LFP battery chatter ( AKA LiFePo4, Lithium Iron Phosphate )

[Sunking]

1. There is a Low Battery Cut Off (LBCO), which is programmable in .1V steps from 40V to 48V. There is a programmable timer up to 600s associated with the LBCO to prevent sags from triggering it. It is said to "shut down" the inverter, but I don't know if the control circuits stay alive.

Great and it should work out great. Commercial 48 volt LFP LVD operate at 2.9 vpc, so a set point of 46.4 volt is in that range. They also have switch selectable delay times just for that purpose. but unless you are discharging in excess of 1C, Ri of LFP is so low voltage sag is not much of an issue in a solar application. Not so with EV's high current demands of 5 to 10C

2. There is an auxiliary input to shut down the inverter.

Great as that gives you another LVD option using an external BMS.

3. There is a programmable output that could be used to power a disconnect relay based on a low battery voltage setting, again at .1V steps.

The natural question is could one of these be used. #2 would require an external voltage sensor. For #1, I am wondering if I need to be concerned about the XW control circuits continuing to be a load even with the inverter shut down. For #3, I'm not sure if breaking the battery circuit kills the XW completely. Seems like it. If an external disconnect relay were NC, it would have to latch open via the output before the XW dies.

Options and more options to work with. All this means is you can do LVD externally, or internally on your Inverter. You gotta figure out which way is best for you. All I can do is consult and call out the Pro's and Cons allowing you to make an informed decision. I am not making a penny off you and unlike a salesman I am not going to push the more expensive options influenced by a commission. However I would never turn down a check for helping.

[Sunking]

Or are you just pointing out I can parallel - for example I might consider 48V @ 600Ah if I can find 300Ah batteries?

Just pointing out how to go large. Honestly I forgot what AH you thinking about with LFP

In your answer, if it is quick to address paralleling pairs or triples and putting them in series vs paralleling series strings, that would be great. I haven't seen that addressed anywhere. With LFP and shunting individual batteries, paralleling pairs might get complicated.

Parallel LFP is no problem unlike FLA. Only difference is you use a Ladder Type configuration. In addition it cuts down on Cell Balance Boards. All commercial EV's parallel Lithium batteries.

[Living Large]

Low Voltage Disconnect Implementation

Sunking - I need to read your recent reply regarding BMS and balancing, etc, but I wanted to ask about implementing LVD with a feature of the charger/inverter I am considering - XW5448. My basic question is, do you think it will do what I need, might I need something external, etc. The results of the discussion I can apply to other candidates. There are 3 features that could be used to possibly implement a LVD, IMO. FWIW, the XW appears to operate (logic functions) down to 32V.

1. There is a Low Battery Cut Off (LBCO), which is programmable in .1V steps from 40V to 48V. There is a programmable timer up to 600s associated with the LBCO to prevent sags from triggering it. It is said to "shut down" the inverter, but I don't know if the control circuits stay alive.

2. There is an auxiliary input to shut down the inverter.

3. There is a programmable output that could be used to power a disconnect relay based on a low battery voltage setting, again at .1V steps.

The natural question is could one of these be used. #2 would require an external voltage sensor. For #1, I am wondering if I need to be concerned about the XW control circuits continuing to be a load even with the inverter shut down. For #3, I'm not sure if breaking the battery circuit kills the XW completely. Seems like it. If an external disconnect relay were NC, it would have to latch open via the output before the XW dies.

So I'm all wrapped up in details. Going back to our previous discussion, might #1 be good enough? Programmable LBCO?

[Living Large]

16S2P = 16 cells in series, and 2 cells in Parallel. In other words 2-400 AH cells in Parallel, and 16 like cells in series for a total of 48 volts @ 800 AH. It is terminology required for LFP. Remember the Ladder connection of parallel batteries I drew up?

