LiFeP04 Batteries for Solar & BMS

[Sunking]

A Lifpeo4 battery pack doesn't necessarily stay in balance, it depends on a number of things I believe.

I do not think that is true. Large format cells are not paralleled. If they are they are ladder connected. I know a lot of EV guys who run EV's Bottom Balanced and do not have BMS and running fine for 20 months. With everything in series and bottom balanced capacity remains equal.

[Living Large]

A Lifpeo4 battery pack doesn't necessarily stay in balance, it depends on a number of things I believe.

What I have been hearing and seeing in videos of users is it tends to stay in balance for extended periods of time (months).

[Living Large]

LL is it possible you are stuck inside a Pb box mentality. It is easy to do. With Pb we work on system battery voltages as a whole. Example charge at 56 volts, and disconnect @ 42 volts on 48 volt system. With LFP we work or control at the individual cell level voltages without much regard for total cell voltages as that lacks the level of detail we need to know.

As discussed we can either balance at the top of bottom. Perhaps Balance is not the right way to think of it because it is really a reference point voltage. A 100 AH LFP cell actual capacity will range from 101 to 115 AH. If we balance at the top, all cells will be 100% SOC voltage of 3.43 volts when rested. As we discharge and we hit say 101 AH discharged, the weakest cell will already be down to 2.0 volts and falling into death zone while all the other cells still have acceptable voltages and pack voltage would still appear to be OK when it is not

If we Bottom Balance we are setting SOC voltage at 0% where all cell voltages are equal. When we charge and monitor for the first cell to hits 100%, it is going to be the weakest cell at 101 AH, and its resting voltage is 3.43 volts and the other 15 are at a lower SOC voltage. However all 16 cells have the same 101 AH capacity which is dictated by the lowest cell.

Sorry if that is repetitive, but very important for you to understand.

As of today there is no solar charge controller made to work with Lithium. When it does I promise will be designed for Top Balance. However we can use what controllers are on the market today if we bottom balance because all we need is the Constant Current or Bulk mode with a BMS to achieve that goal. To do that all we have to do is set Bulk to something equal to or higher than 57.6 volts (16 x 3.6 volts). Once that first cell reaches say 3.4 volts we want the controller to do one of two things. 1. Turn off until the next morning, or lower voltage and go into float mode just below the pack total voltage, so charge current stops until the batteries drain down to Float Voltage level so we can have the power from the panels to be as much as possible while the Sun is up.

I am not stuck in Pb mentality, because remember, I am brand new to this. What I am stuck with is the current state of the art, and that is why I am looking at it the way I am. While a BMS can monitor to the cell level, the question is what can be done with that information. That was the angle I was coming from.

You didn't specifically critique what won't work in what I proposed. I told you one potential issue with your proposed solution - the XW, which appears to be a pretty flexible unit, will shut down using the Aux input. It's definition is "REMOTE POWER OFF". It doesn't suspend activity and wait for the input to change state. So I am stuck with equipment function limitations, and how to operate within the confines. You're the one who taught me this.

I sense this is not the route you would go, but I ask you, IF I have a CC and inverter/charger that shut down using their Aux inputs, would it be [not possible], [no added benefit], [acceptable but not an optimal use of resources], [acceptable] to add a BMS to an LFP system with gen backup as follows:

- use the BMS to only flag crossing dangerous low and undesired high thresholds (with a timer to account for transients, if needed) and shut down the offending operation (charge, load). User intervention would be required to resume.
- rely on only the built in CC and inverter/charger thresholds and operating modes for normal operation.

We started the LFP conversation a month or more ago with your suggestion that you wouldn't suggest an LFP bank without a BMS. At the extreme, one could have an LFP bank with no BMS, monitor it frequently to assure proper operation. My idea, since I am constrained by equipment, is add a BMS to prevent out of bounds conditions that normally would not be seen assuming the cells haven't fallen out of balance. Is this not the kind of protection I would want to add? Does what I suggest not provide such protection, or is your principal objection much more could be done having added a BMS?

[tasman]

What I meant by that is, either the cell has degraded or it has gone out of balance. That's all. Instead of letting things get worse, shut down. Make the monkey (me) come along and figure out what cell has gone out of bounds, for whatever reason. Rebalancing is supposed to be rare, since LFP cells tend to stay in balance.

I want to terminate charge normally at whatever high threshold is set in the CC and charger for the entire pack. We bottom balanced, so I wouldn't have the exact same 90% SOC on each cell, but do I care? The BMS high threshold would be above that, and represent that I detected a high voltage on a single cell, not waiting to see it on the entire pack, which may mask the individual exceedance.

