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 > Onan Generator Cranking Amps

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CA Traveler

The Western States

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Posted: 09/10/19 01:21pm Link  |  Quote  |  Print  |  Notify Moderator

What is the expected cranking amp draw for a Onan 10K generator?

Mine is cranking slow due to excessive voltage drop on the + wire and I want to estimate what the voltage drop should be for the wire size and length. The - side uses frame ground is is OK.


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Posted: 09/10/19 02:02pm Link  |  Quote  |  Print  |  Notify Moderator

My 4KY uses about 100A, very roughly speaking, when cranking. The initial surge is somewhat higher.

If it cranked well in the past, the wire size is presumably not the issue, but rather some poor connection somewhere. It may also be worth checking the starter motor to make sure all the brushes are properly functioning; I had to replace my generator starter a few years ago after it burned out, and discovered that for as long as I had owned it, during which time it was rather lethargic and marginal at starting the generator, one of the four brushes was not working properly and so the starter was only half working. Unlike many motors it has two sets of poles around the rotor and four brushes. Of course, it's entirely possible that your starter is a different design since it's a different generator.





bjbear

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Posted: 09/10/19 02:32pm Link  |  Quote  |  Print  |  Notify Moderator

CA Traveler wrote:

What is the expected cranking amp draw for a Onan 10K generator?

Mine is cranking slow due to excessive voltage drop on the + wire and I want to estimate what the voltage drop should be for the wire size and length. The - side uses frame ground is is OK.


Looked through my generator info and could not find the actual amp draw for the starter.

When I bought my coach, the generator cranked slowly and often would not start at all. Even with the "boost" switch pushed to combine house and chassis batteries, it was a reluctant starter.

I went through the entire 12V supply and cleaned all connections at the battery plus a post on the front wall. It helped a bit, but was not a satisfactory solution. If I remember correctly, the 12V supply fell to 11.5 Volts when cranking.

It is a very poor design to use house or chassis batteries located in the rear to start the generator at the front. Not only because of the long distance with resulting voltage drop, but because, often when you need the generator is when you have run your batteries down to 50% or lower. Just no enough power to start the generator reliably.

What I ended up doing was to install a dedicated battery at the generator and using a diode, charge it from the house system.
[image]

My generator now starts quickly. Even if my house batteries are down, I don't need to worry that my generator will not start.


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garym114

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Posted: 09/10/19 03:55pm Link  |  Quote  |  Print  |  Notify Moderator

Voltage Drop Calculator is at the bottom of the page.
http://www.powerstream.com/Wire_Size.htm


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Ivylog

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Posted: 09/10/19 04:27pm Link  |  Quote  |  Print  |  Notify Moderator

I had similar issues and was considering adding a diode and another battery...bear with me as the size of the cables is not the problem. Check the voltage at your batteries compare to the reading on your inverter while running the microwave...takes 2 people. I had a little over 1.1V difference so cleaned and tighten all the connections I could find including those in the back run box....just tighten them. On your negative house battery cable you have the shunt that measure your amps and cleaning them did the most good. I now have 0.4V difference which was my goal and as a side benefit my 10K will start without using the boost or cranking the engine.


This post is my opinion (free advice). It is not intended to influence anyone's judgment nor do I advocate anyone do what I propose.

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CA Traveler

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Posted: 09/10/19 05:30pm Link  |  Quote  |  Print  |  Notify Moderator

Thanks for the great input. I did some checking at 60F and while cranking I have 0.2V on the negative gen wire which is connected to the frame and 2.1V on the positive wire resulting in about 10.2V at the gen terminals and it will start barely. The gen is on the house batteries and they are in good condition and were 75% SOC. At 50F it MIGHT start with 100% SOC.

I plan to check the entire + wire to determine exactly where the loss is. But in the meantime I wanted to verify the expected drop IF I can determine the amp draw and verify the wire gauge. And a better guess as to length. The wire runs to the very back panel to the fuse area and then a 4/O wire to the 4xGC2s in the next panel forward.

I've already replaced several non related wiring areas where Monaco used undersized wire. So that is a possibility or MAYBE I'll find a loose connection or wire lug.

And I've been considering a battery next to the gen and have room.

I'm traveling and can't do further testing for awhile.

CA Traveler

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Posted: 09/10/19 05:33pm Link  |  Quote  |  Print  |  Notify Moderator

BTW I'm estimating that the starter amp draw is in the 150A range. I don't have a analog meter to measure. I will try the clampon but wondering if the electronic sampling delay will give false readings. [emoticon]

Ductape

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Posted: 09/10/19 06:38pm Link  |  Quote  |  Print  |  Notify Moderator

I expect it’s a lot more than that. Cummins spec for starting batteries:

475 amps down to 0° F (?17° C) 650 amps down to ?20° F (?29° C).


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CA Traveler

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Posted: 09/10/19 10:49pm Link  |  Quote  |  Print  |  Notify Moderator

Ductape wrote:

I expect it’s a lot more than that. Cummins spec for starting batteries:

475 amps down to 0° F (?17° C) 650 amps down to ?20° F (?29° C).
I saw that but couldn't make a correlation between CCA at those temps and the actual amp draw. You may be right.

Ivylog

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Posted: 09/11/19 08:33am Link  |  Quote  |  Print  |  Notify Moderator

This is a common problem on Monaco/HRs, especially in the morning after having to run the heat all night and the house batteries are down around 12.0V. My SWAG on amps is after the initial surge it’s less than 300A BUT in decreasing my voltage drop between the inverter and the batteries I solved my generator cranking problem.

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