LFP on Shore Power with Inverter/charger?

[BFL13]

An inverter/charger requires that you have a battery connected (or some 12v source) so it will supply any 12v to the RV. (Unlike a converter that will do 12v with no battery)

Some say not to float an LFP. If you choose not to float yours, what do you do when on shore power? You can't isolate the battery and run 12v off the charger in the inverter/charger since it would be dead.

You could just not enable the charger, and run 120v off shore power pass through---or can you? Does 120v pass-through happen with no 12v to the inverter/charger?--- And just cycle the LFP for your 12v, thus not floating it.

What do folks do in that situation?

[BFL13]

"If you want to measure your battery capacity, charge your battery full, turn on a load (the current doesn't really matter) and then run until the BMS cuts off. Look at your battery monitor to see how many Ah came out, and that is your capacity"

That makes sense, kind of like my "new plan". It would not quite work with my Trimetric of course, since it gets its 12v from the battery being monitored. When the BMS cuts off the battery, the Trimetric display would go black. Have to take the AH reading just before the BMS shut things down. Have to know when that is about to happen.

time2roll is going to try to camp the same as now using his voltmeter as the only monitor he needs.

I like Itinerant1's approach. He must have done a whole bunch of math before getting his set-up. His 500AH bank was not chosen at random, nor his amount of solar. He figured his daily AH usage and the various rates. He is thus able to do it all, staying above the lower knee, and get his cycles worth of LFP without having to use high rates. He monitors it all like crazy to make sure it is working like it is supposed to. He "adjusts" his solar so it can keep up by moving North and South as required as well as keeping in comfortable climates. None of that happened by trial and error--he did some math first.

time2roll has the luxury of being able to do some trial and error, not being a full-timer. He can get a monitor later if needed.

Anyway, you LFP guys can figure out how to play it each to his own.

[time2roll]

BFL13 wrote:
I suspect time2roll is suspecting he will now need more than just his voltmeter too, so over to you LFP guys on what to do----

Eventually my BMS may have a monitor. However once initially set up I don't expect to use more than a voltmeter. I see no rocket science here. Again more capacity kills the need to watch the fuel tank. Just can't imagine a scenario where the voltmeter would not be able to give a full 24 hour warning that the battery is being depleted and power will be cut.

[FWC]

I enter the nominal battery capacity for my bank into my BMV-712 and enjoy, no real need for any further shenanigans.

If you want to measure your battery capacity, charge your battery full, turn on a load (the current doesn't really matter) and then run until the BMS cuts off. Look at your battery monitor to see how many Ah came out, and that is your capacity.

There is a tendency to make this way more complicated than it needs to be.

BFL13 wrote:
How's this for a plan then?----

FWC was careful by saying, "the resting voltage is 13.2-13.3 " He did NOT say that was the "full resting voltage". He did say that as soon as a load is applied to a full LFP, the voltage drops to 13.3.

So 13.3v means anywhere from 99-89% SOC in effect. Then the voltage/SOC table time2roll posted (from BB's) could be used (except very difficult to get proper voltage with the temperature making that so sensitive, per FWC's note on that)

Then time2roll makes a very important point--you can see where the lower knee starts by watching your voltmeter.

How to use your AH counting monitor or smart shunt?

1. You get the battery as full as you can using its charging specs and guide for that.

2. Pick a number for your amps draw since Peukert is near 1 (FWC again) say 10 amps and hold that constant as you can while your AH counter counts down and you keep an eye on the voltage.

3. When voltage starts to drop off, stop!

Now you have your AH "allowance" that you can operate within. Using a variety of amp draws while camping, you still have the same total allowance. ( which is why the 20 hr rate doesn't really matter with Peukert at 1)

What do you enter as your "full" capacity on the monitor? Doesn't matter as long as it is more than your "allowance". Set your low voltage alarm at that voltage you saw where it started to drop off.

While camping, you can watch the AH counter count down and remember your allowance. You can do the math in your head to tell when you are half way down or three quarters the way down, etc.

