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.

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.

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

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

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.

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.

Onan Generator Cranking Amps | Good Sam Community

Ivylog
Explorer III
Sep 12, 2019
The common cable is the negative that goes to a shunt and then to ground. Once I got started I just tighten any connection I could find.
bjbear
Explorer
Sep 12, 2019
So here is the information CA Traveler was looking for.

I used a Snap-on analyzer specifically designed to measure amperage while a starter is operating.

As shown in the picture below, my generator (Onan 10 KW QuietDeisel) has a dedicated battery that is only used to start the generator.
The 12V+ lead is 4/0 wire 72" long. The ground wire is 48" connected to the frame.
At the start of the test, the battery was fully charged @ 12.7 Volts.
Outside temperature was 15 C.
During the prestart step the generator drew 6.0 Amps, with very little voltage drop off.
When the starter cranked, the amps peaked at 207 Amps, then settled at 197 Amps until the generator fired.
The battery supply voltage (at the terminals) dropped from 12.6 Volts to 10.8 Volts.
Immediately after starting, the battery recovered to 12.3 Volts.

As you may have guessed, I am a proponent of using a dedicated battery for the generator. As per a previous post, I found that even when I cleaned all my terminals from the house batteries to the generator, the generator just did not start reliably. After installing the dedicated battery, I have never had a problem. IMO...
Due to the high current draw, 30 - 40 feet of 4/0 cable just has too much voltage loss.
The house batteries are deep cycle batteries. Therefore, they are not designed to supply the 200 Amps necessary without pulling down the battery voltage significantly.
When you really need to start the generator (i.e. In the morning when you house batteries are at 50% - 12.0V --or-- during cold weather) that is also the time when you have the lowest voltage available and cannot handle further voltage drop as the starter cranks.
CA_Traveler
Explorer III
Sep 12, 2019
bjbear,

Thanks as that is the type of information that I appreciate.

I will measure some more cranking voltages and I want to know gauge wires I have from the gen to the fuse. And it must be the 300A fuse as the next biggest is 200A. From the gen the + wire is not 4/O, maybe 2 but need to check.

aA
bjbear
Explorer
Sep 12, 2019
CA Traveler wrote:
bjbear,

Thanks as that is the type of information that I appreciate.

I will measure some more cranking voltages and I want to know gauge wires I have from the gen to the fuse. And it must be the 300A fuse as the next biggest is 200A. From the gen the + wire is not 4/O, maybe 2 but need to check.

aA

Not sure of my wire size. I used whatever was there originally when it was built by Monaco.
Gjac
Explorer III
Sep 13, 2019
Has this problem always existed or is it a recent issue?
CA_Traveler
Explorer III
Sep 13, 2019
The starting has always been sluggish.
Gjac
Explorer III
Sep 14, 2019
If the starting has not gotten more sluggish over time I would back your tow car next to your genset and with jumper cables hook your starter up directly and see if that makes a difference in how it cranks. If no difference I would suspect the starter, if better I would look first at the cable ends for corrosion especially inside the cable ends(under the shrink wrap). If that looks ok what gauge wire is used from house batteries to starter? The longer the wire the thicker it should be. If ok you know from the tow car test a starting battery will work as someone else already posted.
CA_Traveler
Explorer III
Sep 14, 2019
Gjac,

Great advice and in fact I carry heavy duty jumper cables in the car.

Using the battery boost/emergency start switch improves starting and it's even better with the 200A engine alternator running. and I'm aware that can suggest battery issues. However that does not explain the 2.1V drop on the positive wire.

I will determine the wire gauge used and it appears to be 2 or 4 gauge. However it might go the the front run panel FRP and connect to a larger wire. Once I have this information and estimated lengths with the above 197/207A draw a wire calculator will provide the expected drop - good to know. Corrosion and lugs will be checked. Then I'll know how best to resolve the issue.

We're headed home and other priorities will delay further investigation.
CA_Traveler
Explorer III
Sep 14, 2019
Another testing idea: Remove the gen + wire and using a battery load tester and clampon ammeter measure the draw on the wire and also the voltage and compare that to the wire calculator. As long as I don't exceed the fuse rating it should provide a good comparison. Thoughts?
CA_Traveler
Explorer III
Sep 14, 2019
While faulty connections are certainly possible so is inadequate wiring.

I've already corrected several under sized wiring problems. Most notable was 9.1V (12.7V battery voltage) on the awning motor when retracting due to both Monaco and Carefree undersized wiring. Retraction is now reliable even if the awning structure is totally inadequate for virtually any wind. But the salesman will tell you that you have a great awning while not mentioning wind damage is not covered by warranty!

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Onan Generator Cranking Amps

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