Crazy idea for keeping chassis battery topped up?

[FlyFishinRVr]

Great stuff, thanks everyone!

So, it looks like the right answer for my situation is to leave everything connected and let the solar panel and BIM do their thing to keep all of the batteries happy (just don't forget to turn off the propane!). I don't need to add anything.

[WayneG]

On my 84 I have 400 watts of solar on the roof for the house batteries and seldom use my shore power converter. To keep the chassis battery from going to low I made this circuit to go on the battery isolator on the firewall. It uses a 2.5 amp PTC fuse and a diode to prevent reverse flow. If the voltage difference is to large the PTC fuse limits the drain from the house battery by increasing in resistance thus acting like a trickle charger.
The only problem is that it will not fully charge the chassis battery because of the diode voltage drop in my circuit and the battery isolator diode drop so it can only charge to 1.2 volts below the solar charging voltage.
If the chassis or house batteries get really low I have the jumper solenoid on a manual switch to tie all batteries in parallel or the stock method of the ignition switch. That circuit is in the second image.

[southjk]

They make small solar panels that plug into cigarette lighters for that very thing.

[FlyFishinRVr]

Originally Posted by southjk

They make small solar panels that plug into cigarette lighters for that very thing.

Yes, but my concern was/is the quality of the charge controller built into those panels and if they could do more harm than good. I know first hand what happens when a cheap battery monitor/charger does its evil magic.

It's a moot point however because the good folks on this board have pointed out that the BIM + 100W solar panel in my '17 AI will take care of both sets of batteries automatically so I don't need to worry about it. This wasn't clear from the manual, so it's great to have input from the "veterans".

[Mcrid]

I didn't want to leave you with the impression that the 100W panel alone is sufficient to maintain the house and chassis batteries). You may want to look over the math I did during my Solar/Battery upgrade. By my calculation (big variable is hours of direct light), maybe 45 - 70 days before the house batteries deplete to 50% DOD. If your rig has Service Bulliten SB 164 installed and you turn off the main house switch (which disconnects the parasitic standby draw from the Magnum inverter) then you are probably OK. However SB 164 has its own issues. I think most on this forum would say you need at least 200 Watts to accomplish what you are after.


http://www.airforums.com/forums/f240...ml#post1806655

[weaselbadger]

On your original idea - you could not run an inverter directly off the solar panels - the panels would not supply enough constant current to power an inverter. You would have to trickle charge a battery from the solar and run the inverter off that battery to then run your battery charger to charge the chassis battery. It sounds like a bad joke, but there you go.

[FlyFishinRVr]

Thanks for the replies everyone. Great info and much appreciated. [emoji106]

[FlyFishinRVr]

Originally Posted by Mcrider

I didn't want to leave you with the impression that the 100W panel alone is sufficient to maintain the house and chassis batteries). You may want to look over the math I did during my Solar/Battery upgrade. By my calculation (big variable is hours of direct light), maybe 45 - 70 days before the house batteries deplete to 50% DOD. If your rig has Service Bulliten SB 164 installed and you turn off the main house switch (which disconnects the parasitic standby draw from the Magnum inverter) then you are probably OK. However SB 164 has its own issues. I think most on this forum would say you need at least 200 Watts to accomplish what you are after.

I really only need the solar to keep the batteries charged enough to avoid problems for about 30 days max. If the stock setup will do that then I'm good. If not, then I might be investing in a second panel per your recommendation. [emoji106]

[FlyFishinRVr]

UPDATE:

I wanted to update this thread since I've had some time to let the OEM solar charging system do its thing. Here is what I've seen and learned:

- The solar charge controller "turn on" point seems to be around 12.4v for the house batteries. I just had this confirmed as the "official" set point by AS Tech Support (previously I had been told 12.2v-12.4v).
- I was also told by AS Tech Support that the solar charge controller does NOT charge the chassis battery, in spite of what I was told prior. Previously I had been told that the BIM will switch the charge over to the chassis battery once the house batteries reach a certain voltage. According to this new info, this may not be the case. They are telling me the only things that will charge the chassis battery are shore power, the alternator, and the generator. I'm not sure I believe this based on some behavior that I've seen with my solar charge controller and the SOC of my chassis battery over time.....

