Overcharging current protection which BMS for LiFePo4 DIY

[ke6cvh]

Hello group and thank you for allowing me to join.

Background: I'm not new to electronics nor to solar systems. My entire system is DIY but lead acid. I have built it for a factory I'm building here (garment cut and sew for two production lines and also my house on property). We have hundreds of PV panels into various charge controllers of different manufacture but each CC has it's same PV type/mfg and orientation for that unit. We have a lead acid battery bank with Maxwell boostcap3000 ultracaps/supercapacitors for surge allowing the charge controllers to handle well quick power surges and reduce battery stress. Problem is battery bank is about 7 years old and not true deep cycle instead n200 size "solarmaster" batteries made in Philippines for a 1200aH bank. They are completely shot/worn out.

I want to build my own LiFePo4 bank so I can go completely off grid 24/7. I'm working on de centralizing some of my power requirements so installing allot of solar lights around property/buildings, making small setups for NSS brand DC brushless fans with battery input where solar goes, working on making a standalone system for our CCTV (3 TV's, 3 systems, 41 cameras) etc etc. But our inverters are all Outback. Our non critical is 4 pieces of 3500 watt Outback in series parallel. Our critical (house) is old sealed 2500 watt outback sealed (and yes they no longer can handle the full rating not likely old capacitors inside but not good enough ventilation needing improvement.

So with hundreds of PV panels I can get between 30kw and 40kw at 27.2vdc but never need it so my panels are basically overdone to make up for cloudy/rainy and early as well as late as possible production. We do have generators (two Denyo 25kva into a bank of 3 parallel meanwell supplies giving 9kw DC at our battery voltage when needed but really not ever needed).

My big concern is not if this system works as I've been running it and constantly expanding it for 7 years now but being new to LiFePo4 the massive amount of panels exceeding the maximum charge current into LiFePo4 DIY battery banks. I read on internet some brands limit both charging and discharging current. I really need someone to "elmer" me (HAM radio lingo for teach) so I don't have a catastrophic disaster on my hands. Too late to change my 24v system but I've got so much copper it is rediculous so I can handle hundreds of amps.

Apologize for being long winded on the background but felt I need to get it all out in the discussion.

Best regards and many advanced thanks,
Mike

 

[Substrate]

No problem. Very simple really.

LiFePO4 "likes" being charged at a mild 0.15 to 0.2C for longest life.

But some will allow for charging up to a C1 rate. Sometimes higher for limited amounts of time.

Know that there are "power" cells designed for high-c use, like starter batteries with outrageous things like 35C. But in a starter / power application, that is only done for a few seconds. Thus is would be wasteful to pay for that capability in a "storage" application possibly like what you are seeking. Usual example of a high-c cell are "Headways".

Energy or "storage" cells have a much lower C rating, like only having bursts to say 3C for a limited amount of time. That's what most of us here are using. Using "storage" cells for starting or say motive-power things like electric ATV's hurt the cells. One way to shoehorn your way around this is to use a hugely over-capacity bank of storage / energy cells.

So you pick what is practical. Choose the right cells (power vs energy), and the lower the current, the longer they last.

BMS - despite what the cells themselves are capable of, sometimes the bms will be a limitation. For example, My Talentcell LFP batteries limit the input to only about 0.5C max, although the cells themselves are capable of more.

 

[FilterGuy]

My big concern is not if this system works as I've been running it and constantly expanding it for 7 years now but being new to LiFePo4 the massive amount of panels exceeding the maximum charge current into LiFePo4 DIY battery banks.

Several questions.

* Do all the various chargers on the existing sytem have the ability to do a charge profile appropriate to LiFePO4?
* Will there be one large system where all of the chargers connect to all of the batteries or is it several smaller independent systems?
* What is the total charge current of all of the panels?
* What is the total charge current you see on the system today?
* What system voltage are you running?
* many Amp-hours of LiFePO4 will you use to replace the Lead Acid?

In general, a reasonably balanced system will not have an issue with charge current. A system with a lot of solar panels will probably also have a lot of batteries to spread the charge current across.

 

[curiouscarbon]

hello and welcome to forum!

as substrate mentioned, various LiFePO4 cells have a range of different charge and discharge capability.

many common LiFePO4 cells specify 0.5 C charge rate, so the ampere hours times 0.5 in amps (50Ampere charge for 100Ah cell)

whichever cell, checking the manufacturer data sheet is important.

usually discharge rating is higher than charge rating with the LFP cells i've seen online. many of the ones i see specify 0.5C charge and 1-2C discharge.

good luck with this renovation project, i hope for a successful 24/7 offgrid outcome ?

 

[OffGridForGood]

I really need someone to "elmer" me (HAM radio lingo for teach) so I don't have a catastrophic disaster on my hands. Too late to change my 24v system but I've got so much copper it is rediculous so I can handle hundreds of amps.