OR, if you make your AH allowance your "full" entry on the monitor, and it reads your percentage SOC, you can use that. Just remember to zero/sync the monitor whenever you get the chance when the batt is as full as you can get it.

TA DA! Is that what you LFP guys do? ( Itinerant1 has a more complicated monitoring system that does include some cross-checking, but I am wondering what LFP guys with a simple system do)

jaycocreek is looking for suggestions and I suspect time2roll is suspecting he will now need more than just his voltmeter too, so over to you LFP guys on what to do----

[Housted]

FWC wrote:
How so? You could use lead acid and need 3-stage, temperature compensated charging, do your best to only use 1/2 the rated capacity so that they will last a reasonable period, remember you will only get a fraction of their capacity if you use them hard (peukert), remember to account for their voltage drop as you use them, and make sure you regularly charge them for 12 hours so they don't sulfonate.

The only reason folks think lithium chemistries are more complicated is that they have come to accept all the requirements of lead acid as 'normal'. In an apples to apples comparison the care and feeding of lithium is far simpler, most folks just aren't familiar with it.

Too many people think Lithium batteries are some kind of black magic and need to be treated like a disabled uncle. We have been using Battleborn for about 1.5 years. With 1250 watts of solar and the solar controller set to the voltages recommended by Battleborn they are totally maintained by the solar. I do give them a shot from the PD converter before we leave on a trip. Often do not turn on the converter when hooked to shore power. I have a circuit to disconnect the converter when on inverter power and then reconnect if I start the generator.

Housted

[BFL13]

BB says the surface charge voltage 14.4 -14.6v when the batt is fully charged and is at Vabs drops to 13.6 when that surface charge is gone and the batt is resting at full 100% SOC.

FWC says it drops to 13.2-13.3 but that is under a load, as though somehow the 13.6 was still with a surface charge and now all the surface charge is gone, but he did not say that the batt is still full. Not clear what he really means by that.

My new plan (see earlier post) is to never mind what the capacity is of the LFP you just got. Find out the AH allowance you can have with it, and pretend that is the capacity. Use the AH counting monitor to keep track of what's left of your allowance while camping and recharge as required.

BTW, FWC, you know darn well where that 113AH came from. That was from saying the 1536 Wh was correct. You said that 1536 was fictional, but the 120AH is correct. You never said why to believe that AH number and not the Wh number.

Also "nominal" is in English, so we know what it means. It happens that FLA and AGMs both use nominal voltages , 12v and 6v, eg, while LFPs use 12.8v as "nominal" No problem. We don't use "12v" when using the resting voltage vs SOC tables for those batts either. We use whatever the batt specs say is the resting voltage at 100%.

Many FLAs use 12.7 as "full" while AGMs use 13.0 as full. No problem. eg my AGMs were 12.4 as 50% while my 6s were 12.1 as 50% (it stayed at 0.3 diff for that range of SOCs) AH count still works. Just means use the right voltage for that battery as your cross-check when "matching".

With LFPs it seems you can't believe the Wh or the AH ratings to use with your monitor and you can't really match the voltage with the AH count to do cross checks.

Apparently all you have is your AH counter to go by. So my new plan is to use your "allowance" (as described earlier) and use that as your "capacity" never mind what the real capacity might be in the strange LFP wonder-world of inconsistent numbers. YMMV (and no doubt does! 🙂 )

[FWC]

It is true that the resting voltage depends on the state of charge - but a resting voltage somewhere in the middle of the SOC range is what you should use for your system design.

BFL (who has no experience with lithium as far as I can tell) has been bandying around some misleading math based on a resting voltage of 13.6V, that is leading to some odd claims that a 120Ah battery is actually 113Ah.

This would be the same as saying your AGM battery nominal voltage is something like 13.4V or what ever it is when it comes off the charger. We all know that it not accurate, as soon as it is loaded this surface charge will be gone and the voltage will drop to 12.6 - 12.8V. Same deal with lithium, as soon as you load the battery, the voltage will drop to ~13.2 - 13.3V.