With too much uncertainty surrounding what charges the chassis battery and when, I bought a 5W solar panel with a built in charge controller (NOCO, same company that makes the Genius line of chargers which I use for both AGM and Lithium batteries....really nice units) and have it mounted in my front window between the window and the sun shade. It's plugged into the 12v outlet in the center of the dash below the cup holders/ash tray since that socket is always hot. The NOCO has a really robust multistage microprocessor controlled charging algorithm for most battery types including AGM. The panel/charge controller combo also has built-in back charge prevention so the panel doesn't draw from the battery at night. So far, this unit seems to be doing the job and is keeping my chassis battery right around 12.75-12.80v. For $50, I think it's cheap insurance. Time will tell.....

[Boxster1971]

Originally Posted by FlyFishinRVr

UPDATE:........

- I was also told by AS Tech Support that the solar charge controller does NOT charge the chassis battery, in spite of what I was told prior. Previously I had been told that the BIM will switch the charge over to the chassis battery once the house batteries reach a certain voltage. According to this new info, this may not be the case. They are telling me the only things that will charge the chassis battery are shore power, the alternator, and the generator. I'm not sure I believe this based on some behavior that I've seen with my solar charge controller and the SOC of my chassis battery over time.....

The BIM will allow the coach batteries to charge the chassis batteries no mater which source is charging the coach batteries. The main reason Airstream said that solar does not charge the chassis battery is that the 100 watt solar panel they install is just not enough and can't even keep the coach batteries fully charged.

I assure you my 400 watt solar setup keeps my chassis battery fully charged along with the coach batteries. I added an indicator light on my BIM and can now tell whenever the relay is close connecting the two battery systems. I also changed the momentary boost switch at the driver's seat to a two position ON/OFF switch with an indicator light. Now my PCI BIM works like a Blue Sea ML-Automatic Charging Relay (ML-ACR).


- - Mike
2013 Lounge EXT on 2012 Sprinter

[FlyFishinRVr]

Originally Posted by Boxster1971

The BIM will allow the coach batteries to charge the chassis batteries no mater which source is charging the coach batteries. The main reason Airstream said that solar does not charge the chassis battery is that the 100 watt solar panel they install is just not enough and can't even keep the coach batteries fully charged.

I assure you my 400 watt solar setup keeps my chassis battery fully charged along with the coach batteries. I added an indicator light on my BIM and can now tell whenever the relay is close connecting the two battery systems. I also changed the momentary boost switch at the driver's seat to a two position ON/OFF switch with an indicator light. Now my PCI BIM works like a Blue Sea ML-Automatic Charging Relay (ML-ACR).


- - Mike
2013 Lounge EXT on 2012 Sprinter

Hmmmmm.......

Looking at the schematics in my owners manual, it looks like the only inputs to the Battery Separator (I think this used to be called the BIM...now it's just BS....[emoji2]) are the ignition, generator, inverter/charger, and emergency start switch. The info. re. the Battery Separator says it closes (connects house and chassis batteries together) when it detects that the batteries are charging and opens when it detects they are discharging.

I think what you are saying is the stock 100W panel is a bit anemic but provided the load is small enough, the panel might be able to handle it and still push current into the batteries, in which case the BS is closed, otherwise it's open. In your case, with 400W of solar you are pretty much always providing some charge to the batts so for you the BS (BIM) would almost always be closed.

Is this setup a change on the 17's or did they just change the names to confuse us? [emoji12]

[FlyFishinRVr]

One more thing I see based on the schematics. While AS claims the BS closes when it detects the batteries are charging and opens when they are discharging, I think that's a bit misleading. It looks to me as if the BS closes any time the ignition is on, gen is on, inverter/charger is on, or the dash switch is pressed, regardless of whether these actions actually result in the batteries charging or not. When none of those are on, the BS is open. I don't see anything that actually senses current flow and acts accordingly, so unless I'm missing something, the system isn't as smart as the manual would have us believe. This would certainly explain why my chassis battery was running down......

[toskeysam]

What I have observed is my 100 watt panel will do all it can to charge the coach batteries. If it gets them up to the set point it will switch to the chassis battery. Doesn't do well if I have several straight days of crappy weather. It never gets the coach up to cutoff voltage, so chassis battery just goes down.....and it has.
I am taking care of that this winter with a scheduled upgrade to 400 watts of solar and changeover to lithium batteries.

[amirm]

Originally Posted by FlyFishinRVr

One more thing I see based on the schematics. While AS claims the BS closes when it detects the batteries are charging and opens when they are discharging, I think that's a bit misleading. It looks to me as if the BS closes any time the ignition is on, gen is on, inverter/charger is on, or the dash switch is pressed, regardless of whether these actions actually result in the batteries charging or not. When none of those are on, the BS is open. I don't see anything that actually senses current flow and acts accordingly, so unless I'm missing something, the system isn't as smart as the manual would have us believe. This would certainly explain why my chassis battery was running down......

