Questions about AGM -> LFP

[GallenH]

Actually I think that I read somewhere on the forum that the best life for a lithium was to take it down only to 20% and charge it only to 80% but I'm not sure of it's accuracy. Perhaps it's like the 50% discharge rule with AGMs. As booster has frequently pointed out, occasionally dipping below to, say, 60% discharge does not significantly affect the battery except perhaps by shortening its life a little bit.

[Urlauber]

Yeah, not sure how much of a difference it makes. You/we are not supposed to store/maintain LFP batteries at full charge/100% SOC as that certainly stresses them. That is different from charging/discharging them.

They advertise 2000, no 3000, no 5 or 8000 or even 15000 cycles with those batteries which is of course easy to claim as the general lifespan of them is 5-10 years so most will never even come close to that number of cycles.

Still fairly new technology (LFP) so I guess we'll see in 5 years...

[booster]

Originally Posted by Urlauber

I thought one of the LFP battery advantages is that the full capacity is available. While it would be advantageous for maximum lifespan to not always completely discharge them, it is perfectly fine to do so.

Note - I know over-discharging them is deadly and should be prevented by the BMS.

How did we arrive at the 80% figure?

Some manufacturers do still say you can use 100% of lithium capacity, but many have backed off that and recommend about 10% on each end, top and bottom. It is said to increase cycle life and help keep the batteries in balance.


But the above is contrary to another direction that many manufacturers are going in that they say to charge to 14.6v so that they can do an auto balancing on every cycle. That voltage is essentially full, so violates the idea of leaving some headspace so a bit of a contradiction there.



It has been said in the past as all this stuff has evolved that balancing should only be done if there is a cell voltage difference of some given value (we have seen various voltage differences stated) and that is much harder for the BMS to do all by itself. This may explain why the easy way out is do it every cycle and eat the slightly shorter cycle life. Hard to tell.

[Urlauber]

One of the reasons why, at least in the first iteration, I am planning to get a LFP battery with a BT app showing the distinct cell voltages. This should allow monitoring the balancing (or lack thereof) which one won't know about otherwise.

[MsNomer]

Originally Posted by GeorgeRa

If you would increase your battery to 300Ah AGM your results could be similar.

We wouldn’t have had room for a third AGM even if we had wanted to throw more good money after bad. AGM simply was a p*ss-poor match for the way we want to use the van. Now we can just wander obliviously along, with SOC floating around somewhere about 30-90%SOC, occasionally charging to 100% to withstand 4-5 days' boondocking with full use of appliances even on the last day. Can you imagine the size of the bank that would be required to run a microwave from AGM with 20%SOC?

[Davydd]

Originally Posted by Urlauber

I thought one of the LFP battery advantages is that the full capacity is available. While it would be advantageous for maximum lifespan to not always completely discharge them, it is perfectly fine to do so.

Note - I know over-discharging them is deadly and should be prevented by the BMS.

How did we arrive at the 80% figure?

It's a safety factor. You can deplete a lithium battery down to 99% if you wanted to but complete discharge can kill the battery. ARV sets the BMS at 80% discharge arbitrarily and you can set it to any discharge percentage if you want for their autogen second alternator to kick in. You can also charge your batteries 100% as I routinely do for 8-1/2 years. You almost can't help charging 100% as there is no float period. ARV's system is set to 100% charge and you can do so in a half hour driving on a boon dock overnight stay. As I mentioned the minimum life cycle most stated is 2,000 cycles and I already explained it represents at worst 16 years of battery life and many other factors would enter in other than that worry. MsNomer's comments to me just puts the nail in the coffin for AGMs.

[tlillard23]

I had one battleborn 100ah battery, started the genny, onan 2800 just fine. alternator charging just fine. solar charging just fine. I let the BMS worry about the cells, I just use it.
A year later, I added the shunt to see actual amps, and another 100ah, to start running a small electric fridge along with the propane fridge running on AC through the inverter. *normally dry camping, I switch to propane at night.

I've gone from 2 AGM to 1 lith on 2 different rigs now, the lithium beats AGM and wet cells hands down. I'd recommend a battery with a heater, and from what you want to run, the hot temps from DISCHARGE wont be an issue. I'd still prefer them in the living compartment, instead of out in the elements.
Yes, if you are running a spacecraft, and you are storing the batteries, 100% is bad. But if you are using them, 100% is fine. 100% discharge is a myth, the BMS will kick in when the battery is done and I've never been there.

There is that one little tiny small issue with lithium, thermal runaway. So get a good brand with a good BMS.

[GeorgeRa]

Originally Posted by MsNomer

We wouldn’t have had room for a third AGM even if we had wanted to throw more good money after bad. AGM simply was a p*ss-poor match for the way we want to use the van. Now we can just wander obliviously along, with SOC floating around somewhere about 30-90%SOC, occasionally charging to 100% to withstand 4-5 days' boondocking with full use of appliances even on the last day. Can you imagine the size of the bank that would be required to run a microwave from AGM with 20%SOC?

I am not questioning your decision going from AGM to Lithium, just found comparison 200Ah AGM to 300Ah Li a little misleading.

[GallenH]

Originally Posted by GeorgeRa

I am not questioning your decision going from AGM to Lithium

Neither was I. We all RV in our own ways. I used the microwave so little I removed it for storage and don't miss it at all. My stove is LP.

But I have another question. A RVing relative said he would never go with lithium because of fire issues. Can someone give me a link to an actual study which refutes this? I ask because when my LifeLine goes, I'll replace with lithium.

