Questions about AGM -> LFP

[Urlauber]

This is just theoretical at the moment. I think.

So I have been thinking about replacing the two 100 Ah Group 31 AGM batteries in my Thor Rize with a single 100 Ah LiFePO4 battery. Mostly to save weight - if my calcs are correct it would drop from ~150lbs to ~28lbs.

Couple of pre-qualifications to start this off -

• Temps <32F rarely happen around here, and most likely I will not be camping then. If I do, I can take a separate Power Station inside and run things from there.
• My energy requirements are very humble (therefore the reduction in Ah)
• Where I go it is usually sunny enough for solar panels to do their thing
• We usually don't have a way to plug in (boondocking, dispersed camping)

My thought is NOT to charge the battery from the alternator. So the whole DC-DC and alternator current stuff should be a non-issue. Mostly, I want to charge it from solar, and if needed (expected to be very rarely) via the generator, or even less frequent, via hook-up.

I have been playing with a 100Ah Power Station and without recharging, it allows me to run the fridge and everything else needed (lights, water pump, propane solenoid, roof fan or Truma) for around 3 nights. However, via solar it will fully re-charge during the day. If staying in a shady place for extended periods I would have to run the generator to recharge after two days or so.

Questions...

1. Does this somewhat make sense, considering the specifics mentioned above?
2. Will not charging with the alternator simplify the installation - just need to disconnect/disable whatever charges the batteries during driving?
3. Solar should work by choosing the correct profile - right?
4. I would need to see if/how the converter handles lithium charging. I understand it may not perfectly fill up the battery, but maybe that doesn't matter as long as the solar panel fully charges the LFP?
5. Are there any other electronics/electrics that would be affected by a switch like this?
   

This seems all a bit too simple (besides the physical switcheroo) that I conclude I am missing a few things completely. One thing is that right now, the chassis and house batteries are somehow synchronized, not sure exactly how. I guess I would interrupt that connection and would need to keep an extra eye on the chassis battery.

[MsNomer]

If van and house batteries are synchronized, that suggests an isolator. LiFePO4 does not like isolators. We swapped the isolator for a B2B charger. If it were me, I’d swap that generator for more batteries.

[@Michael]

Originally Posted by Urlauber

1. Does this somewhat make sense, considering the specifics mentioned above?
2. Will not charging with the alternator simplify the installation - just need to disconnect/disable whatever charges the batteries during driving?
3. Solar should work by choosing the correct profile - right?
4. I would need to see if/how the converter handles lithium charging. I understand it may not perfectly fill up the battery, but maybe that doesn't matter as long as the solar panel fully charges the LFP?
5. Are there any other electronics/electrics that would be affected by a switch like this?

(1) Yes.

(2) W/O alternator charging, the design will be somewhat simpler. But a B2B is a fairly simple install if all you do is replace the existing battery isolator.

(3) Yes

(4) What make/model is your converter? Some have an AGM profile which is compatible with LiFePo4. Others have an equalizer mode which is not.

(5) Any charge source needs to be compatible with lithium. Charge consumers will see a voltage that is comfortably within the range that they see with lead-acid or AGM batteries and not be affected.

The exception might be your generator itself. AFAIK, the small generators used by B's draw more than 200A when starting, especially in cold weather. That amount of current likely will trip the LiFePo4 BMS overcurrent protection. In my case, I keep my AGM's around just to start the generator (which I never actually use). If I wanted to dump my AGM's, I'd have to either dump the generator also, or rely on the chassis battery to start the generator.

One of the advantages to LiFePo4 is that when you have to use your generator or alternator to charge the batteries, you'll need less run-time to fully charge LiFePo4 vs. AGM.

[booster]

Originally Posted by @Michael

(1) Yes.

(2) W/O alternator charging, the design will be somewhat simpler. But a B2B is a fairly simple install if all you do is replace the existing battery isolator.

(3) Yes

(4) What make/model is your converter? Some have an AGM profile which is compatible with LiFePo4. Others have an equalizer mode which is not.

(5) Any charge source needs to be compatible with lithium. Charge consumers will see a voltage that is comfortably within the range that they see with lead-acid or AGM batteries and not be affected.

