Victron battery Isolator for charging 2 different battery banks?

[jjchvl]

I checked the other posts on isolator topics and none seem to address my question. So here's some background on my situation: I recently upgraded my 900ah AGM battery bank to an 800ah Lifepo4 battery bank. Rather than stick my nearly new AGM's in a corner and forget about them I've wired them back into my system as back up storage. Ive got them connected via a high amp 5 post solenoid that engages the AGMs as it disengages the lithium's and vise versa. I can activate this solenoid switch from within my RV. The only problem is that the bank of AGM's is not currently connected to any of my Solar arrays for charging purposes.

I had been thinking about using a manual transfer switch to connect the output of at least one of my three solar charge controllers to both battery banks but getting out of my RV to flip the switch when the AGM's needed a charge and having to remember to flip it back later seemed like it would be a challenge for me. Then, while researching/shopping for a better solution, I stumbled across the Victron battery isolators. From what Ive learned about them, one would connect both battery banks to a common ground, a charging source is connected to the input, one battery bank to output #1 and one battery bank to output #2. The isolator keeps both battery banks completely separate from each other yet allows both to be charged by the same charging source. Directing a charge into two different banks of batteries while keeping those batteries separated is all the isolator does. Whatever comes into it from a charging source is what it provides to the batteries connected to it.

I dont think that these isolators were originally designed to be connected to a battery bank of lithiums and a battery bank of AGM's but Im trying to figure out if/how doing so would be harmful to either bank of batteries. Using my admittedly limited knowledge of all of this stuff, it seems to me that since lithium batteries have a lower internal resistance than AGM's, the lithiums would draw most of the current at first and charge up. Once they were full, the AGM's would then start charging. If this is indeed how it would work, its exactly what I would want.

The only thing that the AGM batteries will be connected to full time is a very low draw, wireless temp monitoring system that monitors the temp in my lithium battery compartment. Other than that, the AGM's will be used as back up storage, only to be employed during overly long stretches of cloudy days. This is why I feel that I can keep the AGM bank topped off secondary to the lithium battery bank and using just one of my solar arrays/controllers. I can always add the input of a second solar array/controller to the isolator later if keeping the AGM's topped off requires it(60amp controllers, 200amp isolator).

Currently, my charge controllers are set for "lithium" batteries and at 14.4v charging. For the one controller connected to the isolator, I'd keep that same 14.4v max setting but set the controller to AGM and program a float charge of 13.8v. From what Ive read, such settings should work well enough for both battery types since the Lithium batteries would effectively ignore the float charge anyway.

So does anyone know for sure if this will work? Or not work? And why? Any info or insight about the idea would be extremely appreciated. Thanks!

PS: If this isolator option isn't feasible, Ill probably hook the charge controller(s) up to both battery banks via another $15 high amp, 5 post relay with a switch to activate the relay installed an instrument panel up inside my RV. The panel is already there and houses the gauges and so on that I use to monitor my solar power set up. This would work fine but would mean that Id have to remember to switch back and forth and Ive always been a bit scatterbrained. Having the charging happen automatically would be a huge plus for me.

 

[Rocketman]

For automatic charging- use a Dc-Dc charger to charge the AGM’s from the lithium bank.

I use Victron’s dc-dc charger - it works well.

 

[jjchvl]

For automatic charging- use a Dc-Dc charger to charge the AGM’s from the lithium bank.

I use Victron’s dc-dc charger - it works well.

Let me try to sum up what I think youre saying and please correct me if Im mistaken.

Youre suggesting that I buy another battery charger that taps into my lithium battery bank reserves to charge my AGM batteries instead of using one of my existing charge controllers which, to me is already a configurable battery charger that gets its power directly from the sun via my solar panels? What would be the benefits of doing it that way when Im not hooking anything up to the charging system of my RV? From what I understand, folk mostly use DC to DC chargers to protect their alternators from otherwise unreasonable demands that certain type/confugurations of batteries can put on them. That isnt my situation.

All I want to do is direct an already well regulated charge current from an already existing charger to two different banks of batteries at the same time while keeping the two different battery banks otherwise isolated from each other.

Someting I like about the isolator idea is that my AGMs wont start drawing much of a charge until the lithiums are charged because of the significantly lower internal resistance of the lithium batteries. Im assuming that things would work this way. I dont know for sure, which is why Im posting all this. Im hoping someone can tell me if im on the right track or not.

I dont know too much about DC to DC chargers, but assume that theyd try to keep my AGMs fully charged and wouldnt stop trying until the lithium bank dropped to a certain voltage. If true then that would be almost the opposite of what Id want.

