LiFeP04 Batteries for Solar & BMS

[Living Large]

Can LVD be done on time delay? Say 2.6 volts if greater than 15 seconds? Check your email and PM

I don't know that yet. Worst case, the output would have to be filtered. It's close though, I think. I need to go do some research - the Orion Jr PC software is available on the Ewert site, as are various manuals that may answer the question.

For what it is worth, the TI datasheet for that 6 channel A/D reflected that is has a timer of up to 3.2 seconds, I think. Not saying the Orion has that chip - just an observation.

EDIT The software looks extremely user friendly, and I believe I found a timer that will do what you (I) want.
Here is part of a screen shot that applies to both the charge limits and discharge limits tabs:

[Sunking]

I don't know that yet. Worst case, the output would have to be filtered. It's close though, I think. I need to go do some research - the Orion Jr PC software is available on the Ewert site, as are various manuals that may answer the question.

Oops my bad. That was a PM I sent Bob as a list of questions for him to prepare when you reached out to him. Not sure how it got here for you to see. Well at least it lets you know I am trying to help you in the background.

[Living Large]

Oops my bad. That was a PM I sent Bob as a list of questions for him to prepare when you reached out to him. Not sure how it got here for you to see. Well at least it lets you know I am trying to help you in the background.

Please see the edit and screen shot in post #31. The Orion BMS software is on the Ewert website.

[Sunking]

Please see the edit and screen shot in post #31. The Orion BMS software is on the Ewert website.

Great you found it. That sub menu is where you will set up the LVD and over current trip points.

[PNjunction]

Excellent stuff and great write up. For an EV application.

My whole mantra is to differentiate between EV and solar, and treat the batteries according to usage.

For instance, in a solar bank where each cell may be realistically charged and discharged at 0.05 to maybe 0.1C, voltages need to be set differently.

Consider - if you charge your cells at 0.05C to maybe 0.1C (which could be the case with a regular solar setup), you are *already* charging at what could be considered the absorb current. Then once you reach about 3.45 to 3.5v, you are done. If you do a capacity test at this point, you'll find you have reached full capacity. I have tested it.

Going further - what happens when you set your charge limit for 3.7v, but tickle your cells with 0.05 to 0.1C? TWO THINGS happen, and it is slow enough to actually witness.

Once the cells reach full capacity at about 3.45v, it more or less stalls as the charge current takes a bit of time to actually drop to zero amps. So it sits there around 3.45v with no current flowing. BUT, you are still set for 3.7v. If you watch it long enough, the voltage will *suddenly* ramp up very quickly trying to reach 3.7v, yet there is still no current flowing. So what is making this voltage increase? Electrolyte heating and parasitic reactions. Not good.

In addition, because we are not in an EV application, we have time to do pack-level monitoring (assuming you start out sanely) and run with conservative values. Ie, no more than 3.5v or so at the top. And if you wisely engineer your capacity size so you never reach 90% DOD on a regular basis, your LVD can also be conservative at say 3.2v. There is enough time for the LVD to trip, and no cell will be doing a fast cliff-dive at the currents we are drawing.

This is the point that often gets missed with these batteries coming primarily from EV sources - from a practical standpoint, we WANT to do top balance (a misnomer surely as you have pointed out), and bottom balancing means that you have not sized your bank appropriately for solar.

So the warning here is that when doing solar, essentially tickling our cells, what is good for EV is not necessarily good or may be a case of overkill (depending on your risk factor, which can be mitigated by being conservative.)

But do check out that electrolyte heating phenomenon with low current and high-ish voltage. It was fascinating to watch. Put a cell on the bench and try it. Like a dolt, I actually sat there and watched it. Much like Jack does on his bench.

[Living Large]

Excellent stuff and great write up. For an EV application.

My whole mantra is to differentiate between EV and solar, and treat the batteries according to usage.{snip}

Interesting observations.

I know little about this subject, other than appreciating what you are saying. So I am hoping to see some discussion by the people who are knowledgeable. I do have a few questions about what this means practically for a PV application with LFP, but I will reserve them for the moment.

I did anticipate that now that we have identified a possible tool to implement LVD and HVD, there would be a science to determining the actual thresholds as well as a timer interval to account for sag. I didn't think or know about the implication of a slow charge rate - I only had considered charging by genny.

