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Old 01-30-2016, 11:42 AM   #1
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What? Generator charges the chassis battery?

Interesting but unexpected find. First for the known stuff but it was good to confirm and see it in action:

Coach battery with nothing running: 12.2V

Coach battery with MBS engine running: over 14V

I believe the battery switch was on, but neglected to make a note of it.

Now for the new find (at least to me) (and this was in a different day from the above readings):

Chassis battery with nothing running: 12.16V - could use some juice.

Chassis battery with generator running, key "on" (position 2),
Scrolling through the onboard computer: watched it rise from 12.9 to 13.2 to 14+.

Turn the key off, wait a minute and check again, voltage is down again. Turning the key to position 2, you can immediately hear the generator take on additional load, and the battery meter goes up again.

So - not only does the engine alternator charge the house batteries, the generator charges the chassis battery - but only if the key is "on." This is great to know for boondocking or if we ever find ourselves with a run down chassis battery that needs a boost.

By the way, I just ordered one of these and it should arrive today or Monday:

Zeltauto LCD Cigarette Lighter Digital Car Voltmeter 12V/24V Vehicle Voltage Gauge Monitor for Auto Car Truck (1piece) https://www.amazon.com/dp/B00VL9JZ0K..._uCoRwbEA1PNA3

This will allow immediate battery checks for the chassis via the cigarette lighter socket in the cab (which I believe is "always on" with or without the key), and the house batteries via the socket in the living room with the battery switch on, or even the outdoor 12V socket - without using a multimeter or opening the battery compartment or the hood. It can also be used as a steady state meter, although I won't leave it on when not in use as it would be a slight drain.



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Old 01-31-2016, 09:37 AM   #2
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Just a heads up on the cigarette volt meter. You most likely will find that its not as accurate a reading as a hand held multi-meter. I suggest you take a reading from both units, at the source, so you can adjust the reading from the one your buying. Been there done that. It's just a function of cheap chinese electronic components.

Also, the output on the MB alternator is very impressive. So much so that it can easily charge the house bats during longer driving periods( 2+ hrs.) depending on the state of discharge of course.
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Old 01-31-2016, 12:12 PM   #3
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That is good news either way, but most RV's work the same way. It is generally a function of the battery control center. Paralleling the chassis battery with the house battery at all times is obviously a potential problem since if you kill your house battery you can kill your chassis battery at the same time. Bad karma! The solution is a relay that connects both batteries together is the voltage is more than say 13 volts. This means that something is charging one or the other and they can be safely connected together without risk of discharge. This is the same relay that you manually fire in the "boost" start mode so that a dead chassis battery can be assisted by the house battery to get the engine started.

Your experiment confirmed that the converter will charge both batteries when the generator is running...and that is good. My only thought (and you will find this out when you get your voltmeter) is if it has anything to do with the key being turned on. Perhaps what you saw is the computer voltmeter and display being turned on when you turn the key and not the charge connection at all! when your Amazon order comes in you will quickly find out!
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Old 01-31-2016, 12:57 PM   #4
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That is good news either way, but most RV's work the same way. It is generally a function of the battery control center. Paralleling the chassis battery with the house battery at all times is obviously a potential problem since if you kill your house battery you can kill your chassis battery at the same time. Bad karma! The solution is a relay that connects both batteries together is the voltage is more than say 13 volts. This means that something is charging one or the other and they can be safely connected together without risk of discharge. This is the same relay that you manually fire in the "boost" start mode so that a dead chassis battery can be assisted by the house battery to get the engine started.

Your experiment confirmed that the converter will charge both batteries when the generator is running...and that is good. My only thought (and you will find this out when you get your voltmeter) is if it has anything to do with the key being turned on. Perhaps what you saw is the computer voltmeter and display being turned on when you turn the key and not the charge connection at all! when your Amazon order comes in you will quickly find out!
This OP's initial thread is meant to be equipment specific, in that MB does not allow what other manufactures do, (i.e. bat boost feature) So, not being a Benz owner, what the OP was saying is that the circuit seems to be open in the number two key position between house and chassis.
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Old 01-31-2016, 01:28 PM   #5
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I understood that completely! However, if the alternator charges the house batteries while on the road AND the house does not discharge the chassis battery when off the road, there must be an active component between the two batteries...or, more specifically, either a relay or back to back diode charge splitter. The boost switch was just as an example.

