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 > Onan Generator Cranking Amps

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CA Traveler

The Western States

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Posted: 09/11/19 04:19pm Link  |  Quote  |  Print  |  Notify Moderator

ivylog,

I'll check that out. My inverter is mid ships and the the + and - 4/O wires are routed to the rear, most likely terminated on the common frame ground lug next to the batteries and the RRP. The gen + wire is smaller and may go to the FRP or the RRP, not sure yet. So there are connections to check but my gen and inverter appear to have separate wiring, but I can't yet eliminate some common wiring yet.


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Ivylog

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Posted: 09/11/19 08:41pm Link  |  Quote  |  Print  |  Notify Moderator

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.


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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bjbear

Cold Lake, AB, Can

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Posted: 09/12/19 01:21pm Link  |  Quote  |  Print  |  Notify Moderator

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...
  1. Due to the high current draw, 30 - 40 feet of 4/0 cable just has too much voltage loss.
  2. 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.
  3. 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.


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CA Traveler

The Western States

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Posted: 09/12/19 04:21pm Link  |  Quote  |  Print  |  Notify Moderator

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

Cold Lake, AB, Can

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Posted: 09/12/19 05:02pm Link  |  Quote  |  Print  |  Notify Moderator

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

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Posted: 09/13/19 06:10am Link  |  Quote  |  Print  |  Notify Moderator

Has this problem always existed or is it a recent issue?

CA Traveler

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Posted: 09/13/19 07:33am Link  |  Quote  |  Print  |  Notify Moderator

The starting has always been sluggish.

Gjac

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Posted: 09/13/19 08:33pm Link  |  Quote  |  Print  |  Notify Moderator

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

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Posted: 09/14/19 09:14am Link  |  Quote  |  Print  |  Notify Moderator

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

The Western States

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Posted: 09/14/19 09:21am Link  |  Quote  |  Print  |  Notify Moderator

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?

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