[PNjunction]

I agree wholeheartedly with Sunking for EV use, and a GREAT writeup on that situation.

Generally, although it is not related to Peukert, but only seems similar at the edge-cases, the higher the current you charge at, the higher your cutoff voltage can be. Accordingly, the lower the current, the lower the voltage setting should be. At very low currents such as we do with solar of an appropriately sized bank, you are fully lithiated at a lower voltage.

At very low current, but high cutoff voltages, it is interesting to witness the stall in voltage at zero amps (not something you strive for actually), and if allowed to sit long enough, watch the voltage rapidly rise with NO current flowing! At this stage, one has transitioned beyond the charging stage, and into the parasitic reaction stage causing the rise in voltage with no current.

Having smaller cells allows me to watch this phenomenon in a reasonable amount of time. Aside from a negligible increase in cell capacity, seeing this with my own eyes is what convinced me to be conservative - in this low current application.

Because of this low-current app, I have no need to go beyond the norms one would use with any battery such as an LVD and HVD set conservatively and just making sure I start out sanely. I do an initial top-balance, even though that really means nothing to ensure I'm reasonably sane. However many would argue that last point.

[Sunking]

I know little about this subject, other than appreciating what you are saying. So I am hoping to see some discussion by the people who are knowledgeable.

What PN is saying is an EV charges at a much higher rate. Well I should say some do if they can afford a large format charger. On the DIY side most guys use 144 volt 100 AH LFP. They use a C/10 charge rate of 10 amps which is the maximum a 120 Vac receptacle can handle at 1500 watts. Well guess what? C/12 is exactly what would be used in a solar application.

The jest of what PN is saying if you charge slowly, then the cut off voltage point should be lower. A fully rested LFP battery at room temp at 100% SOC is 3.43 volts. So the conclusion is with a bottom balanced pack at a slow charge is to lower the cut-off voltage which I agree with. Its a tweak to match the application.

Myself on the other hand I have a 48 volt 50 amp charger operating at 240 Vac circuit, so I am charging at C/2, and thus set my cut-off at 3.6 volts. When the batteries rest the weakest cell has the highest resting voltage of around 3.35 volts which is about 90 to 95% SOC which is perfect. The stronger cells are at a slightly lower SOC, but have the exact same capacity because I referenced at the 0% of bottom of the scale rather than the top. The weakest cell is around 102 AH when fully charged. My strongest cell is up around 115 AH. So when I fully charge the weakest cell it has the rated 100 AH along with all the other cells. So when I discharge when I get to 0% all cells arive at 0 at the same time and cannot damage any weaker cells. The cliff is 2.0 volts. So we set our LVD for something higher than 2.0. In an EV with much higher discharge rates I can set for a lower voltage say 2.5 for 15 seconds, or 2.1 instantly. For a slow discharge in solar you can set them a bit higher with longer time delay.

The beauty of bottom Ballance is even if your LVD fails all cells reach 0% at the same time, and if that happens no stronger cell can destroy the rest by reverse polarity. If working with an Inverter you have two fail safes. One is you LVD will operate at roughly 2.9 vpc x 16 = 46.4 volts. If that fails your inverter also has a cut-off voltage at 42 volts or 2.6 volts per cell making it almost impossible to over discharge your LFP battery.

[Willy T]

Well guess what? C/12 is exactly what would be used in a solar application.

Exactly !!!

This might be a good reason for people to talk to someone that actually has a Solar bank of significant size that charges with PV and discharges the bank on a daily basis.

if you charge your cells at 0.05C to maybe 0.1C (which could be the case with a regular solar setup )

Sounds like a 40w panel on a RC battery pack.

[Sunking]

Sounds like a 40w panel on a RC battery pack.

I don't follow you on this. I fly RC planes, and a common battery is 3S 3000 mah or 11.1 volts @ 3 AH. A 40 watt panel would generate 3.6 amps or a 1.2C charge rate which is normal for LiPo batteries because they have extremely low internal resistance and can be charged very fast. Solar is going to be around C/10 as with most EV's. The only real difference between an EV and RE application is going to be the discharge rate. RE applications are going to be very low rates of C/20 or less, where an EV will be 1C and higher.

