Installation up and running

mcgivor

The practical switch from LA to LFP was actually easier than the imagined one, after many months of study, reading comments, both negative and positive, discussion with actual users of DIY systems, I decided to go for it. The primary reason was temperature management of the LA bank which greatly impact the life expectancy especially negatively in my particular case. The top picture is the BMS that actually controls the charging and dischargeing, load negative lower left, charging negative above it, battery negative lower right, having two separate ports allows loads to be drawn whilst fully charged, which was one of the stumbling blocks that needed to be addressed, specifications were sent to a manufacturer who came up with this. Unfortunately there was no means of communication or display. To overcome this I ordered a second BMS which passively monitors the battery using outputs to drive external contactors, I do not use contactors but will use an Omron H3CR timer in a one shot setup when coil power is lost, 0.2W standby load, this will turn the circuit breakers off using a linear actuator, still in progress, a second layer of protection.            
 To minimize the length of load runs I decided to take advantage of the lower weight by mounting them on a platform, this clears up the floor space as well, 90Kg total. When installed the cells were fully charged, three overcast days later, very low production, the sun returned charging back up to close to 100%, 5.2Kw, since SOC is not an issue there was no need for the generator, which is nice. The past number of days have been spent watching the cell voltages to find a suitable setpoint, the BMS charges to 3.6V per cell which is where I left the controller 28.8 bulk, absorption, float but advice from others say there is no need to aim for 100%,  it's now programed at 27.1V which seems to work well, still a work in progress. Temperatures vary 2°C morning to afternoon even with 5Kw worth of charging, the space between cells provides more surface area but it's not that hot right now only 33°C maximum will see later if cooling is nessersary. The one thing about there batteries is there is no long absorption it's bulk all the way to 97% +/- so the amount of time needed is vastly reduced which will help when there are only a few hours of direct sunlight.

So far I'm impressed, they solve many of the issues LA have in terms of performance, temperature management, maintenance (for FLA) and cycle expectancy, the only negative perhaps is complexity, though it's not really complicated. Having safeguards in place is important because if something does go wrong, it's usually in a bad way. Thanks to Simon @karrak and Rick @Raj174 for all your support and guidance your assistance is greatly appreciated. 

Ivor 

MrM1

Yeah, it looks all clean and pretty in the above pic,  but is the wiring so nice with all the cover plates taken off inside the boxes?   Seriously, Wow,  That is a super clean looking setup. IT looks awesome.  Is that 16 3.2v LifePo4 cells in series?  (looking closer at your sig I am thinking 8s2p 200 Ah cells?).  I am following this to see if it meets your expectations.   I am thinking I will get 2-5 years more from my L16s and might switch by then.    BMS is my BIGGEST concern.  As I now the damaging "power" of Lithium batteries.  (honestly I'd be afraid to go on vacation with an attached power room full of them - without some kind of BMS and auto shutdown).

Dave Angelini

Very nice!

mike95490

How does the "Battery Over Full"  cut off work to stop charging ?  Is there a control line for the Solar Charge Controller to shut it down, do you kill the PV Breaker with the actuator or do you disconnect the output of the CC ?  How does it shut down the inverter internal charger if you are running from Generator power ?  Those are the questions I'm trying to figure out before I consider changing

mcgivor

The arrangement is 8 series 4 parrallel, 32 cells total, 100Ah cells, no its not as pretty under the make up but it is functional, if I were to do an original install there would be some changes which wouldn't change the results but would enhance the assthetics in its naked form. Perhaps at a late dater I'll redo the whole thing, as far as leaving unattended goes, I'm still in the observation process, I wouldn't feel confident at this stage, perhaps in a few weeks this will change.

mcgivor

mike95490 said:

How does the "Battery Over Full"  cut off work to stop charging ?  Is there a control line for the Solar Charge Controller to shut it down, do you kill the PV Breaker with the actuator or do you disconnect the output of the CC ?  How does it shut down the inverter internal charger if you are running from Generator power ?  Those are the questions I'm trying to figure out before I consider changing

