Battery Charging Question

[Tim Bentley Co-Admin]

This is not really a problem but I have an idea and would like to know if it will work.  

We have just spent a week dry camping on the desert.  As usual, when we are ready to leave early in the morning, our batteries read 12.2.  Gerald has made it clear that it is hard on alternators to charge low coach batteries as you drive.  Not wanting to damage the alternator, or wait an hour for generator charging, will this work as an alternative?

If I begin my morning drive with the generator running will that charge the coach batteries and take the load off the alternator?  The question is, with the generator running as you drive, what power source is selected for charging the coach batteries.

[Gerald Farris]

When the generator is running and the inverter is charging the batteries while you drive, the coach batteries will be charged by both the engine alternator and  generator via the inverter. Since the inverter will produce a higher voltage in bulk charge stage (the first stage when charging low batteries) than most alternators do, most of the charge current will come from the inverter and therefore not damage the engine alternator.

In other words, if you have low coach batteries and you need to drive, starting the generator and charging them with the inverter as you drive is the very best option.

Gerald

[Richard And Babs Ames]

Plus 1 with Gerald, start the generator first if you can. Inverter/chargers add at least 80 amps to keep the load off the alternator.

[LEAH DRAPER]

Pardon my lack of knowledge/stupidity, BUT should one have the inverter ON when driving etc.  I never have and about the only time it is ON is if I need power not available by land line or generator?

Gerald clue me in!

Thanks.

[Jerry Carr]

Thanks for the info on the inverter charge, although we rarely dry camp its good info. I have never shut our inverter off we have a household frig in our rig and must have 120V to keep it running.
I hope we are doing OK should we shut the inverter off if we have hook ups?

[Edward Buker]

I would opt for having the inverter off unless it is needed.

Each component in the inverter system has a life. The duration of that life depends on the circuit design of the inverter itself, the over design (hopefully) or under design of each component in the circuits and the defects that exist within those components. A good example to visualize this is a filament in a light bulb. The remaining components of the light bulb would generally last forever with the filament being the weakest component. The heated hours of use causes the tungsten filament to thin due to metal evaporation and eventually it breaks and fails. There is a first to fail component lying in wait in the inverter as there is in everything else that is manufactured and the hours that the inverter is on is consuming some of its useful life. The useful life is consumed faster with a heavy load and slower with a light load. Because these Xantrex Prosine units have a known history of failing, usually due to the main FET power transistors being the weak link, the number of hours in use is always getting you closer to where the "Xantrex Tombstone" is to be placed.  

I would also consider having a light load on the inverter when I turn it on and give it a couple of moments to stabilize before I pushed the toaster down...

Hope this helps....Later Ed

[Jerry Carr]

Thanks Ed, I turn the switch to the off spot for now I keep the rig plugged in all the time.

[Gerald Farris]

There are 2 parts to an inverter. The first part is the battery charger or converter. Under normal conditions this part is always left on to keep the house batteries fully charged when on shore power.

The second part is the inverter that produces 120 A/C from the house batteries. There is no set rule on when to turn it off or leave it on. I only turn it off when I am dry camped and do not need 120 A/C current, so I turn it off to save on battery power. On my coach you have to run the inverter when driving if you want to listen to the dash radio or use the back-up camera, because the radio uses the Bose system as a speaker and it and the back-up camera are both 120 A/C. When you are on shore power the inverter turns off automatically and goes to standby.

Gerald

[Joel Ashley]

Jerry-

I don't believe a regular RV refrigerator can run off the inverter.  When dry camping, it runs on propane.  Otherwise you have to be plugged in to AC power for the fridge to run on electricity.  Running the fridge on the inverter would quickly deplete the batteries, so I think they wire it's outlet to be unavailable to the inverter.  Your standard household refer must plug into a different outlet, or that circuit is wired different than most.

-Joel

[Edward Buker]

Tim's question that started this post is a really good question that is hard to answer without a way of measuring the output current of each source. Gerald's is correct in that you need to take load off the alternator. It was never designed to provide the level of current flow for any extended period of time that a large bank of thirsty batteries can draw.
 