[Willy T]

I don't follow you on this. I fly RC planes, and a common battery is 3S 3000 mah or 11.1 volts @ 3 AH. A 40 watt panel would generate 3.6 amps or a 1.2C charge rate which is normal for LiPo batteries because they have extremely low internal resistance and can be charged very fast. Solar is going to be around C/10 as with most EV's. The only real difference between an EV and RE application is going to be the discharge rate. RE applications are going to be very low rates of C/20 or less, where an EV will be 1C and higher.

Where am I going to see the quoted rates when I have 120 amps @ 48 v going into 800 amp hrs of batteries. It's bogus talk without any meaning to " Our Systems " and the way "We charge " . I guess I should have said like a 40 watt panel on a EV.

[northerner]

Would it be desirable to factor in current flow (ie through battery pack) as well as voltage to decide on the trigger point for the LVD and HVD? That way you could have trigger points based on the actual current flows, rather than doing approximations with timing, or does it work fine anyway? I guess that would only apply to someone designing a BMS, unless a unit is available that has this factored in?

I have no experience with LiFePo4, but have done a little research due to my interest in them. My understanding is that some users prefer to top balance, if they find that they more consistently use the top end of the pack, than the bottom. And they may only occasionally go towards a lower charge state and likely stay well clear of the bottom if they do in fact top balance. Sounds to me like it could be a personal preference on whether to top or bottom balance, depending on how you operate. My understanding is that if you go over 90% state of charge on a regular basis with LFP, will increase capacity loss over time, and thus reduce cycle life of the pack. It not top balanced, you could have uneven capacity loss in that case, could you not? (ie the lowest capacity cell(s) becomes even lower over time than the higher capacity ones)

In either case, I agree that it's better to manually balance the cells and not use the current shunts.

[Sunking]

Would it be desirable to factor in current flow (ie through battery pack) as well as voltage to decide on the trigger point for the LVD and HVD? That way you could have trigger points based on the actual current flows, rather than doing approximations with timing, or does it work fine anyway?

Yes, but maybe not for the reasons you are thinking. Remember that dirty word Internal Resistance? Lithium batteries have very low resistance, thus very little voltage sag on discharge, and not much rise in voltage on the charge side.

On the discharge side you sit up the LVD to trigger when either statements are true: 1 Voltage = 2.6 to 2.9 vpc of less for more than 15 seconds. The time delay is used to prevent false trips from large starting loads causing excessive voltage sag. The second condition is a fail safe of Voltage = something like 2.0 to 2.5 trip immediately.

Now here is a legitimate argument one person could make and I touched on it earlier. If you are running everything off a standard battery Inverter they have built-in LVD and they are set for Pb batteries of 10.5, 21, and 42 volts. For example a 48 volt inverter the BMS if you set the time delayed trip point for say 2.6 volts x 16 = 41.6 volts, the inverter would trip before the BMS.

On the charge side is where things get dangerous because to get to 100% SOC requires CC/CV method. You first apply Constant Current until the pack voltage = 3.65 vpc x Cell count. So for 16S would be 58.4 volts. Then hold 58.4 volts and limit current to the 1 amp or whatever value your Shunt Balance Board is rated for until current stops. When current stops all cells are at 100% aka TOP BALANCE.

My understanding is that some users prefer to top balance, if they find that they more consistently use the top end of the pack, than the bottom. And they may only occasionally go towards a lower charge state and likely stay well clear of the bottom if they do in fact top balance. Sounds to me like it could be a personal preference on whether to top or bottom balance, depending on how you operate. My understanding is that if you go over 90% state of charge on a regular basis with LFP, will increase capacity loss over time, and thus reduce cycle life of the pack. It not top balanced, you could have uneven capacity loss in that case, could you not? (ie the lowest capacity cell(s) becomes even lower over time than the higher capacity ones)

You are on the right track, but sounds like you are over looking something. Regardless if you top balance or bottom balance you are still limited by the weakest cell. The danger with top balance is the stronger cells driving weaker cells into polarity reversal, thus destroying them. If you bottom balance you eliminate that threat. Second point and you hit on it is you will get much longer cycle life by only charging to 90% or less. One of the challenges of switching to LFP is getting out of the Pb box. With Pb batteries you want to keep them at 100%, but not Lithium as that does stress them and shorten cycle life. For long term storage you store them at 60% SOC and lithium are best operated and happy between 10 to 90% SOC as it does not weaken them in any way. That is very easy to to with a BMS because the charge/discharge is very flat in that range.