I guess MB owners still need to carry jumper cables!
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Old 01-31-2016, 02:13 PM   #6
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Paralleling the chassis battery with the house battery at all times is obviously a potential problem since if you kill your house battery you can kill your chassis battery at the same time.
!
Typically the battery control center works on a priority system for charging where in if on shore power or genny the coach battery has charging priority and if driving the chassis battery has charging priority. Not sure of the cutoff criteria. The battery control center will have the circuit sensing components to monitor and control charging as allowed. This is the way my GT is set up. Maybe MB went a different route but it is a FR product.
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Old 01-31-2016, 03:05 PM   #7
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Just a heads up on the cigarette volt meter. You most likely will find that its not as accurate a reading as a hand held multi-meter. I suggest you take a reading from both units, at the source, so you can adjust the reading from the one your buying. Been there done that. It's just a function of cheap chinese electronic components.

Also, the output on the MB alternator is very impressive. So much so that it can easily charge the house bats during longer driving periods( 2+ hrs.) depending on the state of discharge of course.
Thanks - on my first test, it was off -.5V against the voltmeter in an auto with key on ACC although it seems to be accurate with car running (not voltmeter tested but just seeing 14.4 then declining to perhaps 13.2 after a bit). I have to run some more tests on this in several modes. If it's way off - or if I can't ID the specific amount or situation where it's off - then it's going back.
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Old 01-31-2016, 03:08 PM   #8
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Originally Posted by ScottBrownstein View Post
That is good news either way, but most RV's work the same way. It is generally a function of the battery control center. Paralleling the chassis battery with the house battery at all times is obviously a potential problem since if you kill your house battery you can kill your chassis battery at the same time. Bad karma! The solution is a relay that connects both batteries together is the voltage is more than say 13 volts. This means that something is charging one or the other and they can be safely connected together without risk of discharge. This is the same relay that you manually fire in the "boost" start mode so that a dead chassis battery can be assisted by the house battery to get the engine started.

Your experiment confirmed that the converter will charge both batteries when the generator is running...and that is good. My only thought (and you will find this out when you get your voltmeter) is if it has anything to do with the key being turned on. Perhaps what you saw is the computer voltmeter and display being turned on when you turn the key and not the charge connection at all! when your Amazon order comes in you will quickly find out!

Yes - more experimentation needed to firm that up. As noted on another reply, the MBS does not have a boost switch.
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Old 01-31-2016, 03:12 PM   #9
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I understood that completely! However, if the alternator charges the house batteries while on the road AND the house does not discharge the chassis battery when off the road, there must be an active component between the two batteries...or, more specifically, either a relay or back to back diode charge splitter. The boost switch was just as an example.

I guess MB owners still need to carry jumper cables!
It's a very intelligent circuit, that's for sure. It mediates both directions. Also if you are running down the road with generator on (for A/C) and the engine alternator is running, it has to moderate that competition.

Booster cables, yes, got em for this reason! However, the ability to charge up the chassis battery if needed seems to mitigate that, at least if you can wait 1/2 hour to power up a bit. Once you get the engine started, it will charge itself of course.
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Old 01-31-2016, 06:53 PM   #10
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I understood that completely! However, if the alternator charges the house batteries while on the road AND the house does not discharge the chassis battery when off the road, there must be an active component between the two batteries...or, more specifically, either a relay or back to back diode charge splitter. The boost switch was just as an example.

I guess MB owners still need to carry jumper cables!
To put it simply, MB does not allow interface with house componets(this argument means electrical). It's the directive of MB. That is why the BCC is wired that way by FR. It would take some serious discharge to drain the chassis bat where you would need a jump, since they are isolated. But, anything is possible once, then the perverbial learning curve comes into play. Besides, think of all the savings in fuel costs so you could easily buy a set of jumpers!
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