The load sharing that actually happens between the Xantrex and the alternator is complex when they are both in use. It really depends on where the voltage sensing is taking place for each source, voltage drops in the charge wiring, and the design of the charging regulator circuits in each of the charging sources. The most important load factors are the size of the battery bank and the battery charge state. All of these are players but we will try and boil it down to just worry about the voltage....

The critical time for the alternator would be when the batteries start out at 12.2v. Until the Xantrex can raise the voltage to approach the 14.2V then both the alternator and the Xantrex will be putting out max current during the lower voltage period. If the full current is being put out by the xantrex and the voltage output is at 12.6V at the battery then the alternator will be at max current also. The alternator current will ramp down as the voltage approaches 13.8 V. which is before the Xantrex current will be reduced at 14.2 V during absorption. The thing to remember is that the voltage level will set the current level from both the sources. Critical alternator time is while the voltage is low.

If the batteries are discharged deeply like 12.2V then without a doubt you want to run the generator and the Xantrex first to start the charging. I would consider looking at the Xantrex voltage output level and if I could, let it climb during absorption, until 13.8V is obtained. What is happening is the Xantrex is putting out max current and the voltage on the battery is slowly rising as it stores the current. The battery voltage at the time you start the engine alternator will set the current level that the alternator will provide. Safest bet might be something like this...

1. Generator /Xantrex only until voltage reaches 13.8V or as long as you have time to wait. The higher the voltage the better.

2. Generator/Xantrex and Alternator until Inverter current output drops to 20-30 amps. That means you are in absorption and the alternator should be able to pick up that added load.

3. O.K. to run the alternator alone

This is a reasonable guess as to a rough guideline one might follow to max the alternator life.

Speaking about voltage drops, wiring losses, charging rates and the like check the following out regarding battery wiring.  Also very interesting and unexpected.....

http://www.smartgauge.co.uk/batt_con.html

Hope this helps....Later Ed

  

[Gerald Farris]

Joel,
Some of the later model Marquis and Patriot coaches are equipped with residential style refrigerators with a compressor that only runs on 120 volt A/C current and can not run on propane like the gas absorption refrigerators in most RVs. In fact most of the 2008 Marquis coaches did not even have a propane tank.

The residential style refrigerators in these coaches are low current draw designs that run well on inverters.

Gerald  

[Edward Buker]

Thank you Gerald for the clarification, it is a very good point that I failed to mention. The inverter unit is smart enough to shut its own inverter circuits off when shore power is available and it will put itself in "standby". The only added circuity being used in "standby" vs "off" is load sensing circuitry that is constantly watching for the need to have the inverter to go back online. What is helpful in extending the Xantrex life is if you do not need the FET Power Transistors to be inverting then your better off turning them off. They are in the off state in both "standby" and when you select "off" for the inverter.

As an aside....I try and sort things out for myself, provide answers to some of these questions, as well as some background that might help us all out. I'm sure sometimes it is more information than some of us really want to know . For that I apologize. I do no profess to be right all the time, just providing my own logic and conclusions. I certainly did not get the C12 40QT capacity oil change correct when I listened to my local Cat dealership service manager. The right answer was 36Qts.

What is great about the forum is that when all the bits and pieces from everyone are added together you have a combination of knowledge and experience that evolves something useful to us all.....for that I am very thankful as a somewhat isolated Beaver owner here in Alabama. I cannot end this without a special thanks to Gerald for his very clear and concise inputs and vast knowledge on these motorhomes. He amazes me...

Later Ed

[Tim Bentley Co-Admin]

Thank you all for your input.  Those of you that are regular contributors to the forum have made it the success that it is.  Most posts are answered within the day and emergences even sooner.  As a group, forum contributers are responsibable for many new members and an increase in revenue.  Great work all, once again we are reminded what an outstanding group our members are.

[Joel Ashley]

Yes, Tim.  We are an outstanding lot, aren't we?  And modest too .  In fact in Oregon for the last week or so we've been very outstanding... out standing in the rain - good thing we're Beavers and Ducks.   Which is appropriate I reckon;  from time to time on this forum I've been all wet.

[Randy Perry]

Huh, I wonder if anything learned here had anything to do with having to replace my altinator about 2 weeks ago :-/