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diy solar

Does the BMS have to be connected to the inverter?

I have read that paper about the degradation of cells. Most of the capacity loss in the testing occurs due to holding the cells at a high voltage. So there is a big trade off between stuffing in more energy and how many cycles you can get. Based on what I have read on the NMC cells I have my absorption voltage dialed down to 57.2 volts for the 14S pack, 4.08 volts per cell, and my CV time is short, just 10 minutes where the current falls from 25 amps to just 8 amps and terminates the charge cycle. The battery only sits at that state of charge for about an hour or two, depending on how long the solar output is running the house before it starts discharging again. I chose the voltage and the shorter absorb cycle to extend the number of cycles. The low charge current also helps battery life. The SEI growth graphs are "normalized" to the maximum growth for a full charge cycle. So it becomes a bit misleading. The RATIO looks worse at 1/2 charge when charging slow, but the total SEI growth when charged at the slower rate is actually less. So if you are doing a partial charge, the faster rate looks better, but for a full cycle, the slower rate is better. This becomes a big issue in a car when you do many partial cycles due to regen braking. This does not happen in a solar storage system. Here is the chart for the actual SEI layer growth over a full charge at the different rates.

Table I.
Percentage of SEI growth from DST driving cycle Charging Rate
Pinson and Safari and Ramadass,
Bazant Delacourt et al.
1C 6.32% 8.48% 7.54%
C/4 3.07% 6.09% 5.67%
C/8 1.62% 3.60% 3.31%

As you can easily see, in all driving cycles with different drivers, the total SEI layer growth is still far less at the lower charging rates. Almost 1/4 for Pinson and Bazant driving their cycles, and still less than half for the others. Still trying to figure out how well this data translates to a cycle a day low rate solar storage. Going from 50% to 90% at 0.1 C rate and only 10 minutes of absorb a day is being very gentle on the batteries. I could basically eliminate the absorb time, and it would not cost me any run time, but at these low rates, it does not seem to add much to the degradation. My system is at about 280 cycles already. What I am doing is about the same as very lightly driving a Chevy Bolt about 100-130 miles every day, and then charging it on a level 1 charger to just under 90%. That would be driving 42,000 miles a year. That is certainly a lot more than an average driver, but this would all be light, fairly steady cruising, and no regen or sudden acceleration. My largest burst current is only hitting 40 amps per string (80 amps total) where the Bolt can hit 500 amps under acceleration on a single string. The Bolt is 96S3P compared to my 14S6P. Yesterday, the absorb charge only put about 2 amp hours (about 150 watt hours) into the battery. That is really nothing. So I will try setting the absorb time to minimum and see if I notice any loss in my run time down to 51 volts. If it nets me a hundred more cycles, then great. At less than a year in, the cells are obviously still acting like brand new.
 
I try not to have a lot of markup but if I were sourcing the unit for a client, based on the last pricing I got, I would be able to sell the SI BMS unit for €345 before accessories.

When it comes to accessories, I don't recommend the screen but I recommend the WiFi module. It's much more useful and only €20 difference. I also recommend the precharge unit for any and all installations where possible.

Wi-Fi module €160
LCD touch display €140
Additional temperature sensor for BMS (max. 3 sensors per unit) €6.5
Precharge unit (2-11s delay @ 11-68V) €35
PC Software BMS Master Control with PC connection cable RS-485 to USB €60
Cable CAN (DB9 to RJ45) (length = 2m) €6.5

You would still need:
1. 50mV Shunt (Best sourced elsewhere. They are expensive from them.)
2. Contactor
3. Fusing etc
Thanks @the_colorist

So all in around probably around €750 plus shipping and fees. Where would that be shipped from? So I can calculate if any fees would be due.

I'll confirm with ZEVA the overall totals too.
 
Hi @the_colorist

I think I'll probably pull the trigger on that ZEVA BMS. Working out quite a bit cheaper even with the customs costs. Thank you for pricing the REC though.

Thanks again for the assistance, and I'll no doubt be back for more at some stage.
 
I have a REC and a ZEVA running on separate packs - i don’t think you’ll be disappointed with the ZEVA.
 
