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Parallel Battery String Topologies, solution needed for JK and other BMS

Manfredi

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Apr 17, 2022
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Hi all. I am designing a multi-house solar PV system, where multiple (2 to 8) DIY LFP 48V battery banks working in parallel. This multi-string battery will have capacity in the 25-100 kWh and feed multiple 48VDC inverters, even of different type and power. Some of the inverters will have also a PV charger function, but in the plant there are also pure LF inverters (without PV charger) and dedicated PV charger/controllers. I think this is a very common and general layout useful for many offgrid plants whose power exceeds 5-10 kW (a single battery) and wanting a large storage capacity.

From what i read in the forum, some BMS makers, like JoKong, explicitly exclude their BMS working in parallel strings, this is a real pity. Some other BMS, like JBD based on contactors, seems more tolerant to use in parallel string, but I doubt that they are capable to control all the issues (eddy currents, inrush currents from strings, lower max capacity, cascading shutdowns, ...) that could happens in a large multi-strings battery bank, potentially leading to damage of the battery and/or BMS, if not greater damages.

I found this detailed doc from ORION BMS that describes all the problems that must be solved to have a multi-string battery working reliably. I think is a good starting reference to address the design issues of parallel batteries.


I particularly like the solution they describe at page 15 named "Parallel strings with 1 charger per string and single discharge DC bus". Unfortunately this solution does not include PV inverter/chargers, and does not address explicitly the problem of having a different state of charge or capacity in different strings. I think we can work on this simple solution to evolve into a more general and universally applicable one. But this will be the subject of a separate post !

Manfredi
 

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It's important to clarify the intent. In some cases, folks have put multiple BMS on ONE battery to increase the current. This "parallel" operation on the same battery is often prohibited. Separate batteries with their own BMS in parallel at the main terminals is rarely prohibited.

Ah... the infamous "eddy current" reference. Ugh.

IMHO, that's a solution looking to invent a problem.
 
The intent is simple: with 20 kW of inverter, you need to supply up to 400A continuous and 800A surge @48vdc to reach the rated power. Practical LFP batteries have 280Ah capacity or less, and for longevity reasons should not be dischardeg beyond 0.5C that is 140A each. This means that to run 20kW of inverters from the same battery bank, you need at least 3 batteries running in parallel.

With smaller inverters, sometimes you want to have many hours of backup power, and this also require a reliable parallel BMS that will maintain the single pieces in good shape.
 
BTW, i want to use JK BMS for the active balancing function, and not happy it cannot (?) be used in parallel configuration.
 
I have set up a few 600ah@48V systems using 3p16s format with 200ah cells.

This coupled to 2 (could be more if required) 8kw SMA Sunny Island inverters, and monitored/controlled by REC BMS.

The system is simple and works with no issues with a combination of AC and DC coupled chargers.

If i was going to go any bigger than this i would separate my loads and build two independent systems.

edit: curious as to why you want active balancing, i am yet to see a well set up system that requires it.
 
BTW, i want to use JK BMS for the active balancing function, and not happy it cannot (?) be used in parallel configuration.
They have since changed their stance after @upnorthandpersonal provided them with solution of using T fuses on each parallel string. They even have diagram upnorthandpersonal drew on their store page showing this for the JK BMS. He has also tested this solution. @Steve_S is converting/converted all of his parallel banks over to using JK BMS using MRBF fuses as well.
 
JK's CAN be used in Parallel there has just been a lot of talk on this and some of it is related to "translation issues". I and many others ARE using them in parallel without issues as long as it is properly done. Using any FET Based BMS in Series comes with extended issues & risk potential issues and is not suggested at all. To be clear I am swapping out almost $4K worth of hardware and replacing it with JK's and this is not being done Cavalierly.

Review this thread starting here: https://diysolarforum.com/threads/jk-4s-200a-bms.38623/page-5

Also a Hankzor has updated their website info on this subject: I expect that within days the ENJ & Other JK vendors will update their pages as that "mistake" cost them sales and for no good reason.

 
Hi all. I am designing a multi-house solar PV system, where multiple (2 to 8) DIY LFP 48V battery banks working in parallel. This multi-string battery will have capacity in the 25-100 kWh and feed multiple 48VDC inverters, even of different type and power. Some of the inverters will have also a PV charger function, but in the plant there are also pure LF inverters (without PV charger) and dedicated PV charger/controllers. I think this is a very common and general layout useful for many offgrid plants whose power exceeds 5-10 kW (a single battery) and wanting a large storage capacity.

From what i read in the forum, some BMS makers, like JoKong, explicitly exclude their BMS working in parallel strings, this is a real pity. Some other BMS, like JBD based on contactors, seems more tolerant to use in parallel string, but I doubt that they are capable to control all the issues (eddy currents, inrush currents from strings, lower max capacity, cascading shutdowns, ...) that could happens in a large multi-strings battery bank, potentially leading to damage of the battery and/or BMS, if not greater damages.