The BMS is bi directional, it supports loads whilst charging, as soon as the batteries are fully charged it continues power to the loads, the charging current is reduced to a 100-300mA pulse every few seconds, depending on demand. There is no need to comand the controller to shut down or any other means of disabling the charging, it'sseamless, as soon as the voltage approaches the setpoint the current drops. With regards to generator charging, the same setpoints would be used, so I assume there would be no difference, but I'm not at that stage at this point in time as I've not needed the generator. The problem associated with most BMS systems is they are primarily designed to charge without loads, the dual port is actually a game changer for offgrid applications, I don't personally take credit as I stumbled across this by asking the right questions. Please understand I'm still in the learning process but so far it's no different than charging LA, as far as I can see. There is in my application a second BMS to provide a window in the form of a display, the addition of signals to to safeguard the primary BMS, should it fail is a work in progress, but it has no function in in daily charging/discharging .Time is needed to digest the question of generator use, I'll do an experiment, ultimately comming up with a solution, but if it should  behave in the same manner as the charge controller, as soon as full charge is achieved the load would be eliminated, manual shut down, auto shut down pass through current? all uncharted territory at its point in time. 

MrM1

So forgive my ignorance, Ivor,  I understand you are running one BMS as primary and one as a passive secondary, primarily for screen display,  but with your primary BMS,   is there more than one unit - say 1 for every string - or just the one?   And does said BMS do the actual "turning off" of charging when the batteries are full?  Or does it communicate with the CC to do so (I think I just answered my own question there). Also  does the BMS detect and correct cell imbalance,  or is the correction left up to the user as Rick (Raj174) has described to me in the past?

Dave Angelini

Ivor, What does the system do if you lower the set-points for the charge controller .5V ?

I understand it is new and you want to let it stabilize. For instance, can you lower the set-points to get to 95% Soc?

mcgivor

Dave Angelini said:

Ivor, What does the system do if you lower the set-points for the charge controller .5V ?

I understand it is new and you want to let it stabilize. For instance, can you lower the set-points to get to 95% Soc?

Dave
When the voltage is set at 27.1V  or 3.385 per cell, the charge current will stay at whatever the array can supply ~48A in my case under clear sky, when the charger reaches 26. 9V the current begins to drop to ~8A, by 27V it's ~ 3A, when 27.1V it's reduced to a pulse of 100-300mA. This is the case with or without loads, all this happens rapidly, it's all about cell voltage though, if the full voltage 28.8V is used the cell voltage will finally reach 3.600V but the cell voltages begin to drift, favoring one cell first then another, the difference in capacity  between 3.400 VPC and 3.600 VPC is so small it's negligible, the charging time between these vaues is a few minutes even with the reduction in current, as opposed to hours to get from 3.100 to 3.30 VPC at 40+A.

When the charging is complete with the small pulse of current whatever the loads are drawing come directly from the panels, if a load is applied, greater than the panels can support, the battery supports it, as soon as the load is removed charging resumes until the setpoint is met then the cycle repeats itself. 

On the first day with 28.8V as the target value, the charging current was low due to overcast conditions, all cells remained within 25 mV of one another, but when clear it would seem the current available causes drift, eventually it will stabilize even allowing a single cell to rise above 3.600V up to 3.900V where it drops charging until, 3.80V then it resumes. Both the BMS and battery manufacturers said this is normal, adding this is where the cell ballancing occurs. The cells can take higher voltage to 4.200 V so it's probably fine to leave it the way it's designed, but for so little benefit it seems hardly worth it, I could be wrong though. Below is the charge curve for a single 180 Ah cell which shows the long plateau then steep rise in voltage, the discharge curve is the reverse, dropping fast to a long plateau.

mcgivor

MrM1 said:

So forgive my ignorance, Ivor,  I understand you are running one BMS as primary and one as a passive secondary, primarily for screen display,  but with your primary BMS,   is there more than one unit - say 1 for every string - or just the one?   And does said BMS do the actual "turning off" of charging when the batteries are full?  Or does it communicate with the CC to do so (I think I just answered my own question there). Also  does the BMS detect and correct cell imbalance,  or is the correction left up to the user as Rick (Raj174) has described to me in the past?