Hi @the_colorist

I think I'll probably pull the trigger on that ZEVA BMS. Working out quite a bit cheaper even with the customs costs. Thank you for pricing the REC though.

Thanks again for the assistance, and I'll no doubt be back for more at some stage.
Sure thing, no worries. Been ultra busy so just getting to reply to this.

Ian makes good hardware. I don't think you'll be disappointed, per @toms comment.

Thanks @the_colorist

So all in around probably around €750 plus shipping and fees. Where would that be shipped from? So I can calculate if any fees would be due.

I'll confirm with ZEVA the overall totals too.
Yes, basically. Ships from Slovenia.

Most of the capacity loss in the testing occurs due to holding the cells at a high voltage.
I've seen similar reports/whitepapers and I'm in agreement that it's a major factor. This is one of the main reasons we prefer to have communication to hopefully reduce or eliminate voltage spikes at the end of the charge cycle by tapering the current on approach to the target. It helps reduce oscillation/overshoots at the top. Active balancing also helps to reduce the time on approach to the target and at the target voltage before switching to a float. The current taper does slightly increase the charging time so I'm attempting to balance both: Overshoot prevention vs overall charging time.

Here are a few further quotes from the article in case any future readers are curious:

Source: Model-Based SEI Layer Growth and Capacity Fade Analysis for EV and PHEV Batteries and Drive Cycles

"The growth of the SEI layer contributes to capacity fade by removing active lithium from the system irreversibly and by increasing the resistance between the solid and liquid phases, creating a layer of lithium carbonate at the SEI. Specifically, the removal of cyclable lithium directly reduces the available capacity, while the increased resistance reduces the power deliverable by the cell."

"The SEI layer growth from the three different growth expressions (in the PDF) can be seen for three charging rates (under constant-current, constant voltage (CC-CV) charging) in Figure 1. Figure 2 shows the SEI growth that occurs during EV use under the dynamic stress test (DST) driving cycle (regenerative charging accounts for almost all of the SEI growth in this case) for all three growth expressions. The kinetically limited expressions from equations 2 and 3 are qualitatively similar, with both showing much greater SEI growth during the later stages of charging, especially the CV portion of charging. As charging rates decrease the differences between the different types of SEI growth will shrink. Additionally as the charging rate decreases, the ratio of SEI growth occurring during driving to the growth during charging decreases as seen in Table I. Table I shows the percentage of SEI layer growth that occurs during the DST driving cycle when compared to SEI growth from CC-CV charging."


Sidenote: Unless I'm badly misunderstanding (anyone feel free to comment if I am), what the comment above in bold is concluding is that in Table I where you see a higher percentage of growth rate, it's comparative. For example, the higher growth rate percentage seen for the DST cycle under Ramadass is actually due to the reduced SEI growth from the faster charging rate, therefore there was more growth attributed to the DST driving cycle, hence the increase in attribution/rise in percentage.

"High rates of cycling have been shown to lead to increased capacity fade. However, for the SEI growth expressions shown above, the amount of SEI growth actually increases with a decrease in the charging rate, mainly due to the increased charging time which allows more time for the side reaction to occur. Other mechanisms can have greater effects on capacity fade during high rate charging beyond SEI growth, such as mechanical stress fractures or overcharging. Stress induced fractures can create fresh electrode surface sites which experience greater SEI growth than portions of the electrode that already have some SEI layer covering them. At lower rates of charging the contribution of SEI growth toward overall capacity fade is greater and while other fade mechanisms are present, SEI layer growth has been shown to be one of the greatest factors of capacity fade. During driving the C-rate applied to the battery is less than 1 C for 80% of the driving cycle."


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Per my previous notes, I believe the amount of stress-induced fractures mentioned above can be reduced by compression and not exceeding rated cell temp during charging.

The low charge current also helps battery life.
I used to be in complete agreement with this (reduced charging rate) however as I have been studying the chemistry more and reviewing more lab tests/whitepapers, I'm finding that several things I had previously thought I now believe to be incorrect. It's my current belief that the charging rate needs to be as fast as possible up to the point that the cells begin to rise above their rated operational temp OR that the physical structure begins to shift at a greater rate than it should (this is harder to measure for sure). As I learn more this may change though. I'm completely open to anything with hard evidence that disproves this.

