Lithium battery BPS lock-out

[Harpist]

Hej folks!


In the beginning of August after not driving the van for approximately four weeks, I went to start the van and had no power in either the start battery or the cabin battery. I thought I had activated the cabin battery disconnect the last time I had driven but perhaps this was not the case?
I jumped the start battery (per Sprinter manual instructions) and ran the engine for about 30 minutes. I then put the van on shore power overnight. In the morning, both battery systems appear to be fully charged. I drove the van over a two-week period without a problem and all appliances seemed to work fine.


On August 23, the van went into MB for service. Afterwards, I checked out all the cabin electronics to make sure everything was working.
On August 22 we drove to Seattle. About half-way over to Seattle, we stopped for lunch (in the van) and all cabin lights and appliances appeared to be working. However, we arrived in Seattle and there was no power in the cabin. I put it on shore power with no results and checked all breakers and fuses without results.


I took it into Airstream of Spokane the next week and the service department reported that the cabin batteries (lithium) had no charge and the battery management system (BPS) would not allow the batteries to be charged by any of the “passive” methods (shore power, alternator, solar or generator). Airstream service charged the lithium battery directly (placing 12V charger directly on the battery) and after the charge everything seemed to be working.


So here is the explanation I am getting:


When the cabin battery is “disconnected” (either using the switch by the sliding door or manually disconnecting under the back seat) there is still a significant draw on the battery. This is “as designed” and it will completely draw down the cabin battery in 3-4 weeks unless the battery is augmented by one of the passive charging systems. Once the cabin battery goes below a certain level, the BPS locks out the passive systems (including shore power). The only way of charging the battery is by direct charging (i.e. getting under the van and connecting the battery charger to the lithium battery).


The service tech claims that there is no documentation or technical note from Airstream and this is the first time they have seen (or even heard of) this issue (although from our discussions I suspect that he is new to working at Airstream service). One work around he suggests is putting in a battery isolation switch that completely disconnects the battery.


Now I am an engineer who spent his life building advanced control systems and from a design viewpoint, this seems wonkey in a number of ways – both the fact that there is no software cut-off (voltage goes below X.X volts, an isolation switch is thrown) and that the BPS forces you to use direct charging. However, I admit that I rarely used lithium batteries as anything but as a UPS.


So, I have two questions for anyone who might have info on this:
1.

Is the explanation I am getting from the tech actually legitimate?
2. If so, is an isolation switch a good idea?


Thanks for your help. After a year and two cross country trips to see our daughter and grandson, we find the van to be a great way to travel. But this has ne shaking my head a bit. Does no one else leave their van for 4 weeks w/o driving it or out in the sun?


Good Roads!
Harpist

[Boxster1971]

Just another example of Airstream’s poor electrical designs. The proper BMS would allow charging from external sources, even after a low voltage shutdown.

An isolation switch would be good - that is what Airstream should have installed in the first place.

I leave my van parked for weeks - but it is outside in the sun and I have added additional solar.

I’ll take a look at the wiring diagrams for a 2021 Interstate 19 and give you some feedback on the techs explanation.

[Boxster1971]

Harpist - looking at the wiring diagrams, found in back of owner's manual, there are several things that are not disabled by the "Main Disconnect Switch".

That includes the solar charge controller and radio power which do cause a low-level drain on batteries. Also, if you only use the "House Manual Disconnect Switch" it appears that leaves power to the Inverter/Charger which also does have a low-level drain on batteries. You have to use the "Main Disconnect Switch" to also isolate the Inverter/Charger.

It also looks like the "Battery Seperator" will continue to function and draw power from the house batteries to charge the chassis battery until it falls below a pre-set voltage.

There is nothing in the owner's manual about the BMS functions.

I have always found it puzzling why Airstream does not provide a true disconnect switch to totally isolate the house batteries. I installed three switches for this function and rewired my Interstate so when I turn off the house disconnect switch(s) there is no drain on the batteries. The third switch I installed completely isolates the solar system.