Anyway, if Ive misunderstood anything you said, please let me know. Ir if Ive assumed anything that sounds rediculous, definitely let me know. Thanks.

 

[MisterSandals]

Other than that, the AGM's will be used as back up storage, only to be employed during overly long stretches of cloudy days.

What is the mechanism for cutting over to your AGMs as backup?
Wondering if this mechanism aids in crafting a possible solution.

 

[Rocketman]

Here’s my reasoning more in depth. You do have the general plan correct. My assumption is your agm pack is separate from the lithium one and the battery combiner does not turn on very often.

I use the Victron smart 30a Dc-Dc charger which is what I am basing this on.

Whenever the charge parameters were correct it would charge the AGM’s. I would get the Victron smart ones (that way you can make adjustments on the phone on your app).

Set it so when the Lithiums are about 13.7v or so it starts charging- this way it is only trying to charge when the sun is out and the lithiums are mostly charged. In my rig, it would charge for a few minutes on most days (about 30 min to an hour). That will be plenty to keep the AGM’s topped off.

Also because you have the charge controller set to agm, it’s charging them correctly.

If/when you use the AGM’s for power and run them down a ways. Then just drop the input side of the charger down to say 13.4v until it recharges the AGM’s or leave it as is, if your AGM’s will recharge fast enough with your solar.

I like easy automatic stuff that I don’t have to fiddle with.

Did I explain it well?

 

[jjchvl]

What is the mechanism for cutting over to your AGMs as backup?
Wondering if this mechanism aids in crafting a possible solution.

From my original post, which is admittedly too long and too boring for any decent attention span: "Ive got them(AGM bank and lithium bank) connected via a high amp 5 post solenoid that engages the AGMs as it disengages the lithium's and vise versa. I can activate this solenoid switch from within my RV."

Then from the PS of the same original post: "PS: If this isolator option isn't feasible, Ill probably hook the charge controller(s) output up to both battery banks via another $15 high amp, 5 post relay with a switch to activate the relay installed on instrument panel up inside my RV."

Conceivably, I could connect the controller(s) up to the same relay that Im using to switch between battery banks since all of the connections would be parallel anyway. But Im concerned about how many amps Id be putting through the single relay. The relay is rated to be able to handle brief bursts of up to 250amps but only 150amps at continuous duty. Running all of my highish amp 110v appliances at once plus whatever I might be running on the 12v side of my RV's electrical(which is connected to the same relay), Im pretty darn close to that 150amp continuous rating already. Adding just one 60amp controller to that mix would put me over the relay's capabilities. Granted, I never run the microwave and water heater at the same time, but who knows if Ill have to some day? Im also limited to a +/- 115amp draw on the inverter side of things by using just a 1500watt inverter anyway.

So yes, Ive considered connecting a controller up through the same relay that I use to switch between battery banks but I dont like how close that gets me to the relay's max continuous current rating. If I was going to use a relay to switch the output of the controllers from one battery back to the other, Id have to set it up on its own relay.

Id really like to not have to remember to switch the charge controllers back and forth though. Thats why the Victron isolator concept seems to attractive to me if its feasible. And I figure it should be feasible IF my assumption that my lithium batteries with their lower internal resistance, will suck up a charge first before my AGMs can start charging. That would make everything so simple, easy and pretty much mindless. It would "happen" exactly as Id like it to happen simply because of the properties of the batteries themselves.

So? What do you think? We've connected a lithium battery and an AGM battery to the same charger. While the two batteries are free to draw from the same charger at the same time, they cannot communicate with each other in any way shape for form. Will the lithium draw its charge first after which the AGM will start charging? I know for a fact that they wont charge at the same rate at the same time. So? What do you think?

 

[jjchvl]

Here’s my reasoning more in depth. You do have the general plan correct. My assumption is your agm pack is separate from the lithium one and the battery combiner does not turn on very often.

I use the Victron smart 30a Dc-Dc charger which is what I am basing this on.

Whenever the charge parameters were correct it would charge the AGM’s. I would get the Victron smart ones (that way you can make adjustments on the phone on your app).

Set it so when the Lithiums are about 13.7v or so it starts charging- this way it is only trying to charge when the sun is out and the lithiums are mostly charged. In my rig, it would charge for a few minutes on most days (about 30 min to an hour). That will be plenty to keep the AGM’s topped off.

Also because you have the charge controller set to agm, it’s charging them correctly.

If/when you use the AGM’s for power and run them down a ways. Then just drop the input side of the charger down to say 13.4v until it recharges the AGM’s or leave it as is, if your AGM’s will recharge fast enough with your solar.

