Mixing lithium with AGM's - bashing around some ideas.

[JohnnyFry]

I just converted my RoadTrek to a LiFePO4 150AH coach system. I was definitely concerned with overtaxing the OEM 145 Amp alternator as the low internal impedance of the Li pack could easily overload it. Checking it out before installing the DC/DC charger it would easily such 70 amps from the alternator, and I suspect that was being limited by the 18 foor #6AWG line to the rear. I looked into various DC/DC units (there are not a lot out there) and ended up using a 20 amp Renogy DC/DC charger. That draws about 28 amps from the alternator when charging because of conversion efficiency. It is quite cost effective at about $130. Next size up is a 40 amp output, which would draw about 60 amps from the alternator and costs $200. They also make a 60 amp model but would probably be advised to install a larger alternator, like a Nations or something similar in order to use this one. The Renogy has several battery profiles available, including LiFePO4, using DIP switches. So far it is performing well and. of course, does not affect the flooded Pb starting battery.

[hbn7hj]

Just an opinion but the converter needs to be removed from the coach battery and tied in to the load side of the A/B/both switch.

[@Michael]

Originally Posted by hbn7hj

Just an opinion but the converter needs to be removed from the coach battery and tied in to the load side of the A/B/both switch.

Thinking about that...thanks.

Made a few changes since the last update.

Portable Solar: I bought a couple of flexible solar panels and set it up so I can charge either the lithium or the AGM batteries, depending on how I connect the panels. The panels are SunPower 110W.

Battery Combiner: Using ideas and information from @Markopolo starting at this post I (temporarily) connected a Victron Argo Battery Combiner between the lithium and AGM batteries.

Here's what I have (minus the Solar A/B):


In theory I can now charge the AGM's from any of alternator, converter, Solar or combiner; the lithium from any of alternator, converter (via the Redarc) and solar.

So far, the known issue is that I can never have both the solid state relay that connects the Redarc, and the On/Off that connects the combiner both 'ON' at the same time. That would create a perpetual motion machine, space vortex, blackhole, and universe-altering time-warp.

I still want to be able to remove the lithium battery in the case where it's below 0F here and I don't trust the battery warmer and furnace to keep the battery above zero. So that limits some of what I'd like to do, as I have to have a functional camper with the lithium removed.

[markopolo]

I think it's great that you're trying the combiner. If it works out then it could help a lot of the RT folks that got the single AGM + compressor fridge and now find that they need more capacity. A light weight 100Ah Battle Born that can be easily removed when the really cold weather sets in (or if they sell or trade in the RV) might be just the answer for them.

I'm hoping that you get really good results with your testing.

[GallenH]

Sorry but I've been gone for a while. Can someone explain exactly what the battery combiner does? It initially appeared to me that it would automatically switch batteries when one got low. Is this not true? And if it is true why would you require an A/B switch. Wouldn't the combiner take care of automating that without user intervention?

[markopolo]

The combiner has two inputs and one output. The two inputs are isolated from each other and both lead to the output. Identical batteries on the inputs would share load support at the output equally.

If dissimilar batteries are used then load support comes from the higher voltage battery until it is drawn down to the lower voltage battery and both batteries share the load.

Lithium batteries operate at a higher voltage than lead acid batteries through most of the discharge curve. If a lithium battery is on one input and a lead acid battery is on the other then load support comes from the lithium battery until its voltage is brought down to the lead acid battery voltage level.

A 100Ah lithium battery combined with a 100Ah lead acid battery could deliver 180Ah for example. No switch & no user intervention needed.

The two very different batteries are cared for individually. The lead acid battery can be on float charge while the lithium battery is at rest for example.

The combiner provides an easy way to use the lithium battery for most things while keeping the lead acid battery in reserve but always ready to go if and when needed.

The Victron Combiner uses Schottky diodes. Here's a link to data I collected with combined lead acid and lithium batteries using Schottky diodes: https://www.classbforum.com/forums/f...tml#post109324

[booster]

I just looked at the spec sheet for Argo combiner, and they talk about .45v drop at full rated amperage, so on an 80 amp unit they would be generating 36 watts of heat which is quite a bit. Voltage drop goes down a bit to .3v at lower loads.


For just running a frig at 3 amps, the power consumption is around 1 watt so not all the that much, but if you were running a microwave or AC unit it could be significant.


Of course everything would be running on slightly lower power because of the voltage drop, which would not likely be an issue for the lithium in the system, but could be for the AGM is any of the things running had low voltage cutouts like some of the frigs do.

[markopolo]

It's worth noting that you don't have to put the lead acid battery on an input. It could used like this: LFP battery to input #1 and the output to the lead acid battery. That avoids the voltage drop on the lead acid side but still permits using the lithium battery more than the lead acid battery.

[markopolo]

Just adding that the primary reason to use the diode (IMO) is to eliminate the risk of uncontrolled alternator charging of the lithium battery. You could combine the two types of battery via a switch but if you forget to separate them then there would be a direct line from alternator to the lithium battery.