[uncle_bob]

Hi

Pretty much all Airstream RV's ( trailers and vans ) have a setup that works pretty much as described above. The magic switch may be called a disconnect. It might be called use./ store. It shuts down *most* of the loads, but not *all* of them. There is always a parasitic drain when the stock switch is used. Storage for a month or so generally is enough to flatten the batteries.

Once you get a lithium battery past a certain point, the BMS internal to the battery cuts it out of the circuit. The output voltage goes from ( maybe ) 10.4 V to zero when this happens. The battery is protected from self destruction, but you have no power. Again, this is very normal for just about all lithium batteries on the market.

Things like the charger section of the hybrid inverter / charger also self protect. They do not want to dump power into a shorted or dead system. Is this a good thing? Maybe it is. They pretty much all do this. They need to "see" a voltage above some limit ( maybe 7 V ) before they will boot up and start sourcing current.

The alternator on a lithium system does not directly feed those batteries. You have a DC / DC converter there to get all the voltages right. They often have the same sort of self protect that the other devices do. Same thing with the solar.

Simple answer is not to totally discharge the lithium system. No, that may not be easy, but that is what you are expected to do. If you do, a fairly quick blast from a plug in battery charger will get things booted up. It does not take much current or much time to get things going. Once the battery is back in the game, the shore power, alternator, or solar all will charge it up "full".

Bob

[StogieMan]

Originally Posted by Boxster1971

I have always found it puzzling why Airstream does not provide a true disconnect switch to totally isolate the house batteries. I installed three switches for this function and rewired my Interstate so when I turn off the house disconnect switch(s) there is no drain on the batteries. The third switch I installed completely isolates the solar system.

Mike, I agree. The only disconnect switch that I’m missing is the disconnect switch to the main disconnect switch - haha.

I added separate disconnect switches for everything that I have tied to our Victron Lynx distribution buss bar in addition to our “main” disconnect. It makes it easier to troubleshoot and isolate every component.

If I remember correctly on our 2015 before we did our Lithium upgrade and complete rewire, when you turned off the main disconnect switch - the old solar controller, lounge seat motor, radio and tank heaters still remained functional but I might be missing one more. They remained functional as they were getting power from the chassis battery via the Airstream installed BIM. As part of our upgrade, we installed the upgraded PC LI-BIM 225 for lithium batteries. Even though it was overboard, we also added a cutoff switch to it even though it wouldn’t have any power to it as it was wired to our Lynx buss bar that will cutoff via the main disconnect. Now, we know that everything is powered down when we flip the switches off.

Airstream never fails to make me scratch my head and say “WTH.” It wouldn’t cost them much extra to add a switch. It’s these small cost cutting measures that tick me off.

[Hawk-ination]

Airstream’s lithium installs have been not known to be the most stellar unfortunately.

At least you aren’t dealing with AGM batteries where you were dead after about 8 hours unless you were plugged in. Still is annoying to find out you have to charge your batteries from an external source if you let your batteries go to long without a charge. Agreed that a Li-Bim would be a good addition.

[Harpist]

Boxter1971,
You must have a better copy of the electrical diagrams than I do. Half of the text is"smudged" even on my electronic version.


But sure enough - a bit of testing shows that the "Master Disconnect Switch" has an additional setting if you rotate it a bit more. The amperage draw drops by a factor of 10. I can see on the wiring diagram that it disconnects the inverter. Does it isolate anything more? It looks like the battery separator is still active as well as the solar system.


Thanks for the help. I haven't decided whether to put in an isolation switch on the cabin batteries. I need to be able to have the system sit idle for a month and I can't count on solar due to our gray winters here.


Good Roads
Harpist

Originally Posted by Boxster1971

Harpist - looking at the wiring diagrams, found in back of owner's manual, there are several things that are not disabled by the "Main Disconnect Switch".

That includes the solar charge controller and radio power which do cause a low-level drain on batteries. Also, if you only use the "House Manual Disconnect Switch" it appears that leaves power to the Inverter/Charger which also does have a low-level drain on batteries. You have to use the "Main Disconnect Switch" to also isolate the Inverter/Charger.