The XW is that, isn't it?

A Lifpeo4 battery pack doesn't necessarily stay in balance, it depends on a number of things I believe. Size of pack, number of cells in lines, where in the pack you connect load and charge to, makes a big difference. Unlike LA, where you connect to dedicated terminals, with lifepo4 there are a number of options. That's why we use active cell balances to compensate for uneven energy distribution, cell acceptance and capacities.

My packs are connected diagonally, which allows for better distribution across the pack compared to connecting to the front terminals. Small packs seem to stay in balance better than large ones because of less discrepancy in distributing charge across all the cell lines and individual cell blocks.

Been thinking of connecting to all 4 points of the pack to get a better more even distribution, but don't know the technicalities and have yet to look at them, so don't know if it will work or be detrimental. My gut feeling is connecting both negatives at each end of the pack and each positive, would allow for an even better distribution and make balancing more stable. Maybe someone can enlighten me on that score.

[Sunking]

LL is it possible you are stuck inside a Pb box mentality. It is easy to do. With Pb we work on system battery voltages as a whole. Example charge at 56 volts, and disconnect @ 42 volts on 48 volt system. With LFP we work or control at the individual cell level voltages without much regard for total cell voltages as that lacks the level of detail we need to know.

As discussed we can either balance at the top of bottom. Perhaps Balance is not the right way to think of it because it is really a reference point voltage. A 100 AH LFP cell actual capacity will range from 101 to 115 AH. If we balance at the top, all cells will be 100% SOC voltage of 3.43 volts when rested. As we discharge and we hit say 101 AH discharged, the weakest cell will already be down to 2.0 volts and falling into death zone while all the other cells still have acceptable voltages and pack voltage would still appear to be OK when it is not

If we Bottom Balance we are setting SOC voltage at 0% where all cell voltages are equal. When we charge and monitor for the first cell to hits 100%, it is going to be the weakest cell at 101 AH, and its resting voltage is 3.43 volts and the other 15 are at a lower SOC voltage. However all 16 cells have the same 101 AH capacity which is dictated by the lowest cell.

Sorry if that is repetitive, but very important for you to understand.

As of today there is no solar charge controller made to work with Lithium. When it does I promise will be designed for Top Balance. However we can use what controllers are on the market today if we bottom balance because all we need is the Constant Current or Bulk mode with a BMS to achieve that goal. To do that all we have to do is set Bulk to something equal to or higher than 57.6 volts (16 x 3.6 volts). Once that first cell reaches say 3.4 volts we want the controller to do one of two things. 1. Turn off until the next morning, or lower voltage and go into float mode just below the pack total voltage, so charge current stops until the batteries drain down to Float Voltage level so we can have the power from the panels to be as much as possible while the Sun is up.

[Living Large]

So far so good.

I'm not so sure about that. I think we have a different vision. Since I am not sure what you suggest can be implemented with the equipment I am looking at, see if what I am suggesting stands any chance, or similarly you don't think it will work. I was thinking the BMS thresholds would be never to exceed high and low values that indicate one or more cells has gone out of balance, started to malfunction, etc. The charger/inverter and CC settings window would be fully contained inside of the BMS window. I was going to use the BMS triggers, both high and low, to shut down the system. The programmable thresholds of the CC and inverter/charger would be the ones "normally" used, both low and high. Your vision is the high BMS threshold is where I would stop charging. You say below that the low BMS voltage would mean it needs to be "recharged," but in my vision this may mean re-balancing might be due because that cell fell lower than the others. We bottom balanced the pack. Recharging is needed at whatever the low threshold is set at in the charger and CC, which I was going to set above the "never to exceed" low voltage.

Note: The input on the XW is a SYSTEM POWER OFF signal - shuts the XW off. I don't know that it will function as "charge is complete" signal as you suggest. It shuts off both the inverter and charger.

OK this is where you start to go off track. During charge cycle be it from solar or generator the BMS is monitoring all cell voltages. If the pack has been bottom Balanced properly, the weakest cell reaches 90 to 95% SOC before any other cell. When that condition is met the BMS sends a signal to the charger to terminate the charge. Nothing gets disconnected, thus nothing is at High Voltage, you are just done charging and everything is normal.

In my vision, something IS high voltage - the cell has exceeded the highest voltage we want it to see.

OK this is where the wheels fly off the track. There is nothing to repair or replace. The LVD is a fail safe to prevent any one cell from being over discharged. If a LVD is issued means you have discharged one or more cells to the SAFE LIMIT, and it disconnects to prevent damage. All it means is you need to recharge before it reconnects, there is no damage.