I am well aware that you proposed a 48V 800Ah solution, but admit I may have missed a nuance that this would be 16 paralleled pairs, as opposed to 2 parallel strings of 16 (would that be 2P16S?), which is it own discussion. I asked about this back when I was considering paralleling strings of FLA and I don't recall an answer. At that time I was curious of whether one or the other is dictated. Someone mentioned they have parallel strings, and can drop entire string or the other off for maintenance, so I assumed 2 parallel strings. But I digress.

In any event, based on my needs, I don't understand why I would consider 48V 800Ah LFP - that is why I said you were hurting my brain. You were the one who suggested 430Ah may fit my needs, based on 700Ah FLA. Everything was falling into place, reasonable gen run time to recharge, "reasonable" cost (<$10K). 800Ah is a huge jump. Or are you just pointing out I can parallel - for example I might consider 48V @ 600Ah if I can find 300Ah batteries? In your answer, if it is quick to address paralleling pairs or triples and putting them in series vs paralleling series strings, that would be great. I haven't seen that addressed anywhere. With LFP and shunting individual batteries, paralleling pairs might get complicated.

I see you addressed the ongoing discussion at length. I don't have time right now, but I will check in later. Thanks!

[Sunking]

But if they are close in SOC and in the relatively flat voltage versus SOC range, I could see this working. Maybe some large sparks when connecting to the bus though....

I have discussed it with a few of the EV guys, searched, and even the distributors recommend it.

When I expressed my concern, it is a legitimate concern and to check that the cells are withing 10 mv. Most of the US distributors, or at least two I know to be reputable put the cells on a charger to either discharge/charge them up to Storage Voltage. Same thing RC modelers do when we put the batteries away. Pretty standard practice for all Lithium manufactures to ship at Storage Voltage. Age and self discharge is the question mark.

So I am OK with it and yeah great idea assuming you know to check the voltages before connecting. Use a wire jumper for temporary and let the sparks fly, followed quickly by the bolted buss bar and let them set for a day.

Now if you want a good debate, the EV guys are split on BMS. I lean more to less is better. I only used Balance Boards and only charge to 90% and have never had to balance or have a board turn on yet in 4 months. Some of the EV guys claim they have never Balanced into 3 years of operation other than initial Bottom Balance at the start. Strange world the DIT EV guys are. Many are pros and fight like cats and dogs when it comes to BMS. One side thinks BMS was invented by engineers to sell a product, and the other side who swears by them. I am kind of in the middle favoring the Passive/Monitor Side vs Active ever shuffling power around side.

[inetdog]

Living Large just got an Attitude Adjustment on another forum for EV's you wil want to hear.

For the Initial Balance when you receive them is to connect all batteries in parallel and let them set for a day using the supplied buss bars. Then connect in series and charge.

Not sure I am crazy about that idea because if there is some significant voltage differences can result is some extremely high currents.

But if they are close in SOC and in the relatively flat voltage versus SOC range, I could see this working. Maybe some large sparks when connecting to the bus though....

[Sunking]

Initial EQ

Living Large just got an Attitude Adjustment on another forum for EV's you wil want to hear.

For the Initial Balance when you receive them is to connect all batteries in parallel and let them set for a day using the supplied buss bars. Then connect in series and charge.

Not sure I am crazy about that idea because if there is some significant voltage differences can result is some extremely high currents.

[Sunking]

Thanks - I have not exhausted the list you previously gave for places that offer EV products, but if you have something more targeted that would be great.

You're hurting my brain. Why would I want to think about that? That isn't something I need, is it? One thing it would mean is $19-20K

16S2P = 16 cells in series, and 2 cells in Parallel. In other words 2-400 AH cells in Parallel, and 16 like cells in series for a total of 48 volts @ 800 AH. It is terminology required for LFP. Remember the Ladder connection of parallel batteries I drew up?

[inetdog]

Sounds like a good protection for a large investments in batteries!

