[RV8R]

The answer to that would be in the details.

I have successfully ( so far ) since 2018 used AGMs as house batteries in Promasters. Mostly from “direct charging methods”.

But, if not for our need for cold weather operations, if building today would probably go LFP ( lithium ).

Recently, I installed a Kisae 1250 DC2DC. Totally user programmable for exactly what FLA, AGM, or FLP battery chemistries.

Much of battery health depends upon one’s understanding of what a specific battery needs / wants to be charged & managing that.

So does Winnebago have an electrical diagram that you can post ,,, if they do, there in those details might be the answers to your question.

 

[RV8R]

So , you did decide to change from the alternator to a DctoDc charger. Whose AGM’s are you using?
Will check for wiring diagram, but pretty sure It is direct with a solenoid to separate them before starting. Good question.

I use 2 of these 6volts in series “Rolls AGMs”;

https://rollsbattery.com/battery/s6-275agm-re/

Note; My actual placards on the actual battery state 83 amps max charge ,,, the above spec states 75 amps ,,, so I assumed the 83 amps is correct.

I was installed the Kisae 1250 from “direct charging” these AGMs because our typical use pattern changed, & we wanted to utilize more of the battery capacity. This required the current to be “capped”, & as the Kisae is very user programmable for FLA, AGM, & LFP I bought one to try it out. I love it & so far like how it works to provide charge to the exact profile the battery manufacturer has stated.


If you want to learn more about our needs changing & the Kisae 1250 it starts at this post;

https://www.promasterforum.com/threads/rv8rs-new-build-tetris.93938/post-838966

 

[RV8R]

Winnebago is quite good with this, all their schematics are online Wiring Diagrams

should be easy for the OP to post the relevant diagram then 👍.

 

[RV8R]

It's not a winnebago thing; probably 95% of rv's use flooded or sealed lead acid batteries charged directly from the alternator. It may not charge the battery "optimally" but it works. Mostly diy'ers like to agonize over this stuff.
If I were you I would leave well enough alone. If later you upgrade to LiFePO4 then you should add a dc/dc charger.

I understand @Pierre O perspective here, however I also am of the believe “an Education in the matter” is not a waste of time.

If you have an interest, learn about it, & then decide if you want to make the system better once you know everything about it.

Unless we are dropped on our heads, we don’t loose the things we learn.

 

[RV8R]

AGMs can't take being chronicly under-charged. If the Solis uses a solenoid system, the house AGM gets what the FLA starter battery gets. The two charge profiles are not the same, but not hugely different either. Before B2Bs (DC-DCs) came along, folks got by with solenoid systems for their AGMs, and many still do. However for longest life, AGMs need to be kept fully-charged most of the time. If you don't drive enough, or have enough solar, chronic under-charging will shorten their life. A B2B provides the correct AGM charge profile and maintains a consistent (faster) charge current. But, that still might not be enough if you don't drive enough, or have enough solar (both panels and sun). Enough charge depends on how much AGM you have.

You also can't manage what you don't measure. Does the Solis have a battery monitor, preferably one that tracks State of Charge (% SOC)? With AGMs, voltage can also be used as a surrogate for SOC. There are tables online. That way you can tell if your AGM is getting fully charged often enough. At least once a week? The more, the better.

We started out with a 500Ah bank of "cheap" AGMs, a relay-type controller for alternator charging, and 400W of solar. Those two charging sources could not fully charge our AGMs often enough. They only lasted 2 years. We switch to a B2B and lithium at the same time. Lithium isn't affected by under-charging.

💯 @SteveSS

IIRC, Steve had a “bad ground” wire while on a van trip & that pooched his AGMs ??

There is a difference of a “non-perfect charge profile” & chronic under charging.

There is what the battery wants & what it gets.

I can say with the Kisae 1250, I get to set the voltage & amperage on different stages. With my limited experience it seems to be just what I wanted for a DC2DC.

@PROMASTER BOB ,, That thread I posted above on the Kisae 1250 shows the before & after voltages & amperages I was getting.

My need was to limit the amps, but I also gained the ability to set my voltage. Where I believe my direct charge method was “ok”, this DC2DC gives my Rolls AGMs what the manufacturer says they want.

 

[RV8R]

We experienced a couple loose connections (+ side) between coach and starter batteries. But their contribution to under-charging was only temporarily until I found them. It wasn't the main issue long-term. Basically, our charging systems (alternator and solar) could not fully charging our AGMs often enough to keep them healthy. There were several contributing factors that resulted in death by chronic under-charging. Hence our migration to lithium, which isn't harmed by under-charging.

They are definitely a different animal & must be cared for differently.

If my AGMs ever pack it in I would deal with the 0C or below temps & find an efficient way to utilize replacement lithiums.

