Question about Onan Generator

[Son_of_Norway]

Something I have never understood about generators (I have an Onan 6.5 Emerald III.) A 12-volt DC battery is used to start the generator, which then produces 120V AC. The battery supplies the energy needed to run the generator, right? So, while the generator is running, does it send a 12-volt DC current back to the battery to keep it charged, or does the generator just continue to drain the battery until the battery no longer has enough voltage to keep it running?

So if the generator drains the battery, then there must be a converter in the circuit to keep the battery charged, just like the alternator on a car engine. OK, I'm no electrical engineer, I just want to be sure that I understand this correctly.

Thanks for your tolerance on this post.

[MrWizard]

wopachop wrote:
I was cycling my onan5500 a couple days ago. Store my batteries in the garage.

Pulled out battery to start the genny. Ran for about 55mins. Genny still running I disconnected the negative cable and it died instantly.

Also had my 12v disconnect switch turned off.

Made me wonder the dealio. If the disconnect switch was turned on, would the converter then power the fuel pump?

Will have to test that next month. The fuel bowl was filled with gas so I guess it wasnt a fuel pump issue but instead a "making spark" issue?

If the disconnect was in use with the Onan running !
You were powering the generator directly from the battery with out charging it from the converter
When you removed the negative cable you removed 12v control, spark, and fuel pump
And left your battery in a partially discharged state

[Chum_lee]

Many RV's have an "AUX BAT" or auxiliary battery switch somewhere on the dash. The purpose of this switch is to temporarily connect the house batteries to the chassis battery(s). When you press this momentary contact switch, provided there is adequate voltage/current available somewhere in the system to operate the relay, you now have the benefit of ALL the system batteries linked together for the purpose of starting the coach engine, or the generator. Often this is enough "juice" to get you out of a jamb.

As others have said, the generator will automatically charge the house batteries through the battery charger built into the converter when running, but generally, it will not charge the chassis battery unless the "AUX BAT" switch is pressed. (not recommended for long term charging)

That said, I often hook up a separate 12 volt (automotive type) battery charger to one of the house 120 Volt outlets to top off the chassis battery if/when the generator is running.

When you start the chassis engine, generally, a few seconds after starting, you will hear another relay engage which now connects the chassis and house batteries together so that you can charge all the batteries while underway using the main engine alternator. (it takes a long time/drive to do this if the house batteries are low)

Chum lee

[DrewE]

wopachop wrote:
I was cycling my onan5500 a couple days ago. Store my batteries in the garage.

Pulled out battery to start the genny. Ran for about 55mins. Genny still running I disconnected the negative cable and it died instantly.

Also had my 12v disconnect switch turned off.

Made me wonder the dealio. If the disconnect switch was turned on, would the converter then power the fuel pump?

Will have to test that next month. The fuel bowl was filled with gas so I guess it wasnt a fuel pump issue but instead a "making spark" issue?

As it died instantly, it's most likely the control board for the generator shutting it down (by killing the spark). That would be a sensible thing to happen, either specifically by design or just because it's no longer powered so ceases to operate.

The 12V system powers the fuel pump, etc. on the generator. If the battery disconnect switch is in the "use" state, and if your converter is on the house side of the switch, then it would need to be on for the generator to (indirectly) charge the battery and not have it run down from the generator's 12V consumption. If the converter is on the battery side of the disconnect switch, then of course the state of the switch doesn't matter. I guess you could say that the converter would power the generator's 12V needs, but it's just part of the 12V system and providing power into that system as a whole.

In general, there's little reason to ever have the disconnect switch disconnected that I can work out; the main one is perhaps when doing maintenance on the wiring or batteries.

(The generator's 12V connection doesn't go through the disconnect switch because it requires a lot of current to crank it; somewhere in the vicinity of 80-120A on my motorhome, if my little digital panel ammeter is to be believed.)

[wopachop]

I was cycling my onan5500 a couple days ago. Store my batteries in the garage.

Pulled out battery to start the genny. Ran for about 55mins. Genny still running I disconnected the negative cable and it died instantly.

Also had my 12v disconnect switch turned off.

Made me wonder the dealio. If the disconnect switch was turned on, would the converter then power the fuel pump?

Will have to test that next month. The fuel bowl was filled with gas so I guess it wasnt a fuel pump issue but instead a "making spark" issue?

[pnichols]

Son of Norway wrote:
Something I have never understood about generators (I have an Onan 6.5 Emerald III.) A 12-volt DC battery is used to start the generator, which then produces 120V AC. The battery supplies the energy needed to run the generator, right? So, while the generator is running, does it send a 12-volt DC current back to the battery to keep it charged, or does the generator just continue to drain the battery until the battery no longer has enough voltage to keep it running?

So if the generator drains the battery, then there must be a converter in the circuit to keep the battery charged, just like the alternator on a car engine. OK, I'm no electrical engineer, I just want to be sure that I understand this correctly.

Thanks for your tolerance on this post.

I don't understand the why of your question ... and some of the responses to it.

Our motorhome's built-in Onan generator is started by power from the coach batteries ... so however the coach batteries are kept charged is also ensuring that they are kept charged up enough to always start our generator.

Aren't all motorhome (and some towables') built-in RV generator setups similar to this?

If so, of course a follow-on question of yours could be "... then how are RV coach batteries kept charged up?"

[dougrainer]

Gas and Diesel RV Gensets do NOT charge the starting Genset battery. They have NO built in charger. So, yes, if you do not have the system set up to charge the Genset Starter Battery it will slowly deplete the battery as it takes 12 volts to run the Genset once started. Almost ALL Gas Gensets in motorhomes are started by the Coach batteries which will be charged by the On Board Power Converter or Inverter/Charger, which ever you have, by the 120 supplied by the Genset. Doug

[MrWizard]

I think !
Everything from the 1990's up
Rv generator depend on the house converter to keep the battery charged and supply the 12v control power, fuel pump etc
And do not charge the battery
Earlier models 60s and 70's did have a 12v supply circuit as do residential models

[fyrflie]

My battery for my generator is the same battery that provides power to my coach and receives a charge from the converter/ charger when the generator is running or when plugged into 120v.

[azrving]

The generator has a stator under the flywheel. It produces AC current which is sent to a regulator/rectifier. The rectifier changes the ac current to DC current which charges the DC battery. The regulator portion controls the DC voltage level to the battery.

[enblethen]

Yes, it sends back a trickle of charge. Main charge comes through the converter battery charger that your coach battery is connected.

[jdc1]

Your battery is being charged by the same converter/charger your coach is. That generator battery only starts the generator...it needs no other source to keep it running....like a lawnmower motor.