I like easy automatic stuff that I don’t have to fiddle with.

Did I explain it well?

I see what your saying and agree that it would work but the cost of the equipment your talking about is pretty significant. At least to my budget it is lol! I can get 80amp Victron isolators for about $35 each. Or one 200amp for about $75(open box on Ebay). So if what Im proposing will work, it would be inexpensive AND "easy automatic". It would literally be a set and forget situation, requiring no further adjustments or fiddling with apps or even extra monitoring above and beyond what I normally monitor of my system. The two banks of different types of batteries would charge exactly as Id want them to charge simply as a result of the natural properties of their chemistries. The lithiums would suck up their charge first as a result of having a lower internal resistance and then the AGM's would charge. It sounds so perfect so Im here waiting for someone who knows better to blow my idea out of the water or tell me that, "Hey. Yeah. That just might work.". Id way prefer to be told the latter.

 

[jjchvl]

What is the mechanism for cutting over to your AGMs as backup?
Wondering if this mechanism aids in crafting a possible solution.

So? What do you think? We've connected a lithium battery and an AGM battery to the same charger. The two batteries are free to draw from the charger at the same time but are completely isolated from each other. Will the lithium draw its charge first after which the AGM will start charging? I know for a fact that they wont charge at the same rate at the same time. So? What do you think?

 

[MisterSandals]

We've connected a lithium battery and an AGM battery to the same charger.

This sounds like it connects the lithium in parallel with the AGMs. Unless your charger has 2 distinct outputs.
I have not connected differing chemistries. I am not sure how they would behave (I wrote this a couple times but could not agree with anything I tried to say, sorry).

I am not familiar with the Victron isolators so cannot comment on how that would behave.

You are on the cutting edge, I have not seen a good implementation of this. I like the idea of charging one battery from the other with configurable chargers.

 

[MisterSandals]

Will the lithium draw its charge first after which the AGM will start charging?

I found the person, Grizzman, who has written quite a few posts with tidbits of his experience on this matter. He has FLA and not AGM but hopefully his experience is relevant enough to be useful.

"In my experience, high current loads are mostly carried by the LFP bank. With that said, the FLA has load shared in my system. Mostly the FLA is held in float day and night. The LFP sucks up the solar amps much faster(that's a big plus on overcast days) biggest issue I see is dealing with a large (short) inrush during reconnection."

Do a search on Grizzman and AGM. Lots of posts. I think the bottom line is that he does not leave them connected all the time but frequently has them connected.

This is also pertinent to your specific question.

Hybrid lithium and lead?

Just ran across this: He advocates using both lithium-iron and lead-acid, tied together. Is he right? Does this have advantages? Or is he all wet?

diysolarforum.com

 

[jjchvl]

I found the person, Grizzman, who has written quite a few posts with tidbits of his experience on this matter. He has FLA and not AGM but hopefully his experience is relevant enough to be useful.

"In my experience, high current loads are mostly carried by the LFP bank. With that said, the FLA has load shared in my system. Mostly the FLA is held in float day and night. The LFP sucks up the solar amps much faster(that's a big plus on overcast days) biggest issue I see is dealing with a large (short) inrush during reconnection."

Do a search on Grizzman and AGM. Lots of posts. I think the bottom line is that he does not leave them connected all the time but frequently has them connected.

This is also pertinent to your specific question.

Hybrid lithium and lead?

Just ran across this: He advocates using both lithium-iron and lead-acid, tied together. Is he right? Does this have advantages? Or is he all wet?

diysolarforum.com

Thanks for pointing out those posts. I will look them up although Im not sure how relevant they will be. You may be missing one of the key aspects of what Im proposing. The Victron battery isolator Im referring to has three connection posts. One is labeled "input". Another is labeled "battery 1". The third is labeled "battery 2". Its as simple as it sounds. You hook up a charging source to "input", one bank of batteries to "battery 1" and another bank of batteries to "battery 2". The isolator completely "isolates" the two battery systems from each other while allowing them to draw current from a single charging source. In other words, current can travel only one way-INTO the batteries from a common charging source. Current cannot travel "backward" from one battery bank to the other.

So thats pretty much the entire purpose of a battery isolator: To keep two battery systems "isolated" from each other while giving them equal opportunity to draw from a common charging source. An isolator doesnt regulate charging current or prioritize a charge to either "battery 1" or "battery 2".