It is a voltage sensitive relay (controlled through an onboard microprocessor) which can then detect when the alternator is running due to its much higher voltage. Current flow is then implied due to higher voltage causing charge current to develop.

[Boxster1971]

Originally Posted by FlyFishinRVr

Hmmmmm.......

Looking at the schematics in my owners manual, it looks like the only inputs to the Battery Separator (I think this used to be called the BIM...now it's just BS....[emoji2]) are the ignition, generator, inverter/charger, and emergency start switch. The info. re. the Battery Separator says it closes (connects house and chassis batteries together) when it detects that the batteries are charging and opens when it detects they are discharging.

I think what you are saying is the stock 100W panel is a bit anemic but provided the load is small enough, the panel might be able to handle it and still push current into the batteries, in which case the BS is closed, otherwise it's open. In your case, with 400W of solar you are pretty much always providing some charge to the batts so for you the BS (BIM) would almost always be closed.

Is this setup a change on the 17's or did they just change the names to confuse us? [emoji12]

Sorry for confusion on terms. BIM (Battery Isolation Manager) is term from the component manufacturer, Precision Circuits Inc. It performs the function of a Battery Separator (BS), the terms used by Airstream in their schematics. I think Airstream is still using the PCI made component in the 2017 models, but I haven't confirmed that. However looking at the 2017 schematics it looks like the same item.

There are two types of connections on the BS. First the main connections across the relay which tie the coach and chassis batteries together. I see on the 2017 schematic that Airstream has added the main cable from inverter/charger, CBL15 (1/0 Ga. RED) to the coach battery lug on the BS relay. In older version they connected this cable directly to the positive battery box lug. But with the batteries now mounted under the van there is no battery box as in the past.

The other connections shown on the BS are control inputs to the electronics module that is separate from the relay. They are from generator, ignition and emergency start switch. The electronic module controls the BS relay and measures voltage in each battery system, and uses varied logic depending on time, ignition state and generator function.

The solar panel charges the batteries independent of the BS and if the coach battery voltage is high enough while the chassis battery is low enough it will close the relay without Sprinter engine or generator operating.


- - Mike
2013 Lounge EXT on 2012 Sprinter

[Boxster1971]

Originally Posted by FlyFishinRVr

One more thing I see based on the schematics. While AS claims the BS closes when it detects the batteries are charging and opens when they are discharging, I think that's a bit misleading. It looks to me as if the BS closes any time the ignition is on, gen is on, inverter/charger is on, or the dash switch is pressed, regardless of whether these actions actually result in the batteries charging or not. When none of those are on, the BS is open. I don't see anything that actually senses current flow and acts accordingly, so unless I'm missing something, the system isn't as smart as the manual would have us believe. This would certainly explain why my chassis battery was running down......

The schematics don’t show the details of the logic inside the BS control module. Here is the logic from a Precision Circuits Inc. technical document I obtained a few years ago. I’m also attaching a copy of the full document.

Detailed Operation:
1) Relay is turned on if:

a) Ignition is on for 20 seconds &

2 minutes have passed since Relay last turned off &

Coach Battery less than 12.6V &
Chassis Battery is greater than 13.2 &
Chassis Battery is Less than 15.5V &
Generator is off

b) Ignition is off &

10 minutes have passed since Relay last turned off &

Chassis Battery less than 12.6V, &
Coach Battery is greater than 13.0V &
Coach Battery is less than 15.5V

c) Generator is On & Ignition is On then the Alternator and Battery charger are fighting each
other and Relay should be turned off.


2) Relay is turned off if:

a) Ignition goes from on to off state
b) Relay has been on for 1 hour
(prevent overcharging and allow to view separate voltages)
c) Anytime Ignition and Generator are both on.
d) Anytime either Battery goes above 15.5 volts for 30 seconds
e) (Coach battery charge can drop to support the engine, in start and stop situations)
While the Ignition is on, the time the Relay will remain on is Voltage dependent

i) High end of time 12.8volts = 40 minutes
ii) Time is scaled between above and below values
iii) Low end of time 11.8 volts = 5 seconds

f) (Thou shalt never discharge Chassis battery for Coach functions)
While the Ignition is off, the time the Relay will remain on is Voltage dependent and shorter
than while the Ignition is On

i) High end of time 12.8 volts = 10 minutes
ii) Time is scaled between above and below values
iii) Low end of time 11.8 volts = 5 seconds


3) Relay Coil will be driven with approximately 4Volts DC. The Solenoid will be turned on hard with
full battery voltage, and then the voltage will be throttled back to reduce battery power and Relay
heat.