[northerner]

Yes, but maybe not for the reasons you are thinking. Remember that dirty word Internal Resistance? Lithium batteries have very low resistance, thus very little voltage sag on discharge, and not much rise in voltage on the charge side.

On the discharge side you sit up the LVD to trigger when either statements are true: 1 Voltage = 2.6 to 2.9 vpc of less for more than 15 seconds. The time delay is used to prevent false trips from large starting loads causing excessive voltage sag. The second condition is a fail safe of Voltage = something like 2.0 to 2.5 trip immediately.

Now here is a legitimate argument one person could make and I touched on it earlier. If you are running everything off a standard battery Inverter they have built-in LVD and they are set for Pb batteries of 10.5, 21, and 42 volts. For example a 48 volt inverter the BMS if you set the time delayed trip point for say 2.6 volts x 16 = 41.6 volts, the inverter would trip before the BMS.

On the charge side is where things get dangerous because to get to 100% SOC requires CC/CV method. You first apply Constant Current until the pack voltage = 3.65 vpc x Cell count. So for 16S would be 58.4 volts. Then hold 58.4 volts and limit current to the 1 amp or whatever value your Shunt Balance Board is rated for until current stops. When current stops all cells are at 100% aka TOP BALANCE.



You are on the right track, but sounds like you are over looking something. Regardless if you top balance or bottom balance you are still limited by the weakest cell. The danger with top balance is the stronger cells driving weaker cells into polarity reversal, thus destroying them. If you bottom balance you eliminate that threat. Second point and you hit on it is you will get much longer cycle life by only charging to 90% or less. One of the challenges of switching to LFP is getting out of the Pb box. With Pb batteries you want to keep them at 100%, but not Lithium as that does stress them and shorten cycle life. For long term storage you store them at 60% SOC and lithium are best operated and happy between 10 to 90% SOC as it does not weaken them in any way. That is very easy to to with a BMS because the charge/discharge is very flat in that range.

A long as one stays well enough away from the top and bottom, I take it factoring in current draw to the voltage state of the cell will not matter so much anyway. Perhaps more of a concern when charging and getting close to 90%.

I think one also has to take an operational viewpoint to determine whether to top or bottom balance. If one keeps their pack state on average at over 50 % state of charge for most of the time and also wants to get to 90% charge state on many days, particularly when the sun is shining (and using solar of course), then I would think that top balancing would make more sense. If you're not balanced at the top, then there is the risk that some cells are getting charged to over 90% on a regular basis, and will suffer capacity loss over time, which will compound. Going to 90% state of charge gives you only a 10% margin from the top, and the problem starts around that level. The top balancing I'm referring to would only be done very occasionally, thus having a minimal effect on the battery.

I'm aware what you are saying about going to low on the bottom, and reason you would stay well away from the cliff. But if you go down to say 20% state of charge, then you have a good safety margin, with no long term effects on capacity degradation, which you could have going close to the top. Do you see now?

[Living Large]

On the discharge side you sit up the LVD to trigger when either statements are true: 1 Voltage = 2.6 to 2.9 vpc of less for more than 15 seconds. The time delay is used to prevent false trips from large starting loads causing excessive voltage sag. The second condition is a fail safe of Voltage = something like 2.0 to 2.5 trip immediately.

Now here is a legitimate argument one person could make and I touched on it earlier. If you are running everything off a standard battery Inverter they have built-in LVD and they are set for Pb batteries of 10.5, 21, and 42 volts. For example a 48 volt inverter the BMS if you set the time delayed trip point for say 2.6 volts x 16 = 41.6 volts, the inverter would trip before the BMS.

A little off topic, but a practical question. The Schneider XW (48V) I am considering has a hard-wired 32V low battery threshhold, but also a programmable low battery cutoff voltage and a timer to account for sags. But this is on the overall battery pack. There is an auxiliary input that I assume is logic level, which is REMOTE POWER OFF, which will shut the XW down completely.

I need my memory refreshed Dereck. With the Xantrex equipment and programmable low and high voltages, the purpose of this BMS would only be to detect faults where I want to shut down? So I could "OR" a signal that indicates an individual cell has decreased below or exceeded a level outside the "normal" range that is taken care of by the overall pack voltage detection, and drive this REMOTE POWER OFF pin? I'd have to see if the XW can tell the MPPT via the Xanbus, or also via an input of the MPPT itself, to shut charging from the array off.