As you stated, AGMs want to be stored / kept 100% charged up. When our van is not in use our AGMs are turned off & in storage mode.

 

[RV8R]

My neighbor has one of those (slightly older - pre-covid. )

Theirs has some solar on it as well, not certain how much right now.

A historical method of dealing with charging batteries is that the alternator charging can do the bulk / higher charge rate portion and solar can do the slower / finishing touches / stages of the charge.

For my own work, I just assume that the power will always be on and that is what the solar is for - dealing with it all automatically and I can be lazy and not even think much about it. Once you are running 400 - 600 watts of solar, many issues just disappear.

Thanks @HarryN

I have thought of this scenario for my van or cabin. My conclusion if I ever purchased LFP is they would go in my van & my cabin would double the Rolls AGMs.

In the van the LFP would be stored @ 50% to 80% SOC with nothing connected but the Victron BM712 ,,, You know Storage Mode.


So why the AGMs in the Cabin ?? 🤷‍♂️ Well because they are on solar. Our Solar is on 100% of the time from April to October. With basically no drain, I would predict every day the batteries would be 100% charged. Even if a slight parasitic drain occurred the recovery of no use would be topped up right after the sun was up. AGMs were meant for that absence of cabin dwellers, but left to run on solar & top up 100% charged all the time. If I threw LFP into that mix they would be 100% charged & trickle charged as long as the sun was shining..

Is that really the scenario I would want for LFP in my cabin? 100% charged & Solar trickle charged all day long ? Again, my understanding is LFP likes to be stored @ say 50%.

For my van, LFP would be stored @ say 50% & the manual switches would disconnect them ,,, basically in storage mode.

 

[RV8R]

The Solaris, like most commercially produced RVs, has about 50% of the battery and solar capacity it needs to keep up with how people want to use them. The pop top adds to the complexity of the situation.

As you are seeing with your own recently added BTB charger, it adds another layer to the charging reliability by making it more predictable. The specific case of a promaster vs other vans on the market make it right on the edge of feasible to not have a BTB charger, but it will never be as good as having one. Solar can be used in some situation to fix this.

_

In my mind, a battery is just a tool, more or less a screwdriver or hammer or tire.

If a screwdriver is just sitting inside of a drawer of an air conditioned home, it will last more or less until you loose it. If you use it in daily commercial use, it will last perhaps 3 - 24 months before the tip is worn out or lost, even a carbide tipped one like I use.

A tire mounted on a van will wear from mileage, etc, but will last longer if being run under reasonable conditions. Nonetheless, just being outside and mounted on a van, it will eventually need to be replaced due to sun exposure and time. I believe every 5 - 10 years or similar?

Any battery sitting in a van, even at a perfectly stored SOC, will also still wear out, so we are really only talking about if your goal is to "use the tool" or just "take pictures of it".

One way to keep a van house battery in good condition is to slightly exercise it. A small trickle load such as an LED light or even just a SOC monitor will keep the battery from being at absolutely full charge 100% of the time. This can be made up for by a small amount of solar trickle charge that will being it back up every day or so during the day time. Even 5 - 10 watts located inside of the vehicle.

Most chargers trigger on a voltage swing, so with an AGM they tend to trigger the charger on with a relatively small change in SOC.

With LiFe, the voltage swing is pretty small, so a solar charger might not even turn on at 95% SOC and very light loads like just a monitor or LED light running, depends on the exact settings.

So for $100 you could more or less skip your planned complex scenario and increase the battery life at the same time.

The cabin is more or less the same scenario.
_
If you are religiously against the above approach, then take advantage of the idea that you regularly use the van.

If you replace the existing 2 batteries with a pair of good quality 12 volt x 100 amp-hr batteries, then they will have a usable capacity of roughly 1500 watt-hrs.

If you have a 1 - 2 watt load that is just left on all of the time, then they will stay below fully charged and run this way perfectly happy for a month+. At that point, it is time to drive the van anyway because the starter battery or other things need to be dealt with.

__

As a practical matter, we are all spending a lot of time thinking about what might be the most reliable consumer item that we own. What else do we own that can be regularly used, every day, and last 10 - 20 years with almost no maintenance? Not much.


Use your tools / batteries and have fun.

Thanks for your words of advice & information @HarryN 👍

I have only briefly looked @ commercially built RV electrical systems, but from my review they seem more “hype” & punch words advertising than substance. I look to the Marine Industry rather than the RV Industry for my “self education” & attempt to understand. I think the Marine Industry it is a much better standard to follow. A Van isn’t a Boat, but I want about as many electrical problems out there on the road while out on a road trip as I would want on the open water ,,, none if I can achieve it.

