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Old 11-21-2014, 07:26 AM   #15
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Here's a side question about battery charging, asking because I don't understand the physics involved, and maybe Lewster can explain:
If my coach batteries are down to 75% for example, and I start the generator or plug into shore power, my display usually tells me within minutes that I'm back to 100% charge. But I know that's not the case, because if I stop the generator or unplug from shore power, it will then display 80% (for example).
So with my lack of understanding of battery physics, I'm concluding there must be a superficial charge and a deep charge going on. And my question is when I have the generator charging the batteries, how do I know when they're fully charged and can turn off the generator?
Does my question make sense?
Your question makes good sense. And apologies to Lewster for pre-empting him.

When you have shore power or generator power as a charging source, your display shows 100% because it's reading the charging voltage, not the battery voltage. It would probably show over 100% if the display was set up to do that. Look at an actual display of volts rather than percent charge, and you'll see. Voltage is voltage, and the display can't tell the difference in where it is coming from.

I don't have a good answer for how long to leave the generator running, though. The only way I know how to tell is to shut off the generator or shore power, let the batteries sit for a while to stabilize, then read the voltage again. And if it's not enough, fire up the generator again to finish the job.
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Old 11-21-2014, 07:53 AM   #16
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No apologies required!!!!

Your generator run time will depend on the amperage output of your charger and the state of discharge of your batteries. If you have the Magnum MMS-1012, it has a 50 amp charging section, which has an adjustable output.

The next variable is the size of your battery bank. Typically, Lifeline AGM batteries use the C/5 rule. That takes the Capacity of your batteries and divides by 5.

If you have the standard 180 hour Lifeline bank, then your preferred charger output is 180/5 or 36 amps. The closest you can set the Magnum is 40 amps or an 80% charge rate.

Next, determine your state of charge level. This is easily done by reading the resting voltage of your batteries. Let's say that they read 12.2 VDC, which is 50% for Lifelines, so at that level you will need to replace 80 amp/ hours into your batteries. At the charge rate of 40 amps from above, you would need to run the generator about 2 hours to fully re-charge your batteries.

Exceeding the run time is no problem as the Magnum will go to a 13.2 VDC float charge when they are full. It is far worse to undercharge them by not completing the charge cycle.

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Old 11-21-2014, 08:46 AM   #17
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Thank you Lewster and Protag, very helpful answers. And yes, I have the standard Lifelines with the Magnum.
Here in the Southwest we mainly dry camp, so generally have to run the generator morning and evening, and the question of how long has been bugging me for a while.
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Old 11-21-2014, 08:56 AM   #18
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Originally Posted by UKDUDE View Post
Thank you Lewster and Protag, very helpful answers. And yes, I have the standard Lifelines with the Magnum.
Here in the Southwest we mainly dry camp, so generally have to run the generator morning and evening, and the question of how long has been bugging me for a while.
By the way, for planning purposes, a full tank of propane in your Interstate will run the generator about 24 hours at half load. At full load that goes down to about 16 or 18 hours. That's if you're not using propane in the water heater or stove.

And when you're running the generator, try to load it up to somewhere between 50% and 75% of rated load. It's most efficient that way. Propane is a bit more forgiving than diesel in that regard; you're not likely to get "wet-stacking" if you under-load it. But in terms of maximum electricity for minimum propane, somewhere between 50% and 75% of the rated load of 2.5kW is best. Or since you aren't really set up to show kW of generator output, figure a load of 10- to 15-amps out of the 20 amps that the generator produces.
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Old 11-21-2014, 08:58 AM   #19
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Originally Posted by UKDUDE View Post
Thank you Lewster and Protag, very helpful answers. And yes, I have the standard Lifelines with the Magnum.
Here in the Southwest we mainly dry camp, so generally have to run the generator morning and evening, and the question of how long has been bugging me for a while.
Peter


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Maybe there should be 200-300 watts of solar in your future
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Old 11-21-2014, 09:38 AM   #20
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Boxter,



There is a part of the schematic missing; and that is the continuation of Cable 1, which drops down the left side of the drawing from the isolator terminal marked 'coach'. What you don't see is the continuation of this cable back up to the coach battery positive connection stud.



The solenoid, when activated, will indeed connect the coach batteries with the chassis battery that is shown entering the solenoid from the right.



It would have been more helpful if they included the entire schematic and hadn't cut off the bottom!

Didn't mean to create more confusion. I thought we had established many times on this forum that both the Airstream installed house batteries an the Mercedes installed starter battery are charged via the BIM. It just depends on the voltages and logic of the BIM.

The newer Interstates have an additional Mercedes installed Auxiliary battery under the hood in front of the driver. I thought the question now was is the Auxiliary battery charged via the BIM? Art's (Rapa Nui) answer from Airstream was yes. That answer is interesting because it means that Airstream has either added another logic circuit or they are defeating the Mercedes relay that isolates the Auxiliary battery from the starter battery when the engine is off.


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Old 11-21-2014, 10:32 AM   #21
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....
Your generator run time will depend on the amperage output of your charger and the state of discharge of your batteries. ....
.... Exceeding the run time is no problem as the Magnum will go to a 13.2 VDC float charge when they are full. It is far worse to undercharge them by not completing the charge cycle.