Depending upon the replies to the above questions, likely you will see that 24V is not going to be the best solution, and your 'so much copper' may be best repurposed or sold for its copper value!
It would be hard to imagine building out a new LiFePO4 system in 30-40kw capacity using only 24VDC.
Hopefully your system is actually several smaller subsystems that you can tackle one by one and upgrade individually. As Filter Guy noted, the older SCC's may not even be suitable for LiFePO4 charging, so they will not serve a function in your new system, but become part of the obsolete equipment along with the old batteries. Perhaps you can sell them off as used but functional smaller packages to recapture some of their value? We see older solar panels from large industrial 'solar farms' being sold off used -while they still have reasonable reduced performance - to allow their owners to capture some salvage value, avoid disposal charges, and reduce the replacement costs of new (better) PV panels in those large installations.

 

[ke6cvh]

Hello again and thanks for the answers so far. I did run across some internet information where it had two main classes of BMS being a 2 wire and a 3 wire system with the difference being the charge and discharge path on same wire for the 2 wire system and separated on a 3 but might be specific cases only and likely I'm really just plain ignorant when it comes to BMS stuff? One reply mentioned separating the system which is "kind of" what I'm trying to do and "kind of" what I've done already. There are multiple buildings one building for each of the production lines. I did a S.W.A.G. and if all the servo motors were on at same time (not going to happen) for the machines being 550w per servo "maxed out pedal to the metal" I'd have 20kw. It's just not happening that way nor even close to that in power drain as these see intermittent use even in operation and I'm not using them all at once or even close. So I have a studio for sample making where most of the solar equipment is located. Then I just run heavy guage wires (2awg) for my 230vac balanced out to the small area where the generators are and MBT/manual buss transfer and from there I go to the buildings. This MBT allows me to run the output of the genset (only one or the other through a different MBT) directly to a building bypassing the solar system and simultaneously another set of 2awg wires with 230vac running to the studio with all the electronics and the 3 paralleled 9kw total output meanwell power supplies. Meanwell supplies rock but get wierd when not running from the genset but a different subject as people use rebar for ground stakes here and I use an Ufer grounding system imbedded into the concrete so my ground doesn't like the grid's ground (which is all over the road so to speak for the balanced power).

I can upload pictures but might not do allot to help clarify. I can say I did my best and it is quality work just a little outside the box maybe allot. Answer one question my charge controllers are a mix of make-sky-blue 60amp that work great after the ground loop was eliminated, midnite solar classic 150's, Outback, and some Intronics (USA made not so common likely a bit rare 75 volt version I think called 75hv). I have 20 pieces of the make-sky-blue and all combined well over 30 charge controllers and north of 300 solar panels. My thought to limit current being I'll have all day to charge is just to set all the CC's output to 27.2 volts (float setting) with any kind of bulk or absorb being turned off. By 5pm the hope would be to have charged and not overdone anything but I'm worried even with a steady 27.2v I could still provide too much amperage. Evening power drain is really becoming less and less as I go to solar lights, not even running the inverter aircon as of late, soon to switch 100 percent to brushless DC fans etc. This would likely facilitate the separation of the system at least in some levels of it likely the big separation being a different bank (LiFePo4) for house 3 wire BMS for that and keeping lead acid wet cells (ughh I really really don't like them) for the non-critical/commercial side of things and a common DC buss for both.

My requirements change drastically at 5pm when everyone is gone and I'm just wanting to run power for the house and off the grid. When I first made this setup I had real problems with the charge controllers having latency issues with sudden loads hence began the addition of allot of banks of Maxwell supercaps. I currently have 5 banks and more going in. To charge them I basically have a clipped resistance to slow charge up until around 20 plus volts then I allow a direct short to the 27.2vdc buss. I use a combination in series of ANL fuse and Bussman DC breakers on output of all capacitor banks at different amperage values with the fuse being a higher but much quicker protection value. My saving grace I'm guessing in past likely has been in the past the higher internal resistance of the wet lead acid cells and forgiving nature of them. Maybe....the only way around this is to completely separate my evening source as being LiFePo4 only and decentralizing it. Possibly a 3kw toroidal inverter wall mounted with the LiFePo4 bank into it being a 3 wire BMS with the load coming off that for the critical power supply for the house and keeping a wet cell lead acid for everything else?

Best regards and thanks again,
Mike

 

[FilterGuy]

Wow. Sounds complicated,

Hello again and thanks for the answers so far. I did run across some internet information where it had two main classes of BMS being a 2 wire and a 3 wire system with the difference being the charge and discharge path on same wire for the 2 wire system and separated on a 3 but might be specific cases only

You are talking about the common port and split port BMS. My guess is that you will not want to go with the split port BMSs. Stick with the common port BMSs

If I followed the description it sounds like all of the charge controllers dump into a single large bank of batteries.