The exception might be your generator itself. AFAIK, the small generators used by B's draw more than 200A when starting, especially in cold weather. That amount of current likely will trip the LiFePo4 BMS overcurrent protection. In my case, I keep my AGM's around just to start the generator (which I never actually use). If I wanted to dump my AGM's, I'd have to either dump the generator also, or rely on the chassis battery to start the generator.

One of the advantages to LiFePo4 is that when you have to use your generator or alternator to charge the batteries, you'll need less run-time to fully charge LiFePo4 vs. AGM.

200 amps for an Onan starter sounds way high to me.


Our Onan, when we had one, was on a 10ga wire which is horribly inadequate for 200 amps. Our van had 80 amp breakers from the engine to back and at 200 amps, even with help from the coach battery, would certainly trip them. Older vans were only 50 amps and they could use the engine running to start balky generators. I have never heard anyone say they had tripped them on either type.


Where did you see the "over 200 amps" statement? It would be interesting to see how they knew, or measured, that.

[Urlauber]

Thank you for the replies, very helpful in thinking this through.

Originally Posted by @Michael

(1) Yes.

(2) W/O alternator charging, the design will be somewhat simpler. But a B2B is a fairly simple install if all you do is replace the existing battery isolator.

(3) Yes

(4) What make/model is your converter? Some have an AGM profile which is compatible with LiFePo4. Others have an equalizer mode which is not.

(5) Any charge source needs to be compatible with lithium. Charge consumers will see a voltage that is comfortably within the range that they see with lead-acid or AGM batteries and not be affected.

The exception might be your generator itself. AFAIK, the small generators used by B's draw more than 200A when starting, especially in cold weather. That amount of current likely will trip the LiFePo4 BMS overcurrent protection. In my case, I keep my AGM's around just to start the generator (which I never actually use). If I wanted to dump my AGM's, I'd have to either dump the generator also, or rely on the chassis battery to start the generator.

One of the advantages to LiFePo4 is that when you have to use your generator or alternator to charge the batteries, you'll need less run-time to fully charge LiFePo4 vs. AGM.

2) Understood, but I feel that most of the time I do NOT want to charge via the alternator. It would tend to always fully charge the batteries on my way back from my usually short trips, and LPF batteries are happier when NOT fully charged all the time while waiting for the next outing. I would make the Solar input (off-) switchable as well to be able to further control that and only charge it to 100% shortly before leaving.

4) It is a WFCO WF-8955PEC

5) Charge sources would only be Solar, and generator/hook-up via the converter.

Good point about the current needed to start up the Generator. I read that too, where someone was doing this switch, but with larger LFP batteries. With a single 100Ah battery it may be iffy. I don't see fuses for the Generator, so not sure. It's one of the quiet-ish Onan 2800i, fed by propane.

I was thinking about re-wiring it to the Chassis battery, that might work, although add a bit to the overall hassle with the additional connection.

The other thing I was wondering about is the switch which somehow connects the batteries to either start up the vehicle with the house batteries or start up the generator with the chassis battery. I assume something would go "boom" if I would do the described drop-in and the LFP battery would be lower in voltage than the chassis Lead Acid one. Maybe it could be re-purposed as the Generator startup button, lol. Being able to recharge a depleted chassis battery from the house battery sounds like a good thing too, although most of the regular cars do OK without it.

I'd definitely need to find someone who is very good and familiar with the electrical setup of these beasts.

[booster]

I haven't found any specific 2800 Onan current draw specs yet or any real testing. I did find people stating a 10KW diesel takes 220 amps.


People were also mixing up actual starting current with the Generator specification for recommended battery cold cranking amp rating. The specifications for all engines will be way higher than actual because they allow a whole lot for output degradation with age as well they may have some voltage drop issues that the Onan can't handle.


For reference, when I was in college I put a 300 amp 8hp car starting unit in the trunk of an old beater Valiant and did car starting before and after classes and on weekends. 300 amps was enough to turn over a 500 cubic inch Cadillac engine at -30*F in Minnesota.