What I meant by that is, either the cell has degraded or it has gone out of balance. That's all. Instead of letting things get worse, shut down. Make the monkey (me) come along and figure out what cell has gone out of bounds, for whatever reason. Rebalancing is supposed to be rare, since LFP cells tend to stay in balance.

So the real question is how do we take equipment made for Pb and make it work on LFP. {snip} So from this point forward let's call it Charge Terminate or CT. OK?{snip} So in theory the INVERTER should trip before the BMS.

I want to terminate charge normally at whatever high threshold is set in the CC and charger for the entire pack. We bottom balanced, so I wouldn't have the exact same 90% SOC on each cell, but do I care? The BMS high threshold would be above that, and represent that I detected a high voltage on a single cell, not waiting to see it on the entire pack, which may mask the individual exceedance.

Let's go one step further. What if we buy an Integrated Inverter with built-in genny support and charger?

The XW is that, isn't it?

[northerner]

If it were me I would send a signal to the genny before that happens. But that is just me. Same consequence if it is Pb or LFP so your point is moot.

You could only do that if you have auto start set up. For my system where I live in a very cold environment, it can take me half an hour or longer to get the generator started manually. My procedure now is to preheat the carburetor with a hair dryer set to high, for 20 minutes or half an hour. Then it's a delicate procedure to get the generator to start on gasoline. After it starts, I switch the gasoline fuel off and switch to natural gas before it quits. Then warm it up for 5 or 10 minutes before I can finally make use of the power.

I may look at preheating the shelter in the future, but issue with that is I have to have it ventilated really well at the same time. But at this stage, auto start with a generator is just a dream!

[Sunking]

Yes, but the inverter/charger won't work if the LVD has shut the system down, unless the voltage of the cell recovers?

If it were me I would send a signal to the genny before that happens. But that is just me. Same consequence if it is Pb or LFP so your point is moot.

[Sunking]

Here goes. I was hearing that you want to never charge a cell over a certain voltage, or drain below another certain voltage. Inside of this window would be the normal operating range, with a higher low and a lower high threshold - these are programmed into the CC and the inverter/charger. Back to the larger window, the Orion BMS (for example) will detect the two "never exceed" events for the first cell that it happens to.

So far so good.

So my thinking was the CC or charger might be charging when the HVD level is exceeded, and I would want it to stop charging. So while not literally a "disconnect", by issuing a signal to both the CC and inverter/charger via the Aux ports,charging would stop.

OK this is where you start to go off track. During charge cycle be it from solar or generator the BMS is monitoring all cell voltages. If the pack has been bottom Balanced properly, the weakest cell reaches 90 to 95% SOC before any other cell. When that condition is met the BMS sends a signal to the charger to terminate the charge. Nothing gets disconnected, thus nothing is at High Voltage, you are just done charging and everything is normal.

Likewise an LVD signal could also be issued to both, whatever condition causes it to arise. In either case, this rare event would cause me to find the system shutdown, and I'd have to perform a repair/replacement.

OK this is where the wheels fly off the track. There is nothing to repair or replace. The LVD is a fail safe to prevent any one cell from being over discharged. If a LVD is issued means you have discharged one or more cells to the SAFE LIMIT, and it disconnects to prevent damage. All it means is you need to recharge before it reconnects, there is no damage.

So the real question is how do we take equipment made for Pb and make it work on LFP. HVD is just a borrowed term from EV's and is a bit misleading. In EV's they use Regen Braking which under certain conditions could lead to a Over Charge. That condition could happen when you take off with a fully top balanced pack and live on top of a hill and use Regen immediately before the pack has had enough discharge to handle the REGEN Braking. Most BMS made for EV's have what is Called a CAN BUS that talks to the charger, motor controller, and all systems on an EV. So as you can see HVD is misapplied to RE systems as we will not physically disconnect the batteries from the charger, we just terminate the charge cycle when the first cell hits the set point. So from this point forward let's call it Charge Terminate or CT. OK?

So how do we implement CT? I can think of two ways. Both involve using the the BMS signal of when the first cell reaches the set point. Either to an Auxiliary Input to a CC if available, or a relay switch to disconnect the panels from the CC.

Next is the LVD. Same thing really we use the BMS to issue the LVD signal to one of two place. Your Inverter if it can accept such a signal, or a relay inserted between the batteries or inverter. There is even a 3rd method that does not need any equipment or BMS signal bu tI would not trust it, and use the Inverter built-in LVD.