Problem is battery bank is about 7 years old and not true deep cycle instead n200 size "solarmaster" batteries made in Philippines for a 1200aH bank.

1200AH is reasonably large, but if they are lead acid there is really only about 600Ah of usable storage. If they are drained below 50% State Of Charge they will fail pretty quickly.

So with hundreds of PV panels I can get between 30kw and 40kw at 27.2vdc

At 40KW, that is 40000W/27.2V = 1470A. That is a lot. (Is that the specification value or is it a measured value?) You will end up building several batteries so the current will be split among several different BMSs, but even that would take a lot of separate batteries.

Let's assume you use the 8S JK BMS that can handle 200A. You would need 7 of them in order to handle all that current. If you used 200Ah cells, that would give you 1400Ah of storage.... and it would be charging the cells at a 1C rate.

Technically that is all in spec, but I really do not like running BMSs at their full spec and charging the cells at a full 1C rate. Consequently, I would want to have more than 7 batteries....perhaps 10 batteries to spread the load a bit more. Also, it would be good to get a more accurate 'max' solar charge current. If you are only doing 30KW of power from the solar, it would be a 25% reduction in charge current and that might be more reasonable for 7 batteries.

Note: If you currently have 1400Ah of Lead Acid, 1400Ah of LiFePO4 is way more than you need)

A few alternatives:
* If you can possibly go to 48V, it cuts the currents in half...... but I suspect you already have too much 24V equipment to make that feasible.
* It sounds like you only need that much solar to ride out rainy days. If your controllers can take it, combine some of the panels onto fewer panels and 'over panel' them. This would clip the total production on sunny days but still give good generation on cloudy days.

Overview of Over-paneling an MPPT controller

To get to the paper, click on the orange button at the top of the screen. There is a misconception that putting more panels on an MPPT controller than the controller is able to use will damage the controller. However, in almost all cases it is...

diysolarforum.com

 

[ke6cvh]

Hi Filterguy and everyone else in group, Yes some of the charge controllers are over paneled and others are not. I have been very careful in this solar salad mix to keep same orientation and panel type in series/parallel strings for each charge controller type. I have 20 pieces of the make-sky-blue as my most recent addition and each one of those are 330w 72 cell panels with 2s2p to not overpower the cc. The make-sky-blue cannot be overpowered in charge as they just will fail with magic blue smoke as the saying used to go. However on the MSB units what I did was to buy aluminum heat sinks almost the size of the CC and with some heat sink grease and a DIY made stainless clamp they are mounted away from the wall for circulation. Some other CC's are most definitely over paneled but they can handle this unlike the MSB units. I've built this system as I can afford it. I will read your links and try to digest it a bit at a time then re-read/think/RTFM again Been studying allot....Maybe an LTO/LiTiO battery system is another avenue of research? I'm reading some can charge in as little as 6 minutes but other places are saying charge rates 10x the discharge rates and almost PFM (pure magic) amount of cycles....almost too good to be true in a solar storage application where the lower power density is not the deal breaker. Need to do my homework....more to follow.
Best regards and many thanks,
Mike

 

[ke6cvh]

FilterGuy
you mentioned likely I will not want to go with a split port BMS. What are the reasons for not going split port and what are the cons of them? It seems I see them mostly used on web pages with RV's so one can program the max input current and protect an alternator in the RV? Reducing input current seems very desirable and if a split port is the only way...... Are split ports typically not for large capacity LiFePo4 battery's? The concept of a split port leaves me wondering not the input for charging but the output that will be paralleled with all the charge controllers and DC buss.

Maybe dedicate some CC's to the input port solely for charging the battery bank and the rest for daytime use for production lines and house? What if the DC buss voltage on the port for inverters is higher than the port with some CC's solely dedicated to battery charging?

It seems the real drawback to the LTO batteries is the cost. B cells don't seem too much of an issue to me as long as they are matched and balanced with one managing their own expectations of capacity from it. My thought on a battery that can handle massive inrush current reducing charge time is that in the morning as the sun comes up the batteries will be long charged by the time full sun is available making a kind of sun regulated charge current that slowly increases with the elevation of sun becoming fully charged long before any substantial elevation in angle. We are much closer to equater than USA is with this cities latitude being Latitude: 13° 45' 15.57" N so buy "high noon" there is lots of sun available if it is not cloudy or raining making the reason for so many panels is to overcome the rain. After a little research LTO batteries are not appearing to be a good or mainstream approach but I can get the 3250 and s168 cells very reasonable. The prismatic cells are much more expensive from research.

Going full circle it seems an S168 might be good if I can use the existing shelf space the N200 size lead acid batteries use which there are 12 of these 12v units in series/parallel. I definitely should be able to build N200 equivalent (or close) size battery banks from 3260's.