I will have to see if I can get the clamp on meter onto our 17hp two cylinder lawn tractor and see what it takes. The Onan is 7hp.

[@Michael]

Originally Posted by booster

I haven't found any specific 2800 Onan current draw specs yet or any real testing. I did find people stating a 10KW diesel takes 220 amps.

I just checked mine, an Onan 2800i. A clamp-on DC ammeter on the 4ft. long 2ga. (+) wire from the AGM's to the starter showed just over 100A in today's warm weather. The 'more than 200A' number is from a cold day last winter. I'm not sure how cold it was, but I'm sure the oil was thicker than it is today. I didn't get an exact number - all I needed to know was whether it was more or less than the 200A continuous discharge rating on my 300Ah lithium battery. It was more.

I know that Sunshine State RV in Florida swaps out the OEM 300Ah AGM on new Coachmen Beyonds for an equivalent lithium & uses that to start the generator. Not sure how well that will work in a Minnesota winter.

[booster]

Originally Posted by @Michael

I just checked mine, an Onan 2800i. A clamp-on DC ammeter on the 4ft. long 2ga. (+) wire from the AGM's to the starter showed just over 100A in today's warm weather. The 'more than 200A' number is from a cold day last winter. I'm not sure how cold it was, but I'm sure the oil was thicker than it is today. I didn't get an exact number - all I needed to know was whether it was more or less than the 200A continuous discharge rating on my 300Ah lithium battery. It was more.

I know that Sunshine State RV in Florida swaps out the OEM 300Ah AGM on new Coachmen Beyonds for an equivalent lithium & uses that to start the generator. Not sure how well that will work in a Minnesota winter.

It would probably also have been too much for whatever lead acid that was in there before was also, wouldn't it?



I still can't fathom that. The V8 cars I have had (5.5-6 liters), and still do only different ones, have tested under 100 amps in good warm conditions. If they are pulling 100 amps to start a 7hp motor that you can easily start with a rope by hand, that is one poorly designed starting system.



Users had been starting Onans with one gp24 deep cycle wet cell for many years without a lot of issues before the current two battery B's showed up as a minimum amount of battery.

[Urlauber]

Originally Posted by @Michael

I just checked mine, an Onan 2800i. A clamp-on DC ammeter on the 4ft. long 2ga. (+) wire from the AGM's to the starter showed just over 100A in today's warm weather. The 'more than 200A' number is from a cold day last winter. I'm not sure how cold it was, but I'm sure the oil was thicker than it is today. I didn't get an exact number - all I needed to know was whether it was more or less than the 200A continuous discharge rating on my 300Ah lithium battery. It was more.

I know that Sunshine State RV in Florida swaps out the OEM 300Ah AGM on new Coachmen Beyonds for an equivalent lithium & uses that to start the generator. Not sure how well that will work in a Minnesota winter.

Thank you very much for testing this! Is yours a gas or propane Onan? Not sure it matters. Maybe.

No doubt 300Ah of Lithium can start a genny - question is whether a single 100Ah could do it. If not, using the chassis Lead Acid could be an option. As far as I can tell most 100Ah LFPs have a 100A BMS so it may get tripped...

I know the two current AGM batteries don't have any issue starting the Onan (I assume they do that and the genny is not connected to the chassis battery).

[Urlauber]

Originally Posted by booster

It would probably also have been too much for whatever lead acid that was in there before was also, wouldn't it?

I wouldn't be so sure about that... quick check tells me that the ProMaster chassis battery has ~850 "Cold Cranking Amps" or 1000 Amps with a 90Ah capacity, so... there.

[@Michael]

Originally Posted by Urlauber

Thank you very much for testing this! Is yours a gas or propane Onan? Not sure it matters. Maybe.

I have an Onan 2.8kw gasoline. Unless the compression ratio is a lot different, I doubt that it matters.

Onan officially recommends a 360 CCA @ 0F battery. That's consistent with what I saw on a cold (but not 0F) day last winter (>200A).