The BMS monitors each cell, which is 4 cells in parallel 8 series in my case, so only a single unit for the bank. The BMS is supposed to ballance in the, upper regions, please read description in my answer to Dave Angelini. The initial cell ballancing is done charging all cells in parallel with a single cell charger before arrangement in the final topography.

Dave Angelini

I just mentioned operating at 95% Soc as many feel it extends life.  Mine will outlive me if I can keep the SPD's happy

mcgivor

There seems little advantage to operating in the upper limits as far as SOC is concerned so I've left the voltage set-point at 27.1V, for now at least, everything seems  to be happy, but today was overcast which hindered my experimentation, oh well there is always tomorrow, don't have to go to work, an added benifit 

bevans

Ivor, greetings from far SE AZ.

Can you provide details of your BMS (make and model)? I'm shopping around and options are few. 

New here and to solar, and trying to wrap my head around general system and safety considerations for running li-ion. Any broad takeaways you have gleaned and can share with me would be immensely appreciated. I have paused my install before connecting the bank to dive further into BMS and battery balancing details. 

Cheers, 
bevans

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mcgivor

The BMS I use is a  Shenzhen Battery Bodyguard model LWS 20S120A (8S130A) Lifepo4 different port
This is not what would be used with Chevy Volt cells however being that they are a different chemistry, NMC, which are a different voltage
.
You could contact them to see if they can supply a BMS based on your particular requirements, cell chemistry, cells in series, charge/discharge currents and so forth, this is what I did, they are very helpful, a message can be sent from their website.
 http://lws-pcm.com/en/new/news53.html.

bevans

OK, thank you for the lead. 

Do you mind also sharing the make/model of the second BMS you chose for external monitoring? Any chance you know if they make NMC compatible BMSs? 

I'm sure you're familiar with "Method 2" of paralleling for balanced charging promoted in this doc (https://www.iotaengineering.com/pplib/balancedcharging.pdf) but your bank appears to be connected sequentially down the line. Can you comment on your decision to do this? And also on the pros and cons of using cables vs. a solid bus bar type conductor? ...ie, calculating that the cross-sectional area of the bars is greater than or equal to the appropriate gauge of conductor that would have connected them. I could guess, but just wanting to hear it from someone with more experience.

mcgivor

The other BMS is a Chargery BMS  16,  http://www.chargery.com contact Jasonwang3a@163.com and ask if they have something for NMC cells 

Another source for BMS is http://liionbms.com/php/bms_options.php#7  there is a selection tool if you contact them don't mention solar, for some reason they think it's not possible, they primarily deal with EV applications.

The connection method you are referring to is for connecting batteries in parallel, with lead acid batteries  ( battery defined as a collection or cells in series ) you series first then parrallel, with lithium it's the opposite parallel first then series. Because of this no matter how much capacity is needed the battery will always be series connected. What you see is 32 × 100Ah cells, 4 cells are connected in parallel to form a single cell or cell block, the 8 cell blocks are then connected in series for 24V nominal.

They are interconnected with bus bar in a U configuration to minimize the overall battery length, the busbar cross sectional is easily calculated, often links are used I however made my own 4 hole busbar links. Using cable would, in my opinion be a mess, what you see in the picture is a PVC duct used to cover the busbar and provide protection against an accidental drop of a tool for example.

http://nordkyndesign.com/assembling-a-lithium-iron-phosphate-marine-house-bank/  This link provides some useful information, which although dealing with Lifepo4 prysmatic cells primarily, the theroy is the same