Best regards and thanks,
Mike

 

[FilterGuy]

you mentioned likely I will not want to go with a split port BMS. What are the reasons for not going split port and what are the cons of them?

It has been a long time since I looked at the split port BMSs so my data may be out of date but:

* The selection of split port BMSs tends to be limited (Particularly for higher current versions)
* The charge current on split port BMSs tends to be very low. If you exceed the charge current, the BMS will shut down. Some (Most?) of the split port BMSs require a manual restart after the shutdown.
* Split port BMSs do not work well for inverter chargers that are both a load and a power source. (This may not be an issue for you)

 

[ke6cvh]

Thanks again FilterGuy !

I'm still behind on my traditional studying.....as a friend said in his description for education he went to the school of odd thoughts....I think I'm like that also.
So I saw a thread from a guy in I think Australia who is using the expensive LTO cells. He likes them because he does not use a BMS and just a balancing circuit on them. Kind of reminds me of my supercapacitor banks. The Maxwell caps require for a passive equalizer a current in the voltage divider 5 times the leakage current which is in the vicinity of 5ma. I got some 5w ceramic resistors at 150ohms and calculated using ohms law a dissipation on the voltage divider passive of just under 1 watt giving me the needed current. I wonder if I'm concerned of burning up BMS units if I'd be better off doing such a scheme? I'm in a warm climate so the cold temp pro's of LTO not needed. I can see these cells available from Yinlong surely B rated or barely B and wonder if I overdo the rating of an active balancer would that be good or not....or maybe a hybrid combination of an active and passive balancer combined.

The other consideration I'm working on now is to eliminate the house and it's evening requirements as much as possible from the portions for the production lines. How to keep the same DC buss would be the stickler on that.

Please forgive me for not yet studying completely the links but I will.

Best regards and thanks again,
Mike

 

[ke6cvh]

Hello group,
I've been rather obsessed with installing solar lights around here lately. Lucky for me the tropical climate means I don't have to worry about any batteries freezing. I've also been obsessed with evaluating different brands of brushless DC fans. I've got 3 brands so far and another conversion 12vdc motor with a small board that has a potentiometer mounted to it. The NSS brand model f87 is by far the most sophisticated. I took it apart and it has two LiFePO4 batteries in series either 32650 or 32700. It can take an external 9vdc solar panel or supply and also can take a mains input. With battery charged on speed 3 of 4 I'm only seeing almost 8 watts draw which is amazing.

My lead acid 7 year old batteries are definitely dead. I saw allot of power being wasted into the battery bank so being I have so many ultracap banks I just completely disconnected the battery bank and left the capacitor banks in place. I have four more to connect (carefully) and was really nervous doing it. It has been running just fine amazingly with ultracap/supercapacitors instead of batteries. No need to worry about excessive charge current here with these. In the evening I'm connected to the grid but have reduced substantially the power usage a huge amount.

After the commissioning of the next capacitor banks my plan is to make some individual battery banks with separate inverters. I purchased a 1kw toroidal SNAT for under 100 dollars and also purchased so far 16 pieces of 60aH S168 cells. I'm completely unable to make up my mind on the BMS and more specifically the balancer as well as what value. This first bank will power our CCTV system that is set up around the production buildings and property. I'm seeing a rise in power due to the IR LED's turning on and expect about 200 watts in the evening so this bank might barely fall short of enough capacity and possibly another 8 cells making it an 8s3p instead of 2p.

So I'll need to take some of the panels and dedicate them to charging the evening battery banks of s168 cells for each inverter and in the day run same system off of the mains from the Outback inverters. Lots more to follow but it appears the real challenge for me is getting the right BMS and active balancing that is available locally preferred to avoid shipping and tax charges. Daly are easily available but from what I read this is likely less than desirable? I'm leaning towards one of these capacitor divider active balancers that are rated at 5.5 amps but concerned that many are seeing theirs fail.

Edited to add a comment about the large amount of capacitance. I'm reading large inverters with large capacitance tend to fry BMS's. Another reason likely I will need to keep things separated......especially all the ultracap banks.

Edited again to add a comment on the meanwell power supplies. I have a Ufer ground set up here DIY. I had a 3kw 24v meanwell and at first I could put it upstream of the MBT to disconnect from the grid and adjust it to around 27vdc. If there was a storm and solar panels started to lag in voltage the power supply would kick in and draw from the grid only as needed. I also have 9kw (3 pieces of 3kw meanwell) off one of the generators. The meanwell powersupplies really work well off the generators. The one upstream the MBT on the grid used to work then stopped. Swapping out power supply does nothing. I tried a trouble ticket with meanwell and the most I could get from them is that there must be feedback from ground to the power supply...I concur it is a ground problem just not certain if it is a ground loop problem, noise on the ground, or lack off good ground with my system ground having degraded.

Best regards,
Mike