[booster]

Originally Posted by Urlauber

I wouldn't be so sure about that... quick check tells me that the ProMaster chassis battery has ~850 "Cold Cranking Amps" or 1000 Amps with a 90Ah capacity, so... there.

That is the chassis battery which would be an SLA, which means it's intended use is "Starting, Lighting, Accessory" powering, battery like nearly all the vehicles come with. They are usually a lead acid wet cell with low/no water use calcium lead used in them and have sealed caps on them. True deep cycle batteries need to have water added regularly. The question is always, when they say 90ah capacity, is just how many cycles can they get if it is used a cycling battery. Most SLA batteries will be done after very few cycles if used for the coach loads repeatedly. Even the "combo" batteries" which in most cases are much closer to SLA than deep cycle won't last all that long in the coach. SLA batteries are designed to give big starting power and recharge most of that use quickly to ready for the next starting event. That is also why the alternators that are computer controlled will tend to run a quite high voltage immediately after starting as that lets them push more charge back into the battery.

The devil is in the details and in the past years, before all the gp24 and 27 batteries stopped being made in true deep cycle like they had in the more distant past, the true deep cycles would have much lower CCA than the current SLA batteries, but they would be able to handle repeated cycling in coach use. That is why I specifically mentioned that the deep cycle batteries were able to start the Onans. That is what I am trouble reconciling. If you take a true deep cycle that is probably sitting at best at 50% state of charge and it is cold out so at reduced output, the amps available for more than a couple of seconds would be very low, AFAIK.

[Urlauber]

OK, so you are saying that Coach lead acid batteries (= deep cycle) were not really made to provide that burst of current/CCA, and so you question if that many amps are needed to start the generator.

All I know is that the two Group31 AGM batteries don't have an issue, but I don't have a good way to measure the current that goes into starting the Generator.

Would you agree that using the Chassis battery once a month or so to start the Onan would be a good solution then? Or is there anything that indicates that the Coach battery should be used (besides existing wiring)?

Which also brings me back around to the little switch which does the connection magic to start the camper via the AGM batteries or start the Generator using the Chassis battery. Does that one just jumper the Isolator/BIM?

[Urlauber]

Originally Posted by @Michael

(4) What make/model is your converter? Some have an AGM profile which is compatible with LiFePo4. Others have an equalizer mode which is not.

The manual of the WF-8955-PEC (actually 8900 series) states

In order to maximize battery life, it is best to charge batteries slowly, keep them topped off
with a trickle-charge when the RV is not being used. The 3-Stage “smart” charger continuously
measures the battery voltage output and regulates the amount of charge using three modes of
operation; Absorption, Bulk and Float modes.
All WFCO power converters are automatic three-stage switching power supplies. The converter
senses which mode it needs to be in by checking the RV system voltage.
The converter normally provides a constant target output voltage of 13.6 VDC (nominal)
to power all the branch circuits. However, it is current limited, and if the output (load)
current reaches its maximum, the output voltage will drop as necessary to hold the
converter’s maximum output current level (the Amperage rating) without exceeding it.
If the output current reaches its maximum (normally caused by a discharged battery),
this will cause the converter to go into Bulk Mode, which means the target output voltage
will change to 14.4 VDC and a timer will start. Although the converter is outputting 14.4
VDC, you will not be able to read that on a voltmeter due to the voltage-current
relationship. From the paragraph above, as load current increases, output voltage
decreases. The actual output voltage will not rise until the load current is reduced, which
happens naturally as the battery charges or if 12 VDC appliances are turned off.
Bulk Mode will be maintained until the current draw drops to approximately five Amps,
or until the timer reaches four hours (whichever happens first). Then the target output
voltage is changed back to 13.6 VDC for Absorption Mode. Lights that are powered from
the output may change brightness slightly at that time

They also recommend to "upgrade" this Power Center with a WF-8955-REP which can detect battery chemistries automatically, incl LiFePO4. Of course, that adds another $250 to the project.

I certainly do not want to risk overcharging the batteries, but not being able to charge them to 100% (and doing that via Solar input) would be alright with me.