StirCrazy wrote:

FWC wrote:
PS the resting or 'nominal' voltage of LiFePO4 is NOT 13.6V. The resting voltage is 13.2 - 13.3 and the 'nominal' voltage is 12.8V.

thats kind of a bold and misleading statment. the resting voltage all depends on state of charge, if you are 100% charged then the resting voltage is 13.6V

Steve

[jaycocreek]

jaycocreek is looking for suggestions and I suspect time2roll is suspecting he will now need more than just his voltmeter too, so over to you LFP guys on what to do----

True...

That's the reason I went ahead and got the smart shunt because I'm not....lol..Just a matter of getting the right numbers into it and it does the thinking for you but like everything else lifepo4,there's different opinions on the correct numbers...lol

[BFL13]

How's this for a plan then?----

FWC was careful by saying, "the resting voltage is 13.2-13.3 " He did NOT say that was the "full resting voltage". He did say that as soon as a load is applied to a full LFP, the voltage drops to 13.3.

So 13.3v means anywhere from 99-89% SOC in effect. Then the voltage/SOC table time2roll posted (from BB's) could be used (except very difficult to get proper voltage with the temperature making that so sensitive, per FWC's note on that)

Then time2roll makes a very important point--you can see where the lower knee starts by watching your voltmeter.

How to use your AH counting monitor or smart shunt?

1. You get the battery as full as you can using its charging specs and guide for that.

2. Pick a number for your amps draw since Peukert is near 1 (FWC again) say 10 amps and hold that constant as you can while your AH counter counts down and you keep an eye on the voltage.

3. When voltage starts to drop off, stop!

Now you have your AH "allowance" that you can operate within. Using a variety of amp draws while camping, you still have the same total allowance. ( which is why the 20 hr rate doesn't really matter with Peukert at 1)

What do you enter as your "full" capacity on the monitor? Doesn't matter as long as it is more than your "allowance". Set your low voltage alarm at that voltage you saw where it started to drop off.

While camping, you can watch the AH counter count down and remember your allowance. You can do the math in your head to tell when you are half way down or three quarters the way down, etc.

OR, if you make your AH allowance your "full" entry on the monitor, and it reads your percentage SOC, you can use that. Just remember to zero/sync the monitor whenever you get the chance when the batt is as full as you can get it.

TA DA! Is that what you LFP guys do? ( Itinerant1 has a more complicated monitoring system that does include some cross-checking, but I am wondering what LFP guys with a simple system do)

jaycocreek is looking for suggestions and I suspect time2roll is suspecting he will now need more than just his voltmeter too, so over to you LFP guys on what to do----

[StirCrazy]

FWC wrote:
PS the resting or 'nominal' voltage of LiFePO4 is NOT 13.6V. The resting voltage is 13.2 - 13.3 and the 'nominal' voltage is 12.8V.

thats kind of a bold and misleading statment. the resting voltage all depends on state of charge, if you are 100% charged then the resting voltage is 13.6V

Steve

[StirCrazy]

BFL13 wrote:
Steve, thanks for the clarification. I was using CCA because it is the only thing I can think of with battery ratings that are a measure of its power. FWC said a battery's power is its max current at its nominal voltage (if I got that right) so 800CCA is twice the power of 400CCA.

I don't know where an RVer cares about power as such. Ads saying a battery has "twice the power" got me wondering what that is supposed to mean. Seems to be "Sales" talk. ?

That Odyssey 31 hybrid battery that allows a high draw makes that claim, and ISTR LFP ads saying that too.

Another puzzle is how they do AH with LFPs, but that only matters to those who operate them trying to keep track of usage while camping if they even do that.

EG, The blurbs say don't use Peukert, but still say the LFP is 100AH because they ran it down at 100 amps in 1 hour so that is 100AH. Then they sort of dance around saying it also takes 20 hours to run it down at 5 amps, so that is 100AH at the 20 hr rate. But no Peukert here folks! 🙂 It makes your head spin.

Then they start in with the Wh, but use the 12.8 nominal voltage to get the AH. OR----did they get the AH first and then invent the Wh????