It also looks like the "Battery Separator" will continue to function and draw power from the house batteries to charge the chassis battery until it falls below a pre-set voltage.

There is nothing in the owner's manual about the BMS functions.

I have always found it puzzling why Airstream does not provide a true disconnect switch to totally isolate the house batteries. I installed three switches for this function and rewired my Interstate so when I turn off the house disconnect switch(s) there is no drain on the batteries. The third switch I installed completely isolates the solar system.

[Boxster1971]

The trick to read those wiring diagrams in the owner's manual is open the pdf file and enlarge the image to read those smudged text better.

The other items left power are listed above by StogieMan:

". . . the old solar controller, lounge seat motor, radio and tank heaters still remained functional . . ."

That has been true of every different Airstream Interstate wiring diagram I've looked at.

Originally Posted by Harpist

Boxter1971,
You must have a better copy of the electrical diagrams than I do. Half of the text is"smudged" even on my electronic version.


But sure enough - a bit of testing shows that the "Master Disconnect Switch" has an additional setting if you rotate it a bit more. The amperage draw drops by a factor of 10. I can see on the wiring diagram that it disconnects the inverter. Does it isolate anything more? It looks like the battery separator is still active as well as the solar system.


Thanks for the help. I haven't decided whether to put in an isolation switch on the cabin batteries. I need to be able to have the system sit idle for a month and I can't count on solar due to our gray winters here.


Good Roads
Harpist

[LB_3]

As you note, no quality engineer would design these systems as Airstream has done. First, the parasitic loads are just lazy. I rewired our 2007 interstate to eliminate them.

But the bigger issue to me is they are still in the dark ages of lithium BMS management. If any cell in the battery is either over or undercharged, it disconnects the battery as you observed.

A quality design would only inhibit charging if the battery reached its charge limit and only inhibit the loads when the battery reached its discharge limit.

Most of the primitive early BMSs had a push button momentary switch to allow you to override the system disconnect to reinitiate charging or discharging as needed until the over or under charge fault condition was cleared. It sounds like Airstream didn’t even bother to implement this simple workaround.

[uncle_bob]

Originally Posted by LB_3

....

Most of the primitive early BMSs had a push button momentary switch to allow you to override the system disconnect to reinitiate charging or discharging as needed until the over or under charge fault condition was cleared. It sounds like Airstream didn’t even bother to implement this simple workaround.

Hi

Time moves on and designs change. None of the major battery outfits do it this way anymore. The switches turned out to be a bigger problem than any other part of the BMS. They went to a system you "wake up" with a charger instead. Given the way the batteries get buried in a lot of installs, getting to a pushbutton on each battery would be a lot harder than plugging the charger.

Bob

[Harpist]

Hej folks!
Thanks for all the responses. At least I will know what to do next time (although it will be a different problem next time).
Good Roads
Harpist

[Harpist]

Boxster1971,
One last question (I hope). If the manual switch is turned all the way off (call that "fully off"), is the battery heater still active? The reason I ask is that the heater keyed shut-off is giving me problems - the entire unit rotates when I turn the key and it is hard to tell if the I have turned the heater off with the key. So I am hoping that the full manual disconnect shuts it off.
I have had the Interstate off shore power and the manual switch in the "fully off" position for a day (at 20 degrees F) and the voltages still look good so it appears that either the manual disconnect isolates the battery warmer or I have succeeded in turning the battery heater off with the key. But I will be leaving it for a week or so and want to be sure I don't deplete the batteries.
Thanks for any and all assistance.
Good Roads
Harpist

Originally Posted by Boxster1971

The trick to read those wiring diagrams in the owner's manual is open the pdf file and enlarge the image to read those smudged text better.

The other items left power are listed above by StogieMan:

". . . the old solar controller, lounge seat motor, radio and tank heaters still remained functional . . ."

That has been true of every different Airstream Interstate wiring diagram I've looked at.