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For Bob Ewing

Yahoo Message Number: 2017
Hey Bob,

Educate me on why there is a difference between 120vac "Shore" and 120vac from your generator. I do not doubt you, I see it with Power Tech and Onan taking 6 hours of generator run to bring batteries to "Float" and 2 1/2 hours on shore power. Same charge from the inverter, 30a per leg on the generator, 60 cycles.

Fred 1998 Intrigue

Re: For Bob Ewing

Reply #1 –
Yahoo Message Number: 2020
Hi Fred:

I have no idea, but I've been told by Onan and others that engine driven generators do not put out like shore power. Run your generator long enough to be solidly in Accept(about 12.8 volts). That type of charging is the only reasonable way to go. Start gen at 12V and shut down at 12.8 or 9. It will take many hours to 13.6V. Not worth it!!!

We dry camp 50-60% of the time, and know this to be true.

Bob Ewing
#6162

For Bob Ewing

Reply #2 –
Yahoo Message Number: 2026
Hey Bob,

We need to have electrical engineers involved if they are out there.
There are three CCs out there that take 4 to 6 hours of generator time to put the batteries into float. Mine will take about 2 hours. I have a 7KW Power Tech and a 2000W Heart Inverter. The generator people might misunderstand and think the house batteries are charging off the generator alternator??? For the life of me I cannot see where there should be any difference in charge time between "Shore" and "Generator". The inverter when in "Charge" mode does not care where the AC power is coming from. I would like to have my curiosity satisfied on this one. I have timed the charge time on a 1999 Concept. 4 to 6 on generator, 2 1/2 on House AC. Someone 'splain it.

Fred 1998 Intrigue Montage 10522

Re: For Bob Ewing

Reply #3 –
Yahoo Message Number: 2028
Hi Fred;

Maybe I've not written what I meant. I have auto genstart set to start at 12V, the generator automatically shuts down at 12.9V this takes 1-2 hrs depending on the load I'm using at the time. I cannot say that shore power would be any faster. Onan says and CC concurs that the generator is at it's best in the above range. Maybe shore power is faster, I don't know. We have one trip we make to the Panama Canal that we dry camp 70-75 straight days. Power mgmt is very important to us.

02 Magna Resort 6162
Bob

Re: For Bob Ewing

Reply #5 –
Yahoo Message Number: 2030
Don't know your name

I am waiting till we arrive for the Class Reunion to discuss with Bryan Keys the matter of solar. While in Ga Bryan told me of a new panel by Siemans, but I've lost the info.

Thanks

Bob Ewing

bobbooewingtoo@...

Bob Ewing

Reply #6 –
Yahoo Message Number: 2031
Hi Bob,

I think we will give this a rest. Hopefully, someone will read the Posts and tell us why it takes longer to charge the batteries from the generator than from shore power.

Fred Sage 1998 Intrigue 10522

Re: Bob Ewing

Reply #7 –
Yahoo Message Number: 2035
someone will...tell us why it takes longer to charge the batteries from the generator than from shore power....
The alternator is referenced to the chassis battery which is charged most of the time. The battery charger, which is powered by shore power or the generator, is referenced to the coach battery which needs charging most of the time. jerry in NM.

Charge times, sine waves, RMS voltage and all that jazz...

Reply #8 –
Yahoo Message Number: 2038
Pay attention, there may be a test. Utility power is advertised as 120 VRMS at 60Hz. Generators are specified similarly. So what is the difference? The answer is the shape of the waveform.
Utility power has a waveform close to sinusoidal, a nice smooth waveform.
When the voltage of the "hot" line with respect to neutral (neutral is always very close to ground potential in a properly functioning system) is plotted against time, a repeating pattern is seen. 60Hz means 60 cycles per second in this situation and thus the pattern repeats 60 times a second or every 16.67 milliseconds (mS or thousandths of a second). In electrical engineering, one cycle is equated to one revolution or 360 degrees. I'm sure you still have your trigonometry table from school so you can easily plot a sine wave to see what it looks like by assigning the time related element (degrees of rotation) to the horizontal axis and the sine of that angle to the vertical axis. Lo, a waveform that crosses zero twice per cycle (0 and 180 degrees), has a positive peak at 90 degrees and a negative peak (or valley) at 270 degrees. A beautiful thing, seriously it is very smooth and just looks nice!

Now for RMS. This is an acronym for Root-Mean-Squared and refers to the manner in which the effective voltage of a sine wave is calculated. Due to power being proportional to the square of the voltage it is trickier than simply calculating the average of the voltage. It turns out, that for a true sinusoidal waveform, the peak values must be the square root of two larger than the effective voltage required. Thus, the 120VRMS provided by the utility has peak positive and negative values of 1.414 x 120 volts or a little under 170 volts peak. An electric heater plugged into 120 VDC (say 10 12 volt batteries in series) would produce the identical heating as when connected to a 60 Hz sinusoidal waveform with a peak value of 170 volts. So far, so good.