[FlyFishinRVr]

Thanks guys. Really appreciate the education on this.

[FlyFishinRVr]

Originally Posted by Boxster1971

The solar panel charges the batteries independent of the BS and if the coach battery voltage is high enough while the chassis battery is low enough it will close the relay without Sprinter engine or generator operating.


- - Mike
2013 Lounge EXT on 2012 Sprinter

I would agree with that statement if you meant "The solar panel charges the COACH batteries independent of the BS", because that appears to be how the solar charge controller output is wired into the bus.

As for the second part of your statement, I looked through the data sheet on the BIM (thanks again for that) and I think I see a flaw in their logic and would appreciate your take on it.

For the ignition off case (1b in Detailed Operation), the data sheet says the relay will be turned on (i.e. closed, connecting the coach and chassis batteries together) if the chassis battery is below 12.6v AND the coach batteries are above 13.0v (but below 15.5v). This is what you are saying above. However, if the coach batteries never get above 13.0v (which mine do not via the stock solar panel/charge controller), according to their condition chart the relay will never close, meaning the chassis battery will never get charged via the OEM solar setup. This would explain why I see my chassis battery voltage continue to decline, even though my solar is indeed turning on and charging my coach batteries (per the solar charge hours counter and voltage level when I check it).

Thoughts?

[Boxster1971]

Originally Posted by FlyFishinRVr

I would agree with that statement if you meant "The solar panel charges the COACH batteries independent of the BS", because that appears to be how the solar charge controller output is wired into the bus.

As for the second part of your statement, I looked through the data sheet on the BIM (thanks again for that) and I think I see a flaw in their logic and would appreciate your take on it.

For the ignition off case (1b in Detailed Operation), the data sheet says the relay will be turned on (i.e. closed, connecting the coach and chassis batteries together) if the chassis battery is below 12.6v AND the coach batteries are above 13.0v (but below 15.5v). This is what you are saying above. However, if the coach batteries never get above 13.0v (which mine do not via the stock solar panel/charge controller), according to their condition chart the relay will never close, meaning the chassis battery will never get charged via the OEM solar setup. This would explain why I see my chassis battery voltage continue to decline, even though my solar is indeed turning on and charging my coach batteries (per the solar charge hours counter and voltage level when I check it).

Thoughts?

You have it right. If your coach batteries never get to 13V from solar then the BS/BIM relay will not close to connect the two battery systems. My solar floats my coach batteries at 13.4V every day so there is plenty of power to also keep my chassis battery fully charged.

I don't see this as a flaw in the logic. You wouldn't want the relay to close at any lower voltage on your coach batteries as it would drain their capacity.

Also keep in mind that the relay can close to connect the batteries when charging from shore power or generator if the chassis battery is low. Both shore power and the LP Generator power the Magnum inverter/charger to bring the coach battery voltage above 13V.

The logic also keeps the possible 15+ volts from equalizing the coach batteries from reaching the chassis battery and possibly damaging the Sprinter vehicle electronics.

[FlyFishinRVr]

Originally Posted by Boxster1971

You have it right. If your coach batteries never get to 13V from solar then the BS/BIM relay will not close to connect the two battery systems. My solar floats my coach batteries at 13.4V every day so there is plenty of power to also keep my chassis battery fully charged.

I don't see this as a flaw in the logic. You wouldn't want the relay to close at any lower voltage on your coach batteries as it would drain their capacity.

I say it's a flaw because it leaves a gap. On paper it's a 0.4v gap, in reality it's about a 0.2v gap. On my rig, the solar keeps my coach batteries around 12.80-12.85v. There just isn't enough solar up there to push over the magic 13v mark to turn the relay on. As a result, this seemingly insignificant gap is a guarantee that my chassis battery will die if I don't intervene. Not smart......

I think if they changed the trip point on the solar charge controller from 12.4v to 12.6v, that may solve the problem because it should reduce that gap to zero, but personally I don't get why they limit the solar at all. It has its own built in charge controller so why not just let it do its thing and provide a charge any time there's sunshine?