My Cabin is 12vdc fully functional from April to October ( if we are there or not ). Automatic & 100% reliant upon Solar. When we are not present, the system uses about 80Wh per 24 hour period, so basically very little. Since we have an automatic solar charger, that energy gets replaced & the AGM batteries get topped up to 100% everyday ,,, & AGMs like that. So basically the AGMs are being “stored” @ full 100% SOC which they like. If I switched to Lithium the new Lithiums would also be stored @ 100% SOC ( at least that is how I imagine that will work ). Further my understanding of Lithiums is they do not like to be stored @ 100% SOC ,,, more like 50% to 70%. Maybe what I have read & this is incorrect or my idea of stored is skewed 🤷‍♂️ ??

My Van; We shut down the AGM house batteries when we are not using the van. So with AGM that is stored @ 100% SOC ( now the charge profile is set by my new Kisae 1250 ). If I swapped out our house AGMs for Lithiums, we would still shut down the Lithiums into “Storage Mode” when the van is not in use. We thus could store future Lithiums at 50% SOC or whatever the Lithium battery manufacturer “specs” as storage. We do not have a need to keep the house batteries in “operational mode” if we are not using the van ,,, just like your screwdriver tool analogy, I do not care about my screwdrivers ,,, I go to where I store them ( tool box ) & I grab one, use it, then once I am done with it I put it back in “storage mode” & forget about it.

some motorcycle dealers I am aware of remove the trickle charger connections when the motorbikes are supplied with Lithium starter batteries on none consuming starter batteries by factory alarm systems etc. Why ?? Because constant trickle charging these motorcycle lithium starter batteries can provide them with an early death. Not so with FLA or AGM as they want to be kept @ 100% SOC.

Overthinking !! ,,, Well I have been accused of overthinking once or twice in my life. I typically just ignore or dismiss such statements. Being curious of my World, Science, & How things work coupled with questioning Authority has worked out well for me. Of all the knowledge out there, I possess very little. One thing that really makes me pay attention & especially if they are in Authoritative Positions is if they attempt to dismiss my curiosity with such statements as “You Are Over Thinking It” 🤔. Learning about “Things & Stuff” is a big part of my life & just the way I tick. Of course, sometimes my Overthinking dissatisfies Mrs. RV8R. 😳

 

[RV8R]

If you have a programmable solar charge controller for your lithium batteries wouldn't it make sense to program an additional profile for storage? If the van will be sitting for 2,3, or 4 months you could switch to your "storage/maintenance" profile and have the max charge voltage set at 60-80% soc. You would have to switch back when you are going to use the van but you wouldn't have to monitor the batteries while in storage.

For my Cabin; that was exactly what I was thinking if I ever went to Lithium. 👍

I have AGMs, so I only need the 1 profile / program.

Here are a couple of screenshots of the cabin daily Solar charging ( listed today was the day we showed up late in the day Oct 23 );

This was 30 days ago ( Oct 23 less 30 days so Sep 23 if I matter right ). So that day we needed 790 Wh to get back to 100%. The shades are Bulk (bottom), Absorption (middle), Float (top);

The Solar Charger is a Victron 100 | 50

 

[RV8R]

I agree that a van and a boat are very similar - certainly from an electrical viewpoint. Another good reason to use 24 volt in vans instead of 12 volt.

Many people disagree with me on this point, but once you install Li batteries in a van, it is no longer a vehicle, it is an extension of your normal living space. You don't trip the breakers in your home to cut off the lights in a room just because it isn't used much. Similarly, when you have Li batteries in a van, you don't necessarily turn off the power. It is now a cabin on wheels that is parked out front.

I have never turned off any of the power systems that I have built except for maintenance or edits and usually neither have my customers. That is both AGM and Li.

Maybe I am wrong, but I don't think that it is all that practical to discharge a battery pack down to 70% SOC every time you use it just to turn it off. You will need to reach an acceptance of imperfection or stay the course.

Maybe do some testing of your system "as is" with just the fridge turned off and see just how much power is really being used in 24 hrs of standby. As I stated in the other post, it might make sense to have a 10 watt solar panel inside of the van connected to your existing charger to off set the parasitic losses if that is a concern.

Thanks for those 2 posts @HarryN ,,, Very Helpful;

We will test out what we have ( both Cabin & Van ), for now & see what to do next n the future. Then revisit these systems down the road when there is a need / want / or hobby decision 😁.

I suppose we all have our experiences & thus biases in how we look at things. I like exploring the ideas of others to expose my biases. I suppose my “Shut’r Down” approach to my Van house batteries & even my Jeep starter battery is my aviation bias of turning off the “Battery Master” ,,, Basically at the end of each flight we turn off the battery via cockpit switch via solenoid. I do not own a boat, but do they have a battery masters ?