[booster]

Originally Posted by Urlauber

The manual of the WF-8955-PEC (actually 8900 series) states



They also recommend to "upgrade" this Power Center with a WF-8955-REP which can detect battery chemistries automatically, incl LiFePO4. Of course, that adds another $250 to the project.

I certainly do not want to risk overcharging the batteries, but not being able to charge them to 100% (and doing that via Solar input) would be alright with me.

Be aware of the inherent problem with most of the battery chargers when charging AGM batteries. They don't know when the batteries are actually full because they aren't measuring what is need to do that.


Sound odd, yes. Does the "fully charged" indicator come (it it has one) yes. Is it correct? Probably not. Most smart chargers leave the batteries undercharged and occasionally overcharged. The idea of detecting "battery chemistries" is a sales hype, I think, and is not necessary to actually get good full/not over charging.


This forum has lots of information on charging AGM batteries with all the research and data to support the conclusions.


Condensed version is that the battery manufacturers will give you a specification, or list it as some do, for an item called "tail amps", "return amps", "float transition amps" or some other terminology. What it is refers to the actual maximum measured amps to the battery after the charging to absorption voltage before the charger goes to float stage. Per the manufacturers that is the only way that can assure getting a full charge without overcharging, on a regular basis. The huge majority of chargers do a timer absorption stage or an algorithm determine timed stage which can be way off most of the time. Some brands of charger do measure the charging amps being generated by doing it inside the charger, but that ignores the coach use amps and makes it inaccurate. It is going to also be inaccurate if you can't set that amp threshold as the manufacturers have different requirements for their batteries.


IMO, don't believe the hype about "perfect charging" unless they are measuring the amps to batteries off a shunt and controlling of that reading. I have tested probably 10 chargers and all were not accurate in charging if not measuring amps at the batteries. There are a very limited amount of chargers that can do that, however. Magnum MS and up chargers with an ARC-50 remote and BMK kit can do it. I am pretty sure a centrally controlled Victron system can do it, and possible some of the Outback chargers might be able to do it. All are very expensive to buy.


Most users that understand this have accepted that they will get less than maximum life from their AGM batteries because the cost of getting the good charging is more than what they save in battery life, but I do think that everybody should be aware of that method and not believe they are doing everything perfectly, no matter what the charger manufacturer or the fully charged light says.



If there is a question about how well any system is doing the charging, it is very easy to determine. Just put on a shunt based battery monitor like a Trimetric or Victron and set it to the battery manufacturer amp based settings and see if it says you got full or not. IMO, everyone probably should have a battery monitor anyway for many reasons. For reference that amp setting is .5%C or 1/2 amp per 100ah of battery capacity at the 20hr rating. There is also the add on spec that says it has to see below that many amps and be at absorption voltage for a settable amount of time before it goes to float. It is very precise compared to all the other methods chargers use, IMO.


I would certainly agree with any system that could reliably leave a lithium bank a bit short of full charge as that idea is what the current best practices for lithium is indicating for most manufacturers now, I think. With lithium you really need to make sure not overcharge them unless it is needed for a limited time to balance the cells periodically. With lithium you can get by terminating charging based on voltage and not amps so that kind of control can be very different than AGM charging.
I still would not trust any charger to determine what profile to use for any chemistry based on just hooking up to it. You just set it once and by done with it and you know it is correct. Why pay $250 to not be sure it did it correctly?

[Urlauber]

Originally Posted by booster

I still would not trust any charger to determine what profile to use for any chemistry based on just hooking up to it. You just set it once and by done with it and you know it is correct. Why pay $250 to not be sure it did it correctly?

Mainly because the existing Converter seems to be suited for SLA/AGM batteries, and I don't know enough about this to determine whether that may damage the LFP batteries or just charge them somewhat incompletely.

And, because the manufacturer (WFCO) recommends that, but of course they will recommend buying more of their stuff.

So... that's the big (converter) question for me here - what is the worst thing that could happen if I would use the existing WF-8955-PEC on a single 100Ah LFP battery. If there is a risk that it would not recognize the charge state at all and somehow over-voltage it then... no bueno. If it maxes out at 14.4 while LFP should be charged with 14.6 (?) then maybe not all that bad. I am not trying to reach 100% at the shortest possible time - but realize of course that some actually have that goal.