[Sunking]

I was confused (maybe I still am). I am getting a clearer picture - thanks for hanging in there. In my "summary" list above, I had 4 items. I believe #2 and #3 are the same now. Doh. The hardware I need is:
1. a CC and charger with software programmable thresholds
2. passive BMS boards (optional)
3. LVD disconnect at correct voltage

And to monitor the individual cells myself to know where I'm at.

OK, now - I believe I understand the simple function of passive shunting of the BMS when things are a little out of balance. Basically, every cell can get charged "fully" (yes, I remember we really don't want 100%). Perhaps "equally" is more appropriate, perhaps not, but I digress.

OK we still have a little Tweaking to do.

1. It is a MUST HAVE a CC that you can use a laptop, PC, inertnet or someway of programming set point voltages to at least 1/10 a volt like 58.4 volts for all stages. No switches to to select Gel, FLA or AGM. You are going to need to fine tune.

2. IMO Balance Boards are not optional as they are too important. PNJunction will disagree with me, but I stand my ground. They will do two very important functions for you. One is give you visual indication when the cells are getting out of Balance. Otherwise you are going to have to get a DMM out constantly and check. More so then with Balance Boards. Second and this is a big one. Without them to balance the cells is going to require you to take the string apart and balance each cell individually and that will takes many days.

Look if you go with LFP you are talking what in terms of money? $10,000? Balance boards are around $15 each and you need 16 of them for a total of around $250. That is a deal of the century for the insurance and convenience your are going to get and will allow you to sleep at night. Ever watch Shark Tank. I am like Kevin and do not want to go to bed at night worrying about my investments. I am out. Let PNJunction loose sleep.

3. Correct a MUST HAVE hardware.

I have two questions about two different concepts.

1. Let's assume I didn't have the passive BMS. Seems like I would have two options when things get unbalanced to the extent of requiring action - either stop and rebalance the cells (see question 2), or manually shunt the higher cells. Is this correct? Obviously rebalancing should be done ASAP if it is believed necessary.

2. Assuming I have passive BMS (which may be immaterial), how would I regularly re-balance the 16 cells once connected in the series string, and installed in the system? Literally, I have my complete system, with gen backup, and now I'm taking it down (I assume) for rebalancing. What would I need (eg auxiliary charger, etc?), and logistically how might it be done?

Thanks! I think I am one step away from thinking I understand. Doesn't mean I do.

Edit Something I just thought of - these passive BMS boards - they can pass currents like 100A around a cell?

I am going to answer all three question in one reply because they are all related.

Initially when you receive the batteries you are going to fully charge each cell. It is a PIA to do and takes a few day or a week. Example a hobby charger set up to charge four at a time. The hobby chargers have built in BMS but you will need a interface cable which is just a simple plug with 5 wires that connect to each cell. So a 1000 watt hobby charger set up for 4S will deliver 60 amps. Do the math on a 800 AH battery. Fortunately they come from the factory at Storage Voltage of roughly 60% SOC. Once you get them all charged up life is easy.

A cell Balance Board is roughly a .5 to .7 amp shunt. Sounds low I know. Once you do the initial Balance, the cells will unbalance so dang slowly the shunt will be plenty fast to keep them balanced even when you are charging at full current. As long as you don't have a cell with an internal short, it shouldn't get far enough out of balance to ever get beyond the shunt's ability. If your pack can't regularly maintain close enough balance so that the shunt can keep up with it, you have bigger problems than just choice of BMS.

Now for some other news. One of my Australian friend I know from another EV forum told me another route you might be interested in. We were discussing your situation. I only have about 4 months experience with a real LFP system in my golf cart, and he has years as a manufacture of BMS systems. He has suggested a true BMS system he sells and makes. In addition to the cell Balance Boards is a BMU. Total cost of around $550 USD. Listen up because it is a good idea and a streamlined BMS without the FuFu.

