJT model 20 Eagle.  The battery box is beside the starter, so the cable run should be 5 ft or less.  I have two 8D batterys with each having positive cable running to the manual disconnect switch.  Should I connect the starter cable at this point, on the input side of the switch, not the load side?

Thanks,  Art

[Sean]

Starter cables should run directly from the battery positive terminal to the starter without passing through anything else first.  It is not uncommon to use the alternator lug, though, as an intermediate terminal.

For a 12v starter on a big diesel I would not use less than 4/0.  With a 24-volt starter you could go to 2/0.

-Sean
http://OurOdyssey.BlogSpot.com

[Lin]

I thought that everything went through the main disconnect on my 5a.  Is the starter cable still hot when I shut that switch off?  I will have to test that when I get the bus back.  It could be very important to know.

[Dreamscape]

Sean, I'm having a hard time understanding your reasoning on having the battery positive running directly to the battery. My Eagle elec. schematics show the positive running to a battery disconnect then to the starter.

Our Eagle is wired according to the schematic and has always worked just fine.

Enlighten me please.

Paul

[Sean]

Paul,

I confess I am not very familiar with Eagles and so I can not speak to how or why the factory would choose to wire that way.

What I can tell you, though, is that generally speaking, main starter cables are wired directly, and with good reason:

Starters can draw immense amounts of current while operating, particularly when first engaged and especially if the engine is cold.  Any extra resistance whatsoever in a main starter circuit can cause problems, including extreme heating of the connections, increased current draw, and heating of the starter motor.

A disconnect switch of any kind adds a minimum of three and usually four extra connections in-line with the starter, each of which is a potential increase in resistance and something else to heat up.  Manual switches left in a partially engaged position can also cause arcing and exacerbate the other problems.

Usually, when working on starters and alternators (which, BTW, should never be separated from the battery by a disconnect switch), safe practice is to first disconnect the battery ground cable, and then the battery positive cable, regardless of the presence of disconnect switches.

Note, BTW, that I am talking here about the main current path from the battery to the starter motor, and not the circuit to the starter solenoid, which may well be routed through a disconnect switch.  This would prevent the coach from being started when the disconnect is open.

Lastly, a main battery disconnect switch for most coaches would be tasked with carrying at most 300-400 amps of current under normal operation (and usually far less than that) and so a switch with that rating would be adequate.  However, starters can draw several times this amount of current, and so if the main starter feed is routed through such a switch, you'd need a switch with a (non-breaking) rating of perhaps 1,000 amps or more.  This, incidentally, is also the reason starter circuits are generally unfused as well.

-Sean
http://OurOdyssey.BlogSpot.com

[bobofthenorth]

While I don't usually argue with Sean in this case I have to take issue.  The original equipment wiring on my 1981 Prevost takes the battery positive through the main chassis disconnect on its way to the starter.  Based on that single data point I'd say there's a lot of Prevosts wired that way.  We used to routinely put battery disconnects on older trucks and those went directly between the battery positive and the lead to the starter.  Older equipment tends to develop a variety of leakage paths due to years of amateur electricians working on it and the only reliable way we ever found to preserve the batteries was to put disconnects directly at the battery.  This is a fairly common practice on older equipment.

[Sean]

Bob,

Again, I am not a Prevost guy and so I can't speak to how they did it or why they made that decision, and I am sure you are more knowledgeable on that subject than I.

There is nothing inherently wrong with it, so long as the disconnect switch is properly rated for the starting load, and the size and nature of all connection points is adequate to ensure minimal resistance in the circuit.  I would assume that, if it was designed and installed by the factory, then such conditions should already be met.  I will say, though, that if you have such a setup, then tightening the connections on the switch, and cleaning corrosion, pitting, and deposits from the contacts, should be added to your PM routine.

What I can not tell from the OP's question, given that he has an air-start coach, is whether any such switch that he already has on board is properly sized and constructed for a starting load, and so I stand by my original recommendation.

-Sean
http://OurOdyssey.BlogSpot.com

[luvrbus]

Yep most Eagles have a cut off  for each battery.The NJT had a cutoff for each system as they had both 12 and 24 V.All  the Cat equipment is cut off at the batteries then goes to the starter.
All power leaves from the starter on Prevost and Eagles don't now about Seans bus 



good luck

[Sean]

Addendum to my previous:

Notwithstanding what I said about factory-designed and installed disconnects probably being fine, I would expect that they break only load circuits from the batteries (whether or not that includes the starter), and never the connection between the batteries and the alternator.