In the previous example, it was 1536 Wh and 120 AH at 12.8v

The battery is full at 13.6v so that is where you have to start. Either:

A. the battery is really 1536/13.6 = 113AH , or
B. the battery is really 120AH so x 13.6 = 1632 Wh

Which is it? If you are monitoring by Wh usage or AH usage, I would think that is important for setting up your monitor.

Not a clue. so Steve, what are you going to do for operating your "280AH" battery for setting up a monitor? Just curious on my part, but other LFPers might be interested. ?

oh where to start, so many questions at once haha..

the first thing you have to come to terms with when dealing with LFP is it isnt lead acid. every other battery flooded, agm, SiO2 and so on is a lead based tech. Most Lithium batteries dont fall into the voltage specs that are simular to the lead based batteries but LFP comes very close. close enough we can use it interchanagbly. having said that its overall volatage is a little higher so conventional ways of measuring wont quite work, and some of the conventions we are use to are out the window. there is pretty much zero Peukert effect with the LFP chemistry so for all intensive purposes you don't worry about it. this is a bonus of LFP as you can run high loads with out losing AH.

As for the 20 hour rate, I imagine there must be a standard voltage that the test is done at but I am not totaly sure on that.

I will probably use the cheepest monitoring thing I can find on Amazon that is LFP compatable. I have a little 50 buck version of the Victron in my camper that says it is LiFePo4 compatable so if that works Ill grab another. plus the few BMS setups I have narrowed my choices down to have bluetooth and an app I can check out the other battery info with. so far it looks like I am going to go with the overkill BMS but there are a couple other ones I am trying to find out more info on.

Steve

[jaycocreek]

I don't pretend to understand a lot of this but reading this explains the 12.8 nominal voltage...
Lithium batteries are an assembly composed of many cells, like lead-acid batteries and many other battery types. Lead-acid batteries have a nominal voltage of 2V/cell, whereas lithium battery cells have a nominal voltage of 3.2V. Therefore, to achieve a 12V battery you’ll typically have four cells connected in a series. This will make the nominal voltage of a LiFePO4 12.8V

Then there's the 14.6 vs 14.4 and resting voltage seems to be 13.6ish...Very confusing sometimes...

[FWC]

time2roll wrote:

FWC wrote:
PS the resting or 'nominal' voltage of LiFePO4 is NOT 13.6V. The resting voltage is 13.2 - 13.3 and the 'nominal' voltage is 12.8V.

Is it?

Yes it is. I have been using LiFePO4 for years - the batteries are at 13.3 - 13.2V for the bulk of their SOC at room temperature. The resting voltage is a much stronger function of temperature than SOC, making it a guess at best. These tables that are floating around are an effort to apply lead acid habits to lithium and really don't work.

[time2roll]

FWC wrote:
PS the resting or 'nominal' voltage of LiFePO4 is NOT 13.6V. The resting voltage is 13.2 - 13.3 and the 'nominal' voltage is 12.8V.

Is it?

[BFL13]

Yes, I did not state very well how my 10hr test is run. I actually use a 50% "marker" like SG or resting voltage /SOC or both. when I get to that marker, I note the AH count. if I hit the marker '"early" (usually happens) then if that is at 9 hours, I know the battery is 90% of its rating.

So with a brand new battery starting full, it ought to go to half its rating in 10 hrs. I would still need the marker. The flatter voltage/SOC just means being more careful to get the real resting voltage.

Running a few of these tests, you get an idea of what loaded voltage will be at the 50% marker that will bounce back after to the real resting voltage by next day. At first you can overshoot or under shoot. If you under shoot you can just restart the load, but overshooting is hard to calculate from.

[3_tons]

FWC wrote:
PS the resting or 'nominal' voltage of LiFePO4 is NOT 13.6V. The resting voltage is 13.2 - 13.3 and the 'nominal' voltage is 12.8V.

Agreed, thats where my LiFePO4 likes to rest...I believe the nominal 12.8 has more to do with customary system voltage using Lead acid, but thats only a guess...

3 tons