Generator start leads

[memilanuk]

So... fixin' to do some electrical upgrades to our TC - LiFePO4 batteries, DC-DC charger, solar... "the usual" ??

One part that has me scratching my head a bit is the starter leads for the generator. Yes, I realize the goal is to not have to use it... but I'll feel better knowing that it's still there if I need it.

Anywho, the pickle is this: as we received the camper (2nd owners) the start leads (#2 AWG) land directly to the battery terminals - no fuses or other OC protection. My clamp-on ammeter shows 350-375 amps draw - for about a second or two. This is my first RV with an onboard generator, and our first TC, so I don't know if this is just the normal way these get wired up by the dealerships, or not.

On the one hand, that current would be considerably above the normal rating for #2 cable - if it were constant. As a result, I really don't know how you could fuse it in such a way to protect the wire, since the start current is already in excess of what you'd fuse it for to protect the wire... on the other hand, if this is 'normal', one assumes that the manufacturer knows what they're doing, going this route... but I haven't any idea what they're basing those decisions on.

Finally, and circling back to my original story... as I'm going about the process of putting in new batteries, installing bus bars (Victron Lynx distributor) and putting a new heavy duty disconnect & fuse on the + lead and shunt on the - lead from the batteries... where do I land the generator starting leads at? On the batteries, un-fused and inside the disconnect & shunt? Or on the bus bar - and fused how, exactly?

[memilanuk]

Well, until someone else actually puts a meter on the start leads a similar Onan 2500 LP generator...





Got everything back together, started the generator with the meter clamped on one of the start leads. Let it run about 15-20 minutes, then shut it down. Pulled it out, took the first pick, cleared the MAX hold, set it again, and about 15 minutes later, started it again.

[JoeChiOhki]

HMS Beagle wrote:
I'm a bit surprised by your measurement of 375A - even momentary - to start a small genset. I wonder if is it an artifact of the sampling rate of the clamp ammeter. A Cummins 5.9L diesel draws only about 300A cranking, motorcycle engines in the 1000 - 1200cc range draw around 150A at stall and 100A cranking. These measured with a 100 KHz amp probe on a 1Ghz recording O'scope. Even if it peaks at 375A for a few milliseconds, that would not heat a thermal fuse enough to blow it.

You might try shutting the fuel off or removing the spark wire from the genset, crank for several seconds and see what the starter really draws. 375A @ 12V = about 6 HP!

Yeah, that sounds like a crazy amount of juice...... My 4000 watt Champion which is an electric start from a tiny battery only has #8 wire so I know it can't be sucking near that much juice for something that's similar or smaller of an engine.....

I would think 37.5 amps would be more realistic...

[deltabravo]

MT BOB wrote:
deltabravo ..which he has.... see original post.

Perhaps you should sharpen up your reading comprehension skills, and your knowledge of Fluke meters.

He said he has a clamp on meter.

Perhaps you should learn how to quote posts correctly.

[MT_BOB]

deltabravo ..which he has.... see original post.

Perhaps you should sharpen up your reading comprehension skills, and your knowledge of Fluke meters.

[MT_BOB]

memilanuk wrote:
My mistake... it's a 36, not a 33. And yes, it's a clamp-on ammeter and yes, it does DC current as well as AC.

Yes, big difference between the 2 meters. Perhaps try the zero set procedure, then test.
They sell inline battery cable circuit breakers, up to 300A. I have them on mine, as they also work well as switches.
Ebay or auto entertainment system installers.

[memilanuk]

My mistake... it's a 36, not a 33. And yes, it's a clamp-on ammeter and yes, it does DC current as well as AC.

[deltabravo]

MT BOB wrote:
A Fluke 33 cannot measure DC amps, without using a DC current clamp add on.

...which he has.... see original post.

memilanuk wrote:
My clamp-on ammeter shows 350-375 amps draw - for about a second or two.

[MT_BOB]

No,your gen starter does not draw that kind of amps,unless the motor is locked up.
A Fluke 33 cannot measure DC amps, without using a DC current clamp add on.

[memilanuk]

Well... the meter does have a few years on it - like 25 or so. It's a Fluke 33 from 'back in the day' when I used to do motor control work in a steel mill and needed something to check start & run currents on large (several hundred horse) AC and DC motors. At the time, the price was pretty salty, but oh so worth it 😉

It did have a fresh battery, but it hasn't seen the inside of a calibration shop for a couple decades...

I've got some stuff apart right now, but I'll check the start current again once it's all back together.

[HMS_Beagle]

I'm a bit surprised by your measurement of 375A - even momentary - to start a small genset. I wonder if is it an artifact of the sampling rate of the clamp ammeter. A Cummins 5.9L diesel draws only about 300A cranking, motorcycle engines in the 1000 - 1200cc range draw around 150A at stall and 100A cranking. These measured with a 100 KHz amp probe on a 1Ghz recording O'scope. Even if it peaks at 375A for a few milliseconds, that would not heat a thermal fuse enough to blow it.

You might try shutting the fuel off or removing the spark wire from the genset, crank for several seconds and see what the starter really draws. 375A @ 12V = about 6 HP!

[deltabravo]

memilanuk wrote:
It's not really ever mentioned as such, but I think towards the end of the Battleborn video I can see some 'extra' cables that look like the generator start leads coming off the battery terminals.

Those are the generator starter leads (on the positive terminal).
The generator B- cable connects to a negative terminal post inside the camper, which is connected to the shunt. I had to ad a terminal post because there were too many B- cable to fit on the shunt.

[memilanuk]

deltabravo wrote:
I put my generator B+ back to the B+ terminal with no fuse when I switched to Battleborn batteries

When I installed my inverter and BMV-712, I put the generator B- cable on the load side of the shunt. I show the shunt wiring in this video or this one

I'm trying to do something roughly along the lines of your build, same inverter charger, bigger batteries, and hopefully less kitchen reno 😉

I'd watched all of those videos a few times... now that you pointed it out, I went back and watched them again. It's not really ever mentioned as such, but I think towards the end of the Battleborn video I can see some 'extra' cables that look like the generator start leads coming off the battery terminals.

[deltabravo]

memilanuk wrote:
where do I land the generator starting leads at? On the batteries, un-fused and inside the disconnect & shunt? Or on the bus bar - and fused how, exactly?

I put my generator B+ back to the B+ terminal with no fuse when I switched to Battleborn batteries

When I installed my inverter and BMV-712, I put the generator B- cable on the load side of the shunt. I show the shunt wiring in this video or this one

memilanuk wrote:
Yes, I realize the goal is to not have to use the genny... but I'll feel better knowing that it's still there if I need it.

I installed one of the new fangled Cummins Onan 2500i generators back in June. It's lot quieter than the old model

[mkirsch]

Engine start circuits aren't protected due to the Amps involved and the impracticality of fusing it. Just look at your truck. Battery cable runs straight to the starter.

Can you imagine in the middle of a snowstorm your car refuses to start because the fuse running to the starter popped?

[memilanuk]

I'm aware of the difference between start and run current. My point was that in this case start current is pretty much all there is, which kind of skews the traditional approach to circuit protection.