If were were to use the Pb mentality and assume all cells are equal voltage, the inverter trips at 42 volts or 1.75 vps on Pb. If we apply that same logic to LFP we disconnect at 2.6 vpc or 41.6 volts. So in theory the INVERTER should trip before the BMS.

Let's go one step further. What if we buy an Integrated Inverter with built-in genny support and charger?

[Living Large]

LL we are talking two separate issue. LVD is physically either a Relay or Logic signal to operate a relay to disconnect a load, or a signal to Inverter to turn it off. There is no HVD in our discussions. Just a signal sent to the CC to turn off. If that cannot be done then a HVD relay to disconnect the panels. However I am confident th eMS controller can be controlled by aux inputs. Sounds like Xantrex can also be controlled. Let us know what they say. I already know what MS is going to say.

Dereck, here was my thinking as a noobie, again with no equipment or experience (please point out where I went wrong). First of all, my misuse of terminology HVD and LVD may be part of the problem.

Here goes. I was hearing that you want to never charge a cell over a certain voltage, or drain below another certain voltage. Inside of this window would be the normal operating range, with a higher low and a lower high threshold - these are programmed into the CC and the inverter/charger. Back to the larger window, the Orion BMS (for example) will detect the two "never exceed" events for the first cell that it happens to.

So my thinking was the CC or charger might be charging when the HVD level is exceeded, and I would want it to stop charging. So while not literally a "disconnect", by issuing a signal to both the CC and inverter/charger via the Aux ports, charging would stop.

Likewise an LVD signal could also be issued to both, whatever condition causes it to arise. In either case, this rare event would cause me to find the system shutdown, and I'd have to perform a repair/replacement.

Please explain how this possibly simplistic thinking falls short. Is this insufficient to accomplish what is needed? Do you really need a hard disconnect in addition to shutting down the CC and/or inverter/charger?

ps Did you see that Northerner also didn't see the Aux port shutdown in the Classic manual? I need to look again. I didn't hear back from them yet.

[northerner]

In off grid system requires a generator.

Yes, but the inverter/charger won't work if the LVD has shut the system down, unless the voltage of the cell recovers?

[Sunking]

In off grid system requires a generator.

[northerner]

About LVD

About LVD, I think that if your power system is essential, it may be worthwhile to have the LVD switch to a smaller temporary backup battery, and then disconnect the main battery from the system. I know that would apply to me who is dependent on a furnace, etc... to prevent the house from freezing.

If you happen to have an LVD event, could you not be stuck down a creek without a paddle if not prepared? It could prevent one from switching on the charge controller, the inverter/charger, plus you would have no power. I would take it that the battery pack voltage may recover enough to be able to run your inverter/ charger to be able to run the generator, in order to recharge the pack. An alternate plan would be to have a separate charger that can recharge the batteries from the generator.

[northerner]

That is where you have to get out of the Pb box thinking. When the first cell reaches 90 to 95% you terminate charge period.

Now you could switch to float providing the float voltage is lower than the pack resting voltage so all charging stops. The LFP battery would have to drain down to Float Voltage before the panels would supply any power. Ideally if you have a day with very little use say gone over night somewhere, is just turn the controller off and not recharge the next day. All that can be done with BMS. Its one less cycle or nail in the coffin. LFP does not need to be 100% SOC. In fact better if it is never 100%.

I agree that you only want to get to 90% or so on the first cell that reaches 90%. All I was suggesting is that it's better if you can force the charge controller into a lower float voltage, rather than shut off the charge controller or panels.

It looks like the way the Aux 2 input works is it will disable charging as long as a voltage signal (ie from the BMS) is present. I know I wouldn't want to permanently latch off charging, but also would not want it alternatively switching from able to disable, as the voltage hovers around the HVD trigger point.

[Sunking]

It depends what you want it to do. Based on the discussion we have been having, I was anticipating that for me it would not be a typical event. But hey, if it could be an added part of normal operation, that would add flexibility in what is possible.

I saw it as a hard error, and was thinking of shutting the whole system (MPPT, inverter/charger) down for either an LVD or an HVD.

LL we are talking two separate issue. LVD is physically either a Relay or Logic signal to operate a relay to disconnect a load, or a signal to Inverter to turn it off. There is no HVD in our discussions. Just a signal sent to the CC to turn off. If that cannot be done then a HVD relay to disconnect the panels. However I am confident th eMS controller can be controlled by aux inputs. Sounds like Xantrex can also be controlled. Let us know what they say. I already know what MS is going to say.