[Urlauber]

Originally Posted by GallenH

Neither was I. We all RV in our own ways. I used the microwave so little I removed it for storage and don't miss it at all. My stove is LP.

But I have another question. A RVing relative said he would never go with lithium because of fire issues. Can someone give me a link to an actual study which refutes this? I ask because when my LifeLine goes, I'll replace with lithium.

Ha, me too! I just removed the microwave. Wife wanted a good looking cabinet door, got that done. Nice big additional storage compartment, plus some weight savings.

I don't have a link to a study, but it is well known that the LiFePO4 chemistry is much more stable than the previous generation of Lithium batteries.

[Urlauber]

Originally Posted by tlillard23

I had one battleborn ....

There is that one little tiny small issue with lithium, thermal runaway. So get a good brand with a good BMS.

Thank you very much for weighing in with some actual experience. Good read!

I believe thermal runaway is less likely with this generation of lifepo4 cells.

[MsNomer]

Originally Posted by GeorgeRa

I am not questioning your decision going from AGM to Lithium, just found comparison 200Ah AGM to 300Ah Li a little misleading.

How is it misleading?

[booster]

Originally Posted by MsNomer

How is it misleading?

I also found it a bit misleading or confusing, and it is now obvious to me why.


In the past, nearly all the talk about battery bank size centered around capacity of the bank and charging time and rates. Many/most of us still think that way when we see posts about battery banks and their sizing, and also apply that thinking to the comparisons between lithium and AGM.


In the past, again, discharge rates where normally quite small, as were the the battery banks and in a system like that lithium and AGM are basically equal in capability. Lithium has some charging speed, not rate just time, advantage in such a system and AGM has other benefits so that part is probably a wash. 200ah is a small battery bank these days, IMO.



If you look at the statement made that was confusing to me from that viewpoint it made little sense.


That statement appears to have been made with top priority for the battery system being able run high discharge rates all the way down in the SOC curve. This changes everything about the system, IMO. It will need bigger cables, fuses, equipment, etc but it also tilts the advantage to the lithium batteries over AGM if you are using that same capacity battery bank.



I don't recall any, but there may have been some, other users that put that much emphasis on being able to use high discharge rates in the past except for running AC, but it is probably something that will be more common in the future. For running AC you need high rate and big capacity so a different issue and solution.



The common solution for the microwave running for users is to run the van engine when using the microwave, if it is allowed where you are camping as that is a very easy and nearly no cost way to do it when compared to getting everything in place to be able run high discharge rates from the batteries, invert it, and not use up too much of you battery capacity.


I think if the statement had emphasized that the calculation was based not on the number of days of offgrid camping desired based on battery capacity, but only on how many days of microwave use would be available, there would have been no confusion for me.

[MsNomer]

"That statement appears to have been made with top priority for the battery system being able run high discharge rates all the way down in the SOC curve."

Yes, absolutely, the ability to use the microwave at will is a necessary part of our equation and running the engine is a bother I would be hard-pressed to tolerate. Besides—it was hard enough charging the AGMs to 100% from 75%. Much lower and it would have been impossible most days.

A fair bit of our travel is in food deserts, so we carry a dedicated 22cf freezer of home-made meals. Heat in the microwave in the dish it will be eaten from. Done.

[@Michael]

Originally Posted by GallenH

Neither was I. We all RV in our own ways. I used the microwave so little I removed it for storage and don't miss it at all. My stove is LP.

But I have another question. A RVing relative said he would never go with lithium because of fire issues. Can someone give me a link to an actual study which refutes this? I ask because when my LifeLine goes, I'll replace with lithium.

This seems to be a decent test:

https://www.fire.tc.faa.gov/pdf/TC-16-17.pdf

[Davydd]

I use my microwave without needing to plug into shore power or running an engine. Pre-prepared packaged dinners like you can buy at Aldi or just re-heating coffee makes a microwave worth it. If nothing else, the Microwave makes a better bread box than an open shelf.

[GeorgeRa]

Originally Posted by MsNomer

How is it misleading?

I agree with Booster.

You are comparing 200Ah battery bank with 300Ah Lithium claiming 5-fold increase capacity. That is misleading, great for marketing folks.

Most likely you 200Ah bank was not good for the job so you are comparing a bad design with your current fix.

[MsNomer]

OK, 5-fold useful capacity for the heavy loads we routinely subject the system to.

It's actually better than that because it wouldn’t hurt to use 270 of the 300Ah Lithium instead of the 250 I used in the calculation.

[GallenH]

Originally Posted by MsNomer

OK, 5-fold useful capacity for the heavy loads we routinely subject the system to.

It's actually better than that because it wouldn’t hurt to use 270 of the 300Ah Lithium instead of the 250 I used in the calculation.

But you understand the confusion, right? You are comparing 200AH AGM to 300AH lithium. If you were comparing equal AH, even at your 25% on the AGM, 300AH would equal 75AH. Using the above 270AH for lithium, the lithium would be less than 4x, not better than the 5-fold capacity you claim. Plus for those of us who don't use high draw appliances, the capacity of 300AH AGM is 150AH-180AH depending on your philosophy of how far you can run a AGM down. Compared to 270AH lithium, that's less than 2-fold.

Again, I don't fault your premise that lithium will provide more AH with a greater charging/recovery rate and at lighter weight. And as I mentioned, when time to replace my. lifeline I will probably go with lithium for those reasons.

[MsNomer]

I compare 3 to 2 because 3 lithium fit in the same (actually a bit less) space as 2 AGM, so maybe I should say that in the same space I got more than 5x…