What? Generator charges the chassis battery?

[MNtraveler]

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.



Land of 10,000 Lakes
2016 Forester MBS 2401R

[Still Kickin]

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.

[ScottBrownstein]

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!

[Still Kickin]

Originally Posted by ScottBrownstein

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.

[ScottBrownstein]

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!

[bubbles]

Originally Posted by ScottBrownstein

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.

[MNtraveler]

Originally Posted by Still Kickin

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.

[MNtraveler]

Originally Posted by ScottBrownstein

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.

[MNtraveler]

Originally Posted by ScottBrownstein

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.

[Still Kickin]

Originally Posted by ScottBrownstein

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!

[ScottBrownstein]

Originally Posted by Still Kickin

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!

All coaches isolate the chassis battery from the house and that is what a BCC is for. Take this in the friendly spirit it is intended but hey, we all love our own coach model, but it does seem that there are some interesting quirks to German engineering here and they aren't constrained just to Volkswagen diesels. http://www.forestriverforums.com/for...des-99476.html

Frankly, this is confusing...jump starts, potentially dead chassis batteries, converters charging chassis batteries and a "lock out" when you haven't filled up on DEF but no message to the operator? Wow!

Sorry but the thread is from this week.

[MOODMAN]

OK. Now you have me wondering..

I have a FR 5er. When I connect my truck to the trailer, I can control the jacks and all 12V appliances, with or without the trailer battery.

What I am wondering now is whether, if I leave the trailer plugged into my truck while camping if it will charge my truck batteries (yes, I have two batteries in my truck because it is a diesel)

Anyone know the answer to this?

[Flybob]

If you leave the truck connected to the 5ER and the truck is running it will charge the 5ER batteries, HOWEVER very very slowly and only if the truck is running. You have two things working against you,
1st the length of the wire run from the Truck fuse block to the 5ER batteries and the small wire gauge used will result in sufficient voltage drop as to make the charging very marginal.
2nd unless the truck is running at highway speeds, the alternator will not generate enough power to do much more than a low trickle charge.

Some newer trucks have a relay in the 12V aux line to the trailer that will disconnect if from the system when the key is off. Without this relay if you left the truck connected and the truck off, the trailer would kill the truck batteries.

So short answer is truck will not do much to charge the trailer batteries without modifications and will do more harm than good if truck is turned off.

[RedLdr1]

Originally Posted by ScottBrownstein

All coaches isolate the chassis battery from the house and that is what a BCC is for. Take this in the friendly spirit it is intended but hey, we all love our own coach model, but it does seem that there are some interesting quirks to German engineering here and they aren't constrained just to Volkswagen diesels. http://www.forestriverforums.com/for...des-99476.html

Frankly, this is confusing...jump starts, potentially dead chassis batteries, converters charging chassis batteries and a "lock out" when you haven't filled up on DEF but no message to the operator? Wow!

Sorry but the thread is from this week.

The MBS does have some interesting quirks... I'm finding new ones all the time!

But hey we don't need no stinking BCC! We can jump start our rigs from inside the unit without one! Just open the house battery step and the chassis battery door, located in the floor just in front of the drivers seat, and off you go...

Another interesting OEM option that could be added is a second chassis battery that fits under the hood. But I hate to think of what M-B would want for retro fitting that...

[Still Kickin]

Originally Posted by RedLdr1

The MBS does have some interesting quirks... I'm finding new ones all the time!

But hey we don't need no stinking BCC! We can jump start our rigs from inside the unit without one! Just open the house battery step and the chassis battery door, located in the floor just in front of the drivers seat, and off you go... k:

Just a heads up on the easy way to jump. Open the hood and you will see MB designed access to the positive terminal. It is covered by a red plastic slip cover, center right of air cleaner. The negative ground is that copper stud screwed into the body on the top right fender wall. Great feature since the chassis bat is where it is in the cabin area under a cover. Just sayin'.

[ScottBrownstein]

Now it would make sense that that would require the key since the charge connection goes through the 7 way. In fact, if there is a relay it might even require the truck to be started. Can't imagine you can get much current into the TT through that route though.

Also...remember...current can go both ways and you can also kill your starting batteries from the TT if it remains connected and you forget to turn on the house battery.

[ScottBrownstein]

Oh...trust me, if your engine charges the house...you have a BCC. Perhaps it is prevented from working the other way around, but that should be pretty easy to change.

The only time I really used my boost switch on my 1997 F-53 was when the 12V magically disappeared from the engine driving down the road at 60 MPH. That was a real exciting experience. Pushed the boost switch and everything came back to life. Started the genny so I could take my finger off the switch and drove for 5 more hours.

Turns out it was a cracked battery lug on the chassis battery!

[RedLdr1]

[QUOTE=Still Kickin;1091469]Just a heads up on the easy way to jump. Open the hood and you will see MB designed access to the positive terminal. It is covered by a red plastic slip cover, center right of air cleaner. The negative ground is that copper stud screwed into the body on the top right fender wall. Great feature since the chassis bat is where it is in the cabin area under a cover. Just sayin'.[/QUOTE


I'll only do it the easy way if its isn't raining...

I carry a "jump box" that makes it really easy...provided I remember to keep it charged up!

[MOODMAN]

Originally Posted by Flybob

So short answer is truck will not do much to charge the trailer batteries without modifications and will do more harm than good if truck is turned off.

I guess what I was asking is if I keep the truck connected to the trailer, and I have shore power connected, will the truck batteries get charged from shore power?

[RedLdr1]

Originally Posted by MOODMAN

I guess what I was asking is if I keep the truck connected to the trailer, and I have shore power connected, will the truck batteries get charged from shore power?

No... The 12VDC feed from the truck to the trailer is a one way path. And when you turn off your ignition there should be a relay cut off to kill that path so your truck battery isn't run down by accident.