So, I know that I can hook up a lithium bank to one of the isolator's "battery" posts and an AGM bank to the isolator's other "battery" post. I can then hook up practically any appropriate charger to the isolators "input" post. None of this is a problem or a mystery. Neither do I think it would cause any significant harm to either battery bank. What Im asking is, under these conditions, will the Lithium batteries lower internal resistance cause the lithium bank to draw most of its charge before the AGM's will begin to charge? This would work out very well, if so.

 

[MisterSandals]

What Im asking is, under these conditions, will the Lithium batteries lower internal resistance cause the lithium bank to draw most of its charge before the AGM's will begin to charge?

From the few posts that i read from Grizzman and his experiences, it sounds like the lithium will draw significantly more but whether it is before the AGMs "begin" to charge i cannot say. I would guess that they both start at the same time but the lithiums might get 80% and AGMs getting 20%.

 

[jjchvl]

I found the person, Grizzman, who has written quite a few posts with tidbits of his experience on this matter. He has FLA and not AGM but hopefully his experience is relevant enough to be useful.

"In my experience, high current loads are mostly carried by the LFP bank. With that said, the FLA has load shared in my system. Mostly the FLA is held in float day and night. The LFP sucks up the solar amps much faster(that's a big plus on overcast days) biggest issue I see is dealing with a large (short) inrush during reconnection."

Do a search on Grizzman and AGM. Lots of posts. I think the bottom line is that he does not leave them connected all the time but frequently has them connected.

This is also pertinent to your specific question.

Hybrid lithium and lead?

Just ran across this: He advocates using both lithium-iron and lead-acid, tied together. Is he right? Does this have advantages? Or is he all wet?

diysolarforum.com

By the way, Victron (and other manufacturers) also make battery isolators that "isolate" three different batteries or battery banks from each other. Also, from what I understand, older versions of isolators used diodes to keep the current traveling in only one direction(into the batteries). For those isolators, diode use causes a slight voltage drop from whatever the connected charging source is putting out but the voltage output of Victron Diode based isolators can be manually adjusted to make up for the loss. More modern(?) isolators dont use diodes to keep current traveling in just one direction so dont have the voltage drop. just some random info that someone might be interested in knowing.

Ive purchased an 80amp, diode based Victron 2 battery isolator for cheap so I guess Ill just have to try my proposal without any input or advice lol! If it doesnt work how Im hoping it should, Ill be out $30 and no harm done. If it does work then that'll be pretty awesome. Ill be connecting just one of my 60amp controllers through it so there should be no concerns about over loading the 80amp isolator. If it works and I need to connect another 60amp controller to both battery banks to keep the AGM's charged, Ill simply get another $30, 80amp, 2 battery isolator.

 

[jjchvl]

From the few posts that i read from Grizzman and his experiences, it sounds like the lithium will draw significantly more but whether it is before the AGMs "begin" to charge i cannot say. I would guess that they both start at the same time but the lithiums might get 80% and AGMs getting 20%.

Ok, that sounds close to what I was hoping would happen. Like I said, I will definitely look up that fellas posts on the topic. Maybe the are relevant after all.

 

[MisterSandals]

Looks like he is around. Maybe he can be conjured: @grizzzman

grizzzman​

Some say "Why" and some say "Why not?"
Joined Sep 21, 2019
Last seen Today at 11:34 AM · Viewing forum list

 

[jjchvl]

Well, I just posted a private message for Grizzman. Ill see what he has to say! Thanks again.

 

[grizzzman]

Thanks MisterSandals I answered his PM.

 

[roly1]

I have been told Lithium charge profile is not ideal for agm’s and they will get progressively under charged using an argofet. So you still need a dc/dc or a solar controller with agm settings to top them off..

 

[jjchvl]

I have been told Lithium charge profile is not ideal for agm’s and they will get progressively under charged using an argofet. So you still need a dc/dc or a solar controller with agm settings to top them off..

Thanks, but that wouldnt have been an issue with how I had planned things. Id still have two charge controllers set on lithium parameters dedicated ONLY to the lithium bank. The third charge controller would be the only one "hybrid" programmed in order to properly charge my AGM bank as well. The problem that I encountered with my proposed set up is that since the battery isolator prevents "backflow" of juice from the battery banks its hooked up to, those battery banks cant communicate with the charge controller in any way, shape or form. And the controller cant properly charge a bank of batteries that it cant "see", even if it can turn on from solar panel power alone.

 

[roly1]

Does that controller have a sense wire or get it through the charge circuit? Why can’t it bypass the isolator? As long as the controller is powered from that battery. Thats how I planned to do it.
And dedicated small AC/DC dock charger.
Which is where boat spends most of its time.