I found this detailed doc from ORION BMS that describes all the problems that must be solved to have a multi-string battery working reliably. I think is a good starting reference to address the design issues of parallel batteries.


I particularly like the solution they describe at page 15 named "Parallel strings with 1 charger per string and single discharge DC bus". Unfortunately this solution does not include PV inverter/chargers, and does not address explicitly the problem of having a different state of charge or capacity in different strings. I think we can work on this simple solution to evolve into a more general and universally applicable one. But this will be the subject of a separate post !

Manfredi
If you want a large system, get a Batrium CORE with K9 slaves. One slave for each bank. Shunt trip breaker.
 
If you want a large system, get a Batrium CORE with K9 slaves. One slave for each bank. Shunt trip breaker.
There was a discussion about using this in another thread and the issue with the Batrium K9's was that if one of the K9's detects an issue with the string it is monitoring the Core shuts down ALL strings, not just the one string the K9 reported the issue. Not sure if this is user misunderstanding or if it is designed like that on purpose.
 
JK's CAN be used in Parallel there has just been a lot of talk on this and some of it is related to "translation issues". I and many others ARE using them in parallel without issues as long as it is properly done. Using any FET Based BMS in Series comes with extended issues & risk potential issues and is not suggested at all. To be clear I am swapping out almost $4K worth of hardware and replacing it with JK's and this is not being done Cavalierly.

Review this thread starting here: https://diysolarforum.com/threads/jk-4s-200a-bms.38623/page-5

Also a Hankzor has updated their website info on this subject: I expect that within days the ENJ & Other JK vendors will update their pages as that "mistake" cost them sales and for no good reason.

I see in the other thread that the solution is using fast acting T-class fuses in all parallel battery branches. Are these "T-class" auto-resettable fuses (current limiters), or need to be changed if they fire for some reason ?

Thank you all for the found solution allowing to use the JK in parallel configurations !
 
There was a discussion about using this in another thread and the issue with the Batrium K9's was that if one of the K9's detects an issue with the string it is monitoring the Core shuts down ALL strings, not just the one string the K9 reported the issue. Not sure if this is user misunderstanding or if it is designed like that on purpose.
It can be done several ways. One large shunt trip breaker and yes, it will shut down the whole system. Or with 2 CORE's, it can be split. Flexibility is key, several configurations if desired.
 
BTW, i want to use JK BMS for the active balancing function, and not happy it cannot (?) be used in parallel configuration.
Definitely possible if you using fast-acting Class-T fuse and if properly sized you will not have issue with Parallel battery banks and inrush scenarios. Just use fast-acting Class-T fuses between each parallel battery bank connecting to the bust terminals in parallel for all banks. Fuse will take hit before bms is going to be fried. Most of BMS manufacturers are advertising against to protect reputation and stupidity with some people who don't take safety seriously and spending money to design it well and have safety hardware to prevent BMS ever dealing with in rush current.
Please don't take my post as offense, I'm only trying to share my knowledge with others.
 
Also for quite advanced users there possibilities to run different Ah in parallel packs . Higher Ah pack will naturally deliver more current in parallel when discharging. Position in the parallel pack storage can be arranged to mitigate this in your favor but this will require more than just fast-acting Class-T fuses. It will require one way diode for discharging and charging diagrams should be changed because you will not be able to charge over one way heavy duty diode. There is also different types to achieve packs in parallel with different Ah ratings to work without problems but it will be quite expensive vs running separate parallel banks with close Ah ratings. Every setup is different but if you running multiple inverters separating them with individual battery banks in parallel is good practice that will save you from having entire system down because of some pack was disconnected from parallel and current is getting critical because you getting one out of service. Also people should benchtop test cells to find voltage threshold ( this is usually when charging to 3.6 V cell with current falling down to 0.1 A to get fully saturated and monitoring Voltage after charging is done for 24 hours will settle around 3.4 V or less ( it could be different between different manufacturers) and this found goldy lock voltage should be your max cell voltage you should run and also will need to figure out absorption time cut by time or current at the end of absorption. If you follow my advice you will be at around 99% SOC with 3.4 V or less with absorption. Usually professional builders don't like to share this type of sensitive information to the ordinary Joe. But I'm not afraid to see someone who wants to learn all little secrets. I will probably get bombarded in private chat by some fellas. So good luck and stay safe.
 
BTW, i want to use JK BMS for the active balancing function, and not happy it cannot (?) be used in parallel configuration.

Technically speaking, there is no real reason you can't parallel them.
The active balancer is good if you have dodgy cells, but of little real world use otherwise (and you do pay extra for it). You should only use it at the top end of charge anyway, and it can be set to operate that way in the JK. If your cells are OK, and you regularly get to full charge (3 or 4 times a week), the smaller balance current of almost all BMS will work fine.
 
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