My goal would be not affect the lifespan of that battery too negatively. I would always try for a ~ 80% SOC, only up it more when actually camping.

[booster]

Originally Posted by Urlauber

Mainly because the existing Converter seems to be suited for SLA/AGM batteries, and I don't know enough about this to determine whether that may damage the LFP batteries or just charge them somewhat incompletely.

And, because the manufacturer (WFCO) recommends that, but of course they will recommend buying more of their stuff.

So... that's the big (converter) question for me here - what is the worst thing that could happen if I would use the existing WF-8955-PEC on a single 100Ah LFP battery. If there is a risk that it would not recognize the charge state at all and somehow over-voltage it then... no bueno. If it maxes out at 14.4 while LFP should be charged with 14.6 (?) then maybe not all that bad. I am not trying to reach 100% at the shortest possible time - but realize of course that some actually have that goal.

My goal would be not affect the lifespan of that battery too negatively. I would always try for a ~ 80% SOC, only up it more when actually camping.

I have actually been looking at a lot of that kind of stuff currently.



Some lithium manufacturers say floating is OK, some don't and say full cutoff charging is better. Those generally would need different chargers unless they are more expensive profile settable charge profiles. Based on mostly a general "makes sense" logic I think it may possibly be best to do a combo of both, especially on small battery bank that needs the charger to provide the power to run the van without discharging the small amount of capacity that might be needed the next day. Perhaps something like one that could three stage charging for when actually camping and full cutoff/rebulk charging when in storage or non use times. But that also runs up the costs, so probably not feasible for many users. There are many of these quandarys for me with lithium and I don't think the answers are finalized yet.


The charge ending point is another one of the not really determined yet things, I think. A lot of information points to not charging to full all the time, which a constant 14.6v would do, but lot of information is also starting to come out about a need to go to 14.6v regularly for at least a small amount of time to generate cell balancing with at least some lithium batteries. That complicates the whole thing a lot, it appears. You would need to charge to maybe 14v most of the time, but sometimes to 14.6v and hold for 15 minutes and then end charging? Not easy for most systems.


As I have mentioned before about AGM charging and now probably applies to lithium charging also, now that costs are down, the best solution if to pick a compromise best fit for you system and accept the lost life of the less than perfect charging. Hard for some of us to accept, however.

[Urlauber]

I am hoping the solar controller (GoPower GP-PWM-30-SQ) would be capable to fully charge the LFP via it's lithium profile. If that is the case, I could once a month or so do the cell balancing act and the rest of the time keep the batteries below 80% SOC (except when actually being out and about).

On that note - can you disconnect a (sun exposed) solar panel from the battery, or will that cause harm? I have read opinions either way. Want to be able to just stop the juice, using a switch before the panel wires enter the solar controller.

Of course, throwing a blanket over it would work in a pinch.

[booster]

Originally Posted by Urlauber

I am hoping the solar controller (GoPower GP-PWM-30-SQ) would be capable to fully charge the LFP via it's lithium profile. If that is the case, I could once a month or so do the cell balancing act and the rest of the time keep the batteries below 80% SOC (except when actually being out and about).

On that note - can you disconnect a (sun exposed) solar panel from the battery, or will that cause harm? I have read opinions either way. Want to be able to just stop the juice, using a switch before the panel wires enter the solar controller.

Of course, throwing a blanket over it would work in a pinch.

I think what you have been hearing/seeing about shutting off solar to the batteries while in the sun was referring to disconnect the solar controller output to the the batteries to stop the charging. This is considered a bad thing to do because the panels can through a voltage spike into the controller an possibly damage it.



You can safely just shut off the panels with a switch in one of the panel wires to the controller according to all the information I have seen. We have done that often when we have full batteries and don't need or want any more charging. We have had no issues at all with doing it that way.

[mloganusda]

This is the wiring on my 2017 WGO Trend for AGM batteries. Yours will be somewhat similar. I hope it helps. On starting my generator if the house batteries are low, WGO states to start the engine to boost starting the gen.