Battery chargers are not rocket science, in fact they are pretty much brute force devices that can easily (and inexpensively) be designed to match battery needs fairly well. The open circuit voltage of a typical battery charger is often around 17 volts. The open circuit voltage of a somewhat discharged battery may be 11 to 12 volts while a fully charged battery might indicate 13.5 to 14 volts. A battery has internal resistance and so does a battery charger. When a battery is connected to a charger the resistances are put in series with each other and are effectively added together and act to limit the charge current. When the battery is first put on charge the voltage difference may be 17 - 11, or 6 volts and this voltage drives a fairly large current through the combined circuit resistance, charging the battery at a fairly high rate. As the battery charge is replenished, the terminal voltage increases such that at 14 volt the voltage difference has been halved and the charging current diminishes accordingly. At the full charge point, the charging current is reduced to a "trickle" and there is no further increase in the battery voltage.
How is the open circuit voltage of the battery charger maintained at 17 volts? Very simply. In its simplest for, a battery charger is no more than a transformer and a rectifier. The transformer "transforms" the 120 VRMS to about 12 VRMS and the rectifier effectively inverts the negative half cycle to produce a series of positive half sinusoid at the rate of 120 per second.
And, guess what, the peak voltage of these voltage pulses is 1.414 * 12 = 17 volts. For those who are curious to know why the charging current is not in fact proportional to the difference between the terminal voltages, consider how the charge period during each of these half cycles widens when the battery voltage is low and becomes narrower as the voltage increases.
Now for the tricky part. RV generators are purchased from the lowest bidder and are not designed or built to produce utility company quality waveforms.
Instead, a typical generator waveform will contain a large amount of "harmonic distortion" which is to say that along with the fundamental 60 Hz waveform it is simultaneously producing 120, 180, 240... Hz waveforms in various proportions (2nd, third, fourth... harmonics). Of these, the third harmonic is usually the largest and, in the battery charger concept, the most significant. The effect of the third harmonic (indeed all odd harmonics) is to reduce the waveform peak and "square off" the waveform generally. (Plot this if you want to confirm it). Thus, the elegant sinusoid is reduced in amplitude and "fattened up" some by the third harmonic. In the extreme, it can be shown that by adding odd harmonics in the correct proportion, a square wave can be produced. However, for a square wave (a fixed positive voltage for a half cycle followed by a fixed negative voltage for the second half cycle and repeating ad infinitum), the peak value required (the "steady voltage in this case) will be only 120 volts. This will produce exactly the same heating effect as the 120 VDC situation mentioned above and, on reflection, what else would you expect? So, here's the deal. To prevent the generator from prematurely destroying 120 volt appliances and light bulbs in the vehicle, the output has to be set no higher than 120 VRMS. Due to the somewhat squared off waveform, resulting primarily from the high level of third harmonic, the peak voltage is set at less than 170 volts. 10% of third harmonic would require the peak voltage to be as low as 157 volts for example.
Back to the battery charger. Even with 6% of third harmonic in the generator waveform, the open circuit voltage of the battery charger will be reduced by 1 volt. While this has little effect when connected to a discharged battery (the charger puts out well here since the reduced voltage is somewhat offset by the "fatter" waveform), as the battery voltage rises, the generator waveform is much less able to "complete the job" than the utility waveform that is producing an additional volt.
So, is everyone now more confused than before? As my kids say, never ask him the time, he'll tell you how to build a clock!
By the way, there was a flurry of posts a while back concerning burned out rope lights and the question was asked as to how the life of a lamp is related to the applied voltage. It turns out that the life of an incandescent lamp is proportional to the reciprocal of the 13th power of the applied voltage, that is 5% over voltage will reduce the life by 47% while 10% over voltage will result in an alarming 71% reduction. This is why you will find 130 volt lamps at the hardware store for use in security lighting or other situation requiring extended use. A 130 volt lamp on 102 volt circuit should last about 2.8 times the normal life.

Peter

Britannia Inc.
800-274-5245

2000 Intrigue 11066, GotR

Re: Charge times, sine waves, RMS voltage and all that jazz...

Reply #9 –
Yahoo Message Number: 2039
Wow Peter, I wish I was in your class! I remember learning all of this in college... just didn't retain it!

Re: Charge times, sine waves, RMS voltage and all that jazz...

Reply #11 –
Yahoo Message Number: 2048
Peter! Hello! How are you today? I have sort of an idea of what you were telling us in your message, but could you PLEASE say it again in Laymen's plain English? Please? It would help a bunch of us out; I'm sure. Thank you for trying to answer the question," why does it take longer to charge batts......................by gen, than shore?"
 

Re: Bob Ewing

Reply #12 –
Yahoo Message Number: 2188
The Heart charging circuitry (as with others) is heavily dependant on 'peak' AC voltage. Peak AC voltage will vary significantly on generator power depending on the load because the generator is a limited resource. Peak AC voltage on shore power is generally stable unless you are in a crowned campground where everybody is running their air conditioners.

Batteries should always be charged until they attain float. If the batteries remain undercharged for a long period of time, a lead crystall can grow until it punctures the plates inside the battery resulting in a dead cell.

Enjoy.