Questions?

Class dismissed! :-))

Lew Farber
Lew - excellent point on undercharging. This is why you must use either the generator or external AC power to get the Magnum to properly charge the coach batteries. Neither the Airstream installed solar or the Mercedes alternator will get the Lifeline coach batteries to full charge. It is an inherent design weakness IMHO.

If you want to dry-camp without having to use the generator add more solar with a proper charge controller.
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Old 11-23-2014, 02:15 PM   #22
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Ok, we now have a better understanding how the alternator, shore, generator and both chassis and house batteries interact together with the BIM. However, we are still not sure what the airstream solar does if anything. Let's assume we add enough solar (200 to 300 watts). Now what. If I am plugged in to shore power, does the BIM use the shore power and/or the solar power to charge all batteries. Does the power source with the most amps dictate what source is used to charge both batteries? Also, say it's a sunny day and the solar is providing 10 amps or more. Is that enough to charge chassis and house batteries on its own?


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Old 11-23-2014, 02:51 PM   #23
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That depends upon your solar controller. The stock controller on my 2013 Ext is a shunt controller. It will not come on until battery voltage drops below set point (12.6v for me). I thought my solar was not working until i read the manual AND then let the voltage drop until it kicked in...it works.


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Old 11-23-2014, 05:15 PM   #24
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I was told by Airstream that my solar controller kicks in when the battery voltage drops to 12.45 volts.
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Old 11-23-2014, 06:27 PM   #25
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Ok, we now have a better understanding how the alternator, shore, generator and both chassis and house batteries interact together with the BIM. However, we are still not sure what the airstream solar does if anything. Let's assume we add enough solar (200 to 300 watts). Now what. If I am plugged in to shore power, does the BIM use the shore power and/or the solar power to charge all batteries. Does the power source with the most amps dictate what source is used to charge both batteries? Also, say it's a sunny day and the solar is providing 10 amps or more. Is that enough to charge chassis and house batteries on its own?


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Depends on what solar charge controller you have, if you are parked with the engine off and 'plugged in' with direct sunlight exposure for your solar panels and the state of charge of your batteries.

With 200-300 watts of solar thru the Blue Sky 2512iX-HV controller and a Magnum MMS-1012 inverter/charger, we have seen the inverter essentially 'yield' to the solar. The voltage sensor of the inverter 'sees' the charging voltage from the solar controller and 'thinks' that the batteries are full, since the solar controller is set for 14.3 VDC bulk/absorb charge and 13.2 VDC float and is wired directly to the batteries thru an appropriate over current device.

The Magnum remote will show 'float charge' but at zero amps......which essentially means it is in standby mode. If you have sufficient solar radiance on your panels and say.....you are running a large DC amperage draw that is still drawing from your battery bank even with the solar charge addition, then the Magnum should kick in at around 75-80% state-of-charge (SOC) and begin it's charging routine.

Haven't tested this scenario yet, as most of the solar I install manages to 'keep up' with the amperage draws placed on the batteries in full sun.

Another aspect of the Blue Sky solar charge controllers is the variability of the output charge to the batteries, which changes depending on the SOC of the batteries. If the batteries are near or nearing full charge, the amperage will taper off to a value of 0.5VDC per each 100 amp/hours of battery bank capacity, but if you place a DC load on the batteries, it will increase the amperage from the controller to the batteries to maintain the state of charge, or continue with it's charging algorithm.

Did that help or confuse???
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Old 11-23-2014, 06:55 PM   #26
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Lew, how does all this electrical talk maintain my chassis battery?


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Old 11-23-2014, 08:25 PM   #27
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The Precision Circuits BIM is a 'smart' device that interconnects the chassis and engine batteries when certain set of parameters are met, and disconnects them for others.

According to the company literature, if one of your batteries is receiving charge, the solenoid will close to join the two battery banks together. If the BIM sees long term charging of both batteries, it will allow both battery banks to remain connected and allow the charging system to do it's job.

Once the batteries are fully charged and have reached the 'float' state for one hour, the BIM will open the solenoid and isolate the batteries to prevent overcharging. It will only re-connect the two battery banks for charging if one of the batteries drops to approx. 80% of it's capacity, based on battery voltage.

If the batteries are not being charged, the BIM isolates the two batteries to prevent one system from depleting the other system.

In your case, if you have your solar or Magnum operating to charge the batteries, the two systems will be linked together until BOTH systems are full and in float charge for an hour, when the batteries are again isolated and are separate. This will also work if you are driving after dark (no solar) and your Magnum is not charging the batteries (no generator) but the engine is running and the alternator is charging the engine battery. The BIM will again link the two battery banks until BOTH are full and have been in float mode for 1 hour and will then disconnect them again.

It works in either direction.
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Old 11-23-2014, 08:54 PM   #28
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I thank everyone for their help. My understanding of the RV's electrical system has improved 500% in the last few days. Knowing what to expect from the RV makes the unknown of my future travels not as intimidating.



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