[booster]

Originally Posted by markopolo

Just adding that the primary reason to use the diode (IMO) is to eliminate the risk of uncontrolled alternator charging of the lithium battery. You could combine the two types of battery via a switch but if you forget to separate them then there would be a direct line from alternator to the lithium battery.

In general, isn't better to also get the alternator charging off the lithium, or at least reduce the voltage way down, which most alternators don't do?


I haven't dug into the high end lithium system information about allowable charge rates, but it sure seems like the drop in types are quickly adding some pretty severe current limits. I think Battleborn is at .4C, which is in AGM territory. It it is a big bank like 800ah, it doesn't matter as you can't supply that much anyway, but in the smaller banks, it is very easy to go beyond .4C. I wonder if there is any more recent research work being done on the charge rates, or if some of the manufacturers have seen what they think are charge rate related failures?

[GallenH]

Originally Posted by markopolo

A 100Ah lithium battery combined with a 100Ah lead acid battery could deliver 180Ah for example. No switch & no user intervention needed.

Why is it less than the total of both batteries combined?

[hbn7hj]

Originally Posted by markopolo

A 100Ah lithium battery combined with a 100Ah lead acid battery could deliver 180Ah for example. No switch & no user intervention needed.

It would be nice if the combiner works that way (maybe it does, I have never used one.) Certainly, using the two batteries separately, I never get that much out of them. The lead acid voltage drops too low for anything but lights even before it gets to 50%. The lithium I’ve had below 45%.

Let us know the results when you do a test like that.

[markopolo]

Originally Posted by GallenH

Why is it less than the total of both batteries combined?

I figured you could use 100Ah from the Battle Born and 80AH from the lead acid battery. 100% DOD for the lithium & 80% DOD for the lead acid.

Harry makes a good point. It's only theory if it hasn't been tested.

[GallenH]

So if your setup was:

alternator——————AGM————————loads

Could you introduce the lithium with the combiner as:

DC->DC—Lithium—-|
| combiner———loads
alternator—|—————AGM—-—|

[GallenH]

ok that way of diagramming it didn't work. Sorry

[GallenH]

What I was trying to text/draw was that:

the incoming from the alternator would be split; one part going to the AGM as it was originally and one part going to a DC-->DC charger which goes to the lithium.

The AGM then goes to one side of the combiner and the Lithium to the other with the output of the combiner to the loads.

[markopolo]

Sounds like you have a good understanding of it. There will be some variation as to exactly where the DC-DC charger goes but it will ultimately be after the alternator and before the lithium battery as you indicated.

[GallenH]

Originally Posted by markopolo

Sounds like you have a good understanding of it. There will be some variation as to exactly where the DC-DC charger goes but it will ultimately be after the alternator and before the lithium battery as you indicated.

Not really. I'm just shooting in the dark and asking questions. I deliberately made the alternator the only charging source. Right now it goes directly to my AGM and it seemed from my past forum reading that it was a good idea to have the DC->DC charger to the lithium to get the proper charging profile. Once I add the shore charger and the solar into the system the equation probably changes. But I'm not sure. I don't know enough about the charging profiles. I know my shore charger can be set for lithium as can the solar. But what happens then to the AGM?

[rowiebowie]

Originally Posted by GallenH

Not really. I'm just shooting in the dark and asking questions. I deliberately made the alternator the only charging source. Right now it goes directly to my AGM and it seemed from my past forum reading that it was a good idea to have the DC->DC charger to the lithium to get the proper charging profile. Once I add the shore charger and the solar into the system the equation probably changes. But I'm not sure. I don't know enough about the charging profiles. I know my shore charger can be set for lithium as can the solar. But what happens then to the AGM?

Nothing happens to the AGM if you don't want it to. At least if properly isolated from the lithiums. My only non-lithium is my engine battery. It is isolated from my lithium coach batteries by the DC to DC charger. Engine battery still charges from the alternator while driving just as in any other verhicle. And the engine alternator can stlll charge my lithiums at the proper profile through the DC to DC charger.

I see a lot of over-think on the forum. I bought lithium compatible components, set them to proper settings for my battery type, then leave them to do their job. It's working. And with no additonal fuss than when everything was lead-acid.

[GallenH]

Originally Posted by rowiebowie

Nothing happens to the AGM if you don't want it to. At least if properly isolated from the lithiums. My only non-lithium is my engine battery. It is isolated from my lithium coach batteries by the DC to DC charger. Engine battery still charges from the alternator while driving just as in any other verhicle. And the engine alternator can stlll charge my lithiums at the proper profile through the DC to DC charger.

Understood. But I believe that your current setup is a bit different from what is being proposed with the combiner. In my case I was talking about the potential to use my existing coach AGM plus a coach lithium. The combiner presented an interesting solution.

The confusion might have been created from my discussion, though. I meant for the incoming alternator to be after the isolator involving the chassis battery.

It all stems from my reluctance to toss a 2-3yr old LifeLine and go completely lithium in the coach. If to try some hybrid combo ends up costing more for the added equipment (say, dedicating a 2nd shore charger for one of the batteries and similar adjustments from solar, plus an additional monitor for one of the batteries) it might just be less costly to toss the LL.