Here is another interesting article discussing expansion and its effects on the SEI layer growth. It's also attached as a PDF.

Connecting the irreversible capacity loss in Li-ion batteries with the electronic insulating properties of solid electrolyte interphase (SEI) components.


So I will try setting the absorb time to minimum and see if I notice any loss in my run time down to 51 volts. If it nets me a hundred more cycles, then great. At less than a year in, the cells are obviously still acting like brand new.
Very interested to hear the results of this!
 

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Thanks @the_colorist

I've ordered the ZEVA with monitor. Ian is kindly going to set it up with my battery details so it should be ready to go and I'll have the monitor in case anything changes in the future.

Still need to source a suitable contactor.

Would future battery expansion be advisable with this pack? For example if I bought another 16 cells in the future could I add them into the existing pack for packs of 3 parallel and then 16s?

Or would the older batteries cause problems mixed with newer ones? Would it depend on the condition of the older ones?

I have plans to expand my panel capacity in future from 6.2kwp to 12kwp so additional storage would be useful.
 
I used to be in complete agreement with this (reduced charging rate) however as I have been studying the chemistry more and reviewing more lab tests/whitepapers, I'm finding that several things I had previously thought I now believe to be incorrect. It's my current belief that the charging rate needs to be as fast as possible up to the point that the cells begin to rise above their rated operational temp OR that the physical structure begins to shift at a greater rate than it should

I think this is the most misunderstood way to damage LiFePO4 cells.
 
I think this is the most misunderstood way to damage LiFePO4 cells.
Thanks for your input. Do you have some research on this?

Would future battery expansion be advisable with this pack? For example if I bought another 16 cells in the future could I add them into the existing pack for packs of 3 parallel and then 16s?
It really depends on the capacity and resistance of the new vs old cell. The farther the IR difference between the banks, the more issues you will face with eddy currents etc. We usually tell clients we prefer to do it within the first year or so otherwise we'll add the new bank using a new inverter and parallel on the AC side with F/W control etc.
 
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Thanks @the_colorist

I was thinking the same. If I do add batteries it will be because I'm adding more panels so I'd need another inverter anyway.

So makes sense to just add them to it.
 
Here are two of my charge cycles, one with absorb, and one without.
The first is March 22
BattChargeAbsorb.PNG
This was charging at 27 amps. That is just 0.075C rate. At 1:11 PM it hit the absorb voltage of 57.2 and switched to CV mode, the current dipped and tapered from 13.3 amps, down to 8 amps when it terminated the absorb cycle. This is just over 4 hours of Bulk Charge at 27 amps, putting 113 amp hours into the battery. That is just under 1/3 of the battery capacity to bring the battery up from 51.5 volts to 57.2 volts (3.68 to 4.09 per cell). My loads running off the battery are not constant, so I can't use this as a true capacity meter. And the amount of solar coming in after 4 pm can greatly delay how long it takes before it starts using battery power. That night, it didn't start using battery until 5:47 pm, but it ran all of the house load to 9 pm, and kept running the backup loads all the way to 1:31 am.

I am running the battery down to 51.0 volts now (0.5 volts lower), and charging a little slower. Here was the charge cycle from March 29 with the new settings.
BattChargeNoAbsorb.PNG
With the battery run down just 0.5 volt lower, and lowering the charge current from 27 amps down to 23.3 amps, it now took 6 hours and 5 minutes (9:23 am to 3:28 pm) of Bulk Charge to reach the 57.2 volt Bulk limit. It still did 6 minutes of Absorb charge at about 8 amps. This bulk cycle put 142.3 amp hours into the battery, almost 40% of the full capacity. The BMS does report very close to 60% remaining each night when it goes into standby, so that seems very close.

On this day, we also had good solar. The battery did not start running the house until 5:48 pm, just one minute later than the previous chart. Again running the whole house until 9 pm, and then the backup loads only after that. This time it ran the backup loads out to 1:50 am. That is 18 minutes more on battery, but it was more amp hours put in. On Mar. 22 it ran for 464 minutes on 113 amp hours, 4.11 minutes per amp hour. On Mar. 29 it ran 482 minutes on 142.3 amp hours 3.39 minutes per amp hour. But this is not a fair test as the load running in the house could be very different. And even though the solar dipped to low enough to start the inverter at about the same time, this does not account for how fast it fell to zero as it was surely still making some power out to 6:30 pm.