As I wrote two posts ago, the batteries and alternator should never be separated by a switch.  That's because inadvertently opening the switch while the alternator is running could, in fact, destroy the alternator.

So, while I understand the reasons why Bob says he used to routinely put disconnects at the batteries, I have to strongly advise against this particular practice.  At minimum, the battery to alternator path should be direct, continuous, and unswitched, and any switch should be installed between this pathway and the loads.

FWIW.

-Sean
http://OurOdyssey.BlogSpot.com

[Sean]

...
All power leaves from the starter on Prevost and Eagles don't now about Seans bus 

On my coach and, AFAIK, most Neoplans, the battery positive is wired to a terminal post, and from there one cable goes to the alternator, which in turn is wired to the starter (so both the alternator and starter lugs are always hot), and another cable goes to the main disconnect switch and from there to the loads.

So on Eagles is the alternator output also wired to the far side of the disconnect from the batteries?  That would seem like a poor choice.

-Sean
http://OurOdyssey.BlogSpot.com

[bobofthenorth]

I understand the risk you are referring to Sean but given the application I think the risk of a completely discharged battery is likely quite a bit more serious on our older vehicles.  In the case of my coach I just had a look at the switch and the service manual and both indicate that the current situation is also the way it shipped from Prevost.  When the disconnect is off everything is disconnected - absolutely everything.

In the case of the field installed disconnects that we (and a large portion of the population of old truck users) used they are typically installed somewhere close to the battery case so that a short piece of cable can connect them to the positive post.  I guess in theory some idiot could walk by a running truck and disconnect one of them - and our staff complement always seemed to have its share of idiots.  All I can say is that we routinely lost batteries due to discharge and never to my knowledge lost an alternator due to an idiot performing that specific action.  Other idiot-induced actions .... all bets are off.

[Sean]

I understand the risk you are referring to Sean but given the application I think the risk of a completely discharged battery is likely quite a bit more serious on our older vehicles. ...

Absolutely agreed, Bob, and so long as there is a significant load present (for example, headlights on), opening the disconnect in such a circumstance is unlikely to do damage.

But can we all also agree that (1) replacement batteries, should they remain connected and discharge completely, would cost perhaps $200 while a replacement 50DN alternator would cost between five and ten times that amount and (2) since we're here discussing it, and people are asking how to do it, that we should be recommending "best practice" rather than "expedient"?

In which case, I will again stand by my recommendations:

1.  The alternator and the batteries should be directly connected, without an intervening disconnect switch.

2.  There should, indeed, be a disconnect and it should be used any time the coach is stored, but it should be "downstream" of this alternator-battery connection, and

3.  The starter should either be connected directly to the batteries without a switch, OR any such switch should be (a) sufficiently constructed to handle the load and (b) properly maintained on a regular basis.

And, just to be clear, as far as I can see, there should be no cost difference to having the disconnect after the alternator connection rather than before it.  It's a simple matter of on which side of the switch the alternator feed is connected.

As long as we are discussing it, I should point out that many of us have 50DN alternators with external Delco-Remy (or compatible) regulators.  These regulators have a "battery sense" lead, which, per manufacturer's recommendations, should be connected directly to battery positive.

IF you choose to connect the alternator output to the non-battery side of a disconnect switch (contrary to my recommendations above), THEN you should also connect the sense lead to the same point.  Otherwise you risk, in the event the switch is inadvertently opened, the regulator not seeing what the alternator is doing at all, thus increasing the possibility of a "runaway" and alternator damage.

FWIW, YMMV, JMO, etc.

(BTW, to get back to the OP's question, again, absent specific information about the rating and construction of any existing disconnect switch, I would recommend direct connection between the battery and starter.)

-Sean
http://OurOdyssey.BlogSpot.com

[Lin]

Again, I do not have the 5a home to be able to test, but I do believe the starter goes through the main switch.  As I remember it though, the alternator is direct to the battery.  These are certainly things I will confirm.

[Gary '79 5C]

Lin,

On my 5C, both alternator & starter are downstream of the shutoff switch. Having replaced the alt. & rebuilt starter, I cut the power with the switch and not pulling a batt cable. IIRC.

Have a great day.

[Dreamscape]

Sean, Thanks for you explanation.

I'm leaving mine the original way, it's worked thus far.

Paul