It is a single BOX with 16 Cell Balance Boards, and a real Fuel Gauge. The Balance boards do communicate over a single pair serial cable so no rats nest. Here is the beauty of it. It interfaces with any Solar Charge Controller and LFP battery up to 80 amps with built in LVD. It has 3 DC Power Ports. One for the CC, Battery, and Load. The port for the CC can be any charger type, makes no difference as long as it is a 3-stage charger which any good Solar CC is. It will used the Bulk and Absorption stages.

Check this out. Here is the company website.

[Living Large]

{snip}-= Don't get to worked up about over discharge becaus eit will only be rare occasions when there is no sun for 2 or 3 days until you approach the cliff. But that is where you will have to manually monitor things.

Could I not have my generator set to come on at a point just below where I would normally intervene, in case I were absent for some reason? Assuming there was not a malfunction, it seems like that would be good insurance.

Then perhaps as time goes on and you save a few coins you can add a monitor to see all cell voltages. Perhaps 5 years from now Midnite Solar or some of the other manufactures will off Charge Controller made for LFP and Active BMS integrated into their product. Until then you just have to work around it.

Now you and others understand why I say LFP is not ready. It can be done but you had better completely understand what you are dealing with. Otherwise one ignorant mistake and your very expensive battery is a BRICK.

Well... I didn't see the part where you said LFP "is not ready". I may have missed that post. It seems close, though absolutely, one mistake and a whole lot of money down the drain. I can see why that would make you say "not ready". It appears more ready than AHI in the operating window respect, but not the "I'm about to self destruct, please pay attention and lend me a hand" respect. My brother just pointed my to liquid metal batteries, which appear even farther off.

This whole ballgame seems to be one of a half to full time job, so I am not too afraid of having to babysit it so it doesn't implode. From the description, it doesn't sound that bad, and I'd rather take the risk and not deal with watered Pb batteries. Maybe I'm being short sighted.

I don't carry collision insurance on my cars - not even the new ones I have owned, because I don't plan to hit anything. So I am a calculated risk taker.

[Living Large]

Tell you what I got some chores to do right now, but when I get back I will look around for some US distributor that offer packaged systems.

Thanks - I have not exhausted the list you previously gave for places that offer EV products, but if you have something more targeted that would be great.

In the mean time think what 16S2P means using 400 AH packs.

You're hurting my brain. Why would I want to think about that? That isn't something I need, is it? One thing it would mean is $19-20K

[Living Large]

Don't worry about it, it was a dead end thread going no where



The Balance boards will give you a clue, and your volt meter will confirm it. You will start noticing one, two, or three Balance Boards will light up long before any of the others. When ther light comes on the cell is full and the Balance Board is in Bypass. Unlike Pb LFP cannot be equalized as they stop passing current when fully charged up so they have to be Bypassed. Understand?

To balance the cells, you use the Balance Boards by applying a low current charge until all Balance Boards light up. That is one of the Big differences between Active and Passive BMS. An Active BMS actually takes energy from higher cells and gives it to lower cells continuously. A Passive system is a Bleeder aka Shunt that bypasses the full cells. Active systems takes Smarts and Communications with all other cells and the charger. Passive is just an inexpensive dumb circuit board unitasker taking care of it one lonely cell and knows nothing else.

Think of it this way. The Balance Boards make LFP behave like Pb when being EQ'd. A Pb cell still passes current when fully charged to allow lower cells to keep charging. A LFP cell quits passing current when fully charged.

I was confused (maybe I still am). I am getting a clearer picture - thanks for hanging in there. In my "summary" list above, I had 4 items. I believe #2 and #3 are the same now. Doh. The hardware I need is:
1. a CC and charger with software programmable thresholds
2. passive BMS boards (optional)
3. LVD disconnect at correct voltage

And to monitor the individual cells myself to know where I'm at.

OK, now - I believe I understand the simple function of passive shunting of the BMS when things are a little out of balance. Basically, every cell can get charged "fully" (yes, I remember we really don't want 100%). Perhaps "equally" is more appropriate, perhaps not, but I digress.