Just 2 days, with a different cut off voltage is not enough data points to see which was a better charge, but they are close enough to show that both are doing well. The energy in to energy out of the battery is coming out to better than 94% return DC from the battery in both cases. Even including the charging and inverting efficiency losses, I get back very close to 90% from AC watt hours in to AC watt hours back out. I need to find a better way to log the actual watt hours as the Schneider dashboard resets that each day at midnight.

I have drawn up a basic plan to add 6 more solar panels, need more detail though if I am going to put in for the permit. These 6 new panels would just go to a DC charge controller to the battery bank and I would not use the AC charging mode of the XW-Pro inverter under normal use. 6 x 355 watt panels x 5 sun hours a day woks out to just over 10 KWH a day. At an average voltage of 54.1, that would be over 180 amp hours into the battery. That is more than I am putting in with the AC charging from the XW now. So I would not be pulling any AC power from the existing panels, all that power would run the house or push back to the grid. The battery would get full charged each day by solar alone. This will be a very different charge curve though. It will start at very low current, ramp up to 30-40 amps at solar noon, and ramp back down as the sun falls. The inverter will start pushing power to the backup loads while there is still solar coming in, so hopefully I won't be throwing too much of the new solar power away.

My maximum charging rate with the grid up will likely never exceed 40 amps, or 40/360 = 0.1111C If the grid goes down, that is a completely different story. My backup loads only need about 800 watts. In full sun, my AC solar tops out at 3,900 watts, and the DC solar could potentially make another 2,000 watts for 5,900 watts coming in, -800 watts, means up to 5,100 watts of charging. At 51 volts, that is 100 amps going into the battery.

With only one battery bank, and one cycle a day, it will take years to see any real trend for how it is holding up. What I am doing is so much easier on these cells than what they see in a Chevy Bolt. How many years do they run without needing a new battery?
 
Thanks for your input. Do you have some research on this?

One of the manufacturers i use has done extensive testing on this while evaluating the optimum initial charge regime used during the SEI formation.
 
One of the manufacturers i use has done extensive testing on this while evaluating the optimum initial charge regime used during the SEI formation.
Would love to see a whitepaper on that. Both on initial charge and extended cycling vs measured SEI growth under varied charging parameters.

Here are two of my charge cycles, one with absorb, and one without.
Very cool feedback. We're currently cycling Nissan Leaf modules across a variety of installations and in various cell configurations from 12S to 16S. 16S has proven so far to provide the most stability for surges/startup on a 48V inverter but not all inverters are capable of a 65.4V voltage target. It's too early to tell yet (we still need to add remote monitoring and data logging for a couple of systems) but I'm hoping within a years time we will begin to have enough data that we can make some predictions about degradation and how we can adjust our charging parameters further (beyond what the scientific community has published). I'm hoping though it will take years (per your comment) to see any meaningful degradation.

I was thinking the same. If I do add batteries it will be because I'm adding more panels so I'd need another inverter anyway.

So makes sense to just add them to it.
Sounds like a plan. ?
 
BMS has arrived from Australia in about a week.

Batteries will probably be another month unfortunately.
 
I'm preparing to connect a 20kwh LiFePo4 pack (when it arrives) to a Solis hybrid inverter.

I've been advised by Solis support that I have 2 options on connecting them (unofficially that is. Officially they don't support DIY batteries).

1 - connect the batteries using the PylonTech option in the Solis menu. Use a Can cable to connect the BMS to the Solis and it should (but not guaranteed) communicate OK.

2 - connect them using the default Lead Acid setting on the inverter, and don't bother connecting the Can cable. The battery parameters can be entered on the Solis and it will then be able to estimate the SOC.

However a friend of mine who built his own battery has told me that there is no point in having a BMS, if it isn't communicating with the inverter because the BMS's only function is telling the inverter to stop when it's supposed to.

