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?

[greenno]

To the original poster. Your correct there is typically no fuse for the generator leads going directly to the battery's. Mine have #2's going back to generator box. But since I don't have a built in unit they are just safed off and not hooked up in the battery compartment.

[jimh406]

I’ll try to look soon, but I’m pretty sure there is a 12V fuse for the generator.

[memilanuk]

jimh425 wrote:

Everything is fused as 12V or 110V as far as I can tell. I have a fuse box with easily accessible fuses or breakers in the case of 110V..

I'm betting your generator start leads ain't coming from the house AC/DC distribution panel...

[jimh406]

memilanuk wrote:
The original question - in my mind, as the OP - was for people who actually have (or have had) onboard generators, about how they found them wired from the factory/dealer. Were they fused, or not?

Everything is fused as 12V or 110V as far as I can tell. I have a fuse box with easily accessible fuses or breakers in the case of 110V..

[memilanuk]

The original question - in my mind, as the OP - was for people who actually have (or have had) onboard generators, about how they found them wired from the factory/dealer. Were they fused, or not? The discussion was never intended to be about what the starting current is or isn't.

As I build out the new system, there seems to be strong recommendations to use appropriately sized fuses to protect various segments - but as found, the cables are landed directly on the battery terminals, completely inside any form of overcurrent protection.

Looking at the OEM install manual for the Onan 2500 LP generator(s)... it recommends #2 AWG for installs up to 45 ft (total round trip length). It recommends *against* using the frame for the negative return if possible, and while it calls out specific torque values for the connections, it doesn't show or recommend anything specific as far as OCPD. Also, it mentions having a battery with a minimum of 360 CCA, on up to 1000 CCA.

[Grit_dog]

So 3 pages later, I’m not understanding the issue other than OP thinks his starter may draw too much for the fancy new system being installed?
Why not just hook it up and go. 2 big lfp batteries even half dead will start a little generator. One lfp that is about the size of a spice tin with 135cca and 4ah reserve will start my 450 bike with like 12.5:1 compression.
Idk why it shows 300-400A. But unless there’s cause to think something is wrong with the starter, this seems like a non issue in search of problems

[memilanuk]

Depends on how long the duration is. Think of it like a slow-blow fuse 😉

[jimh406]

My guess is a 1250cc BMW requires a lot more power to run the starter because the compression is likely a lot higher.

I’ve jump started my Onan 2500 LP with 14 gauge wire that was 15 ft long. That was clipped to my TC battery with alligator clips and to my truck battery. So, I don’t think the Onan’s take that much to start.

No idea how much that is, but I don’t think very much or wouldn’t the wire burn up?

[HMS_Beagle]

memilanuk wrote:
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.

I'm still quite skeptical of the measurement. Even as a transient. I've measured a few small engine starters with equipment designed to record the transients and never seen anything that high. That meter was not designed to measure rapidly varying currents, and a starter motor, even when cranking at steady state has rapidly varying currents.

Here is an example of the voltage and current starting my 1250cc BMW motorcycle (100 HP 4 cylinder). Current varies between about 160 and 60 amps with each compression stroke. Either your meter is wrong or your genset starter is seriously damaged.

[memilanuk]

The LFP batteries I'm building have a JBD 150A BMS that is rated at 160A for 10 seconds, or 890A instantaneous (each). Both of them in parallel shouldn't have a problem starting the generator. One by itself... it'll be interesting to see.

[time2roll]

First make sure that LFP battery is rated for 500 amps. I would use a 350 amp Class-T from the battery to the bus. Connect the generator direct to the bus. The fuse should pass 500 amps for at least a minute, while a direct short should open the fuse before the wire melts.

[memilanuk]

ticki2 wrote:
Is the shadow on the screen hiding a decimal ?

On the 1000A DC scale there is no decimal point.

[MT_BOB]

Well,a 4k onan starter draws 100-145 amps.Could not find the specs for yours.
BUT, the proper way to check starter draw is-- disabling the fuel or ignition, and continuous cranking. Same as with an auto, although newer automotive testers are calibrated to ignore the "inrush" current. What you might have is a meter too sensitive for the job, and it might even be "seeing" that much current, but only for a millisecond. Try the meter on your truck.

[ticki2]

Is the shadow on the screen hiding a decimal ?