I have two questions about two different concepts.
1. Let's assume I didn't have the passive BMS. Seems like I would have two options when things get unbalanced to the extent of requiring action - either stop and rebalance the cells (see question 2), or manually shunt the higher cells. Is this correct? Obviously rebalancing should be done ASAP if it is believed necessary.

2. Assuming I have passive BMS (which may be immaterial), how would I regularly re-balance the 16 cells once connected in the series string, and installed in the system? Literally, I have my complete system, with gen backup, and now I'm taking it down (I assume) for rebalancing. What would I need (eg auxiliary charger, etc?), and logistically how might it be done?

Thanks! I think I am one step away from thinking I understand. Doesn't mean I do.

Edit Something I just thought of - these passive BMS boards - they can pass currents like 100A around a cell?

[Sunking]

EditThe CALB battery "standard" charge current is 0.3C. I would be a little lower than this - around 100A+/- each, from the PV or generator.

No problem. All the LFP can be charged at 1C max, but will sacrifice cycle life if you do that. The .3C they reccomend is the max to get maximum cycle life. Key word is MAX. You can go as low as you want with no problem. Unlike Pb minimum is C/12 because that is what it takes to gas and stir the electrolyte to prevent stratification. LFP has no minimum to speak of.

Tell you what I got some chores to do right now, but when I get back I will look around for some US distributor that offer packaged systems. In the mean time think what 16S2P means using 400 AH packs.

[Sunking]

OK, what I am hearing is that the LVD is built into many inverter/chargers, however here we have one of the current incompatibilities in technologies - that the voltage threshold is fixed, and designed for Pb. Being an EE, I would lean towards modifying the built in detection circuit. Why add complexity, and another device in series with the power source? And I do see this wouldn't be part of the CC.

Excellent suggestion to contact manufacturers before backwards engineering, breaking out the soldering iron, and voiding the warranty. Sounds like LVD is done on the entire bank voltage, and not the individual cells. Seems like potentially one cell may dip below the death threshold, and not trip the LVD. But the assumption is someone (me) is monitoring regularly to detect a trend, and the cells will generally track together. Thanks again. Very useful info!

You got it down and understand completely now. Absolutely correct all the equipment you will use is designed for Pb Charge and Discharge curves. 48 volt load device like an inverter are designed to operate between 40 (LVD) to 60 volts (EQ). LFP operating range is 2.9 vpc (46.4 volts @ LVD) up to 3.65 vpc (58.4 max charge voltage). As you can see LFP is inside Pb operation limits, just narrower from a flatter discharge curve.

So on your Charge Controller you want to set it up for 2-stage charging. To do this we just ignore Float and EQ settings. Set Bulk, the constant current mode, to roughly 80% SOC = 56 volts. Set Aborb, the constant voltage mode to 95% = 57.6 volts. Set Float = EQ = 53 volts. By not taking the LFP to 100% is going to significantly increase cycle life from 2000 cycles up to 2500 to 3000 cycles. But DO NOT let that fool you into false sense of security no cell will ever be over charged or get to the SLOPE OF DEATH 2.5 volt cliff because the cells are not perfectly matched and after several cycles will become unbalanced. If you use Passive Balance Boards they will tell you which cells make to 100%, but will NOT tell you which is the lowest cell flirting with death. Don't get to worked up about over discharge becaus eit will only be rare occasions when there is no sun for 2 or 3 days until you approach the cliff. But that is where you will have to manually monitor things.

Then perhaps as time goes on and you save a few coins you can add a monitor to see all cell voltages. Perhaps 5 years from now Midnite Solar or some of the other manufactures will off Charge Controller made for LFP and Active BMS integrated into their product. Until then you just have to work around it.

Now you and others understand why I say LFP is not ready. It can be done but you had better completely understand what you are dealing with. Otherwise one ignorant mistake and your very expensive battery is a BRICK.