So who is right?
I did try the solis inverter but the issue was it did not support the user defined setting for lead acid batteries.
I also did extensive research on diy lifePo4 batteries, the communications link/protocol from the batteries has to be the same as the solis inverter and because this is not the case, it will not work. (I’ve tried it).
there is a couple of options I have one is try out the Phocos all grid inverter (I’m using it now, working well except for a minor issue), or try out the sofarME3000 inverter which seems to be working well for one of the members here. (Shaverncspud).
 
Time will tell soon enough, my batteries are due in about 2 weeks so I'm back to harass @the_colorist with more questions

This is my current battery connection.

20210504_114442.jpg20210504_114543.jpg
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Do I need anything more here or can I literally just disconnect the PylonTech and connect up the LiFePo4 set? The emergency switch should function the same?
 
Also, any suggestions on storage? Given the reduced thermal runaway potential with LiFePo4 can they just be stored in a shelving unit rather than a cabinet?

Do they require compression? I see some saying yes and some no!
 
Have you received your cells Phil? Have you bought the Solis yet?

I have some cells now and need to get a cabinet for them to go next to the garage. Havent bought the solis or the cabling yet.
 
I'm preparing to connect a 20kwh LiFePo4 pack (when it arrives) to a Solis hybrid inverter.

I've been advised by Solis support that I have 2 options on connecting them (unofficially that is. Officially they don't support DIY batteries).

1 - connect the batteries using the PylonTech option in the Solis menu. Use a Can cable to connect the BMS to the Solis and it should (but not guaranteed) communicate OK.

2 - connect them using the default Lead Acid setting on the inverter, and don't bother connecting the Can cable. The battery parameters can be entered on the Solis and it will then be able to estimate the SOC.

However a friend of mine who built his own battery has told me that there is no point in having a BMS, if it isn't communicating with the inverter because the BMS's only function is telling the inverter to stop when it's supposed to.

So who is right?
You friend is partially right, how ever the bms does more than just tell the inverter to stop charging. It has many other functions which are to many to list. Solis inverter on the other hand does not support lead acid or any other battery type apart from lithium iron which has a can bus protocol related to the solis inverter. I have tried all types and this not possible.
in my case I cut my loses, sold on the solis inverter and have a Phocos all grid. Which is working perfectly. I have successfully installed a 50kw lifePo4 48v battery bank. All is working well.
 
You friend is partially right, how ever the bms does more than just tell the inverter to stop charging. It has many other functions which are to many to list. Solis inverter on the other hand does not support lead acid or any other battery type apart from lithium iron which has a can bus protocol related to the solis inverter. I have tried all types and this not possible.
in my case I cut my loses, sold on the solis inverter and have a Phocos all grid. Which is working perfectly. I have successfully installed a 50kw lifePo4 48v battery bank. All is working well.
Importance of bms. Please extract below
Functions of a BMS

  • The BMS protects the battery from over charging, deep discharging, over loading, under temperature, over temperaure and short circuit.
  • The BMS ensures that all the cells in the battery are at the same State of Charge (SoC) making the battery to run at the full capacity. The SOC is usually calculated by the coloumb counting, which is nothing but calculating the current going inside and outside the battery. The BMS transfers the extra charge to a hig charge cell to the lowest charge cell, helping in increasing the battery life by up to 25%.
  • The BMS can communicate with other devices helping to check the battery health and historical date online. The communicaltion is enabled through protocols such as BLE / CAN / CAM. The data that are ususally monitored are minimum and maximum voltage, maximum current limits, open cell voltage for state of charge, weak cell thresholds, charge controller status, thermal management status, etc.
  • The BMS consumes very lower self power which is about 5 milliwatt, making the shelf life of battery to be one year.
  • The BMS provides thermal management to the battery, safegaurding it against over and under temperature.
The BMS also may have a feature to protect the connection from the battery to the load. This is achieved either through seriesly connected power resitors until the capacitors are charged or the SMPS connected in parallel. These are used to precharge the load before the battery, ensuring a safe way to connect the battery to the load. This is called precharge system.
 
Gulam i will be using the Solis in the same way DrPhil is, with the Lead acid setting and custom voltages. My BMS will be seperate and operate to protect the battery if any problem with individual cells or faults etc. I will have no connection between BMS and inverter.

How is the phocos for you? What BMS have you used with it? What comms did you use to communicate between the 2?
 
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