Midnite MNSICOMM W/ SMA 6048 and Lithium BMS

Started by CayoPV, May 21, 2020, 01:12:00 PM

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Hey all,

Got a question for anyone who knows. I have an installation coming up where the client would like to re-use thier Midnite Classics but they are replacing thier outdated Magnum with a pair of 6048's. We are also going to be installing a lithium (LiFePo4) battery bank with a BMS that controls the SMA's charging parameters in realtime via CANBUS communication per SMA requirements. In a Sunny Boy+Sunny Island configuration this is great however in a 6048 + CL150, will the commands sent to the 6048 be relayed to the CL150?

This becomes a tricky situation I think because the SI is designed to back down the current passing through it from the SB's as the BMS instructs it too however this by design doesn't affect current to the loads.

However in a purely DC-coupled system where the CL is handling charging current + loads, this becomes an interesting situation. If the BMS instructs the charging current to 0.02C to top off the charging cycle, does the CL compensate for this? Does it allow load current + charging current? How would it know load vs charging current? Again the BMS is monitoring the cell voltage and sending commands over the CANBUS every 100MS.

Just curious if anyone has any thoughts on this? Perhaps the SI compensates for the loads when forwarding commands to the CL? And that is IF the BMS commands and translated and forwarded at all.



You can program the Classics to only charge a certain current but let it provide all other available current to the loads . Look in the knowledge base on Midnite site for more info and there has also been discussions on the forums here too.

system 1
Classic 150 , 5s3p  Kyocera 135watt , 12s Soneil 2v 540amp lead crystal for 24v pack , Outback 3524 inverter
system 2
 5s 135w Kyocero , 3s3p 270w Kyocera  to Classic 150 ,   8s Kyocera 225w to Hawkes Bay Jakiper 48v 15kwh LiFePO4 , Outback VFX 3648 inverter
system 3
KID / Brat portable


Thanks Larry, appreciate the feedback. In this specific case, I need to ensure that the CL is backing down the charging current based on the realtime instructions from the BMS/SI so that no single cell is pushed past 3.65V during the end of the charge cycle and the balancing phase. Since the CL is not capable of reading each cell voltage individually, I need it to back down based on external instructions.


Quote from: jakobw on May 22, 2020, 10:40:52 PM
Thanks Larry, appreciate the feedback. In this specific case, I need to ensure that the CL is backing down the charging current based on the realtime instructions from the BMS/SI so that no single cell is pushed past 3.65V during the end of the charge cycle and the balancing phase. Since the CL is not capable of reading each cell voltage individually, I need it to back down based on external instructions.

Look in the Lithium battery section of the forums - Steve S had found a BMS that had a lot of features .

system 1
Classic 150 , 5s3p  Kyocera 135watt , 12s Soneil 2v 540amp lead crystal for 24v pack , Outback 3524 inverter
system 2
 5s 135w Kyocero , 3s3p 270w Kyocera  to Classic 150 ,   8s Kyocera 225w to Hawkes Bay Jakiper 48v 15kwh LiFePO4 , Outback VFX 3648 inverter
system 3
KID / Brat portable


The Chargery BMS which is what I use, utilizes an RS232 port for communications and so you'd need an intermediary (Raspi with Node-Red ? ) to take the signals and action them and send out CanBus signals to devices.  My particular BMS' uses Relays (or combinations of them) and can be configured for Common-Port or Separate-Port modes.

There are many types of BMS' with many various capabilities and functions and prices points as well.  There are FET Based BMS' (most common and lowest prices) but they typically cap out at 250A Discharge / 100A Charge "ratings" which are usually the edge case of capacity, we usually suggest derating by 20% to leave a safe margin. 

Here comes the big rub. 
48VDC LFP (LiFePo4) pack consists of 16 Cells in Series to make the 48V pack.  Therefore a 16S BMS is used so that you can manage each cell.  If only using a single pack then it's one thing to have the BMS interact with the SCC (Solar Charger Controller) and the Inverter/Charger components.   The tricky stuff starts when you pave more than one pack in a battery bank, where a pack is an independent battery pack with it's own BMS.  Now you have to have a coordinator / intermediary to intercept all the BMS's signals and make decisions & to take actions accordingly.  For example, if you have 2 or 3 packs in parallel and one shuts down to to Low Volt Disconnect but the other Pack(s) are still pushing juice OR when one pack reaches FULL before the others while being charged.  You cannot have one BMS decide for all packs as each have their own "state" of charge.

As stated previously, there are many types of BMS' with varied functions & capabilities. Some cheap FET based ones can cost as little as $75 USD but BMS' can go into the thousands of dollars too.  Some are very limited as far as discharge & charge handling while others can handle 1,000A constant discharge (provided battery type supports it). 

Known Good BMS' to consider for reviewing & comparing of options:
I use Chargery BMS http://chargery.com/balancer.asp
TinyBMS :  https://www.energusps.com/shop/category/battery-management-4
ElectroDacus BMS :  http://www.electrodacus.com/
Orion BMS $ :  http://www.orionbms.com/?gclid=EAIaIQobChMIscrf4uLJ6QIVgYbACh18_wGQEAAYASAAEgKH5vD_BwE
Batrium BMS $$$ :   https://www.batrium.com/

Good Low Cost FET Based: (much more limited capabilities)
* Known good from our membership using them and being mostly happy with them for their particular application.  I do NOT use these and so I cannot recommend any in particular
DALY BMS (caution when ordering and configuring during pre-order):    https://daly.aliexpress.com/store/4165007?spm=a2g0o.store_home.pcShopHead_11835965.0
Smart ANT BMS :  https://www.aliexpress.com/item/32826820690.html
LLT Power BMS' :  https://www.lithiumbatterypcb.com/product-instructionev-battery-pcb-boardev-battery-pcb-board/ev-battery-pcb-board/smart-bms-of-power-battery/

An Excellent General Resource on BMS's and all things related to them.  Very worthy of serious perusal for a general overview understanding of BMS' and the issues / quirks / foibles and such.

A note regarding LFP cells / packs.  LFP is the most stable type of cell to use for ESS/HSS systems and also the safest available at this time.  Typically the cells can handle 1C Charge & Discharge rates, some up to to 3C.  If you have a pack of 280AH cells that means you can pull 280A but also fast charge at the same rate (makes heat) ... now if you have 100AH cells then your limited to the 100A ...  (basically and simplistically)  ALWAYS REFER to Manufacturer Spec Sheet !   Typically, we use 80-90% capacity to maximize lifecycles up to 5000 cycles.  Also suggested to charge at no more than 0.5C rate so as to not create excess temps or potential issues.  Also VERY Important that the LFP's are not charged below 32F/0C, LFP does not like freezing and slow charge rates only below 5 Celsius, so some thermal management watching is required in certain regions where freezing is possible.

Hope that Helps,
Good Luck


Thanks Steve & Larry. Again appreciate the input.

The concept of Raspi with Node-Red is very interesting. I'm currently attempting to solve the SMA SI+CL issue but in the future was considering using an industrial Aurdino as an intermediate to translate in realtime between a BMS and the Magnum inverters/Midnite controllers just as you mentioned.

My concern here is real-time current control at the end of the charge cycle/during the balancing phase. I would prefer not to have any of the cells exceeding 3.5V-3.6V if I can help it, even if it is just during the end of the charging phase while the other cells finish charging.

If I end out installing multiple banks on a single system, I'll likely use master+slave BMS units communicating via CAN/RS485 so non-issue there. Good point you made though.

That said, for my current installations (I'm an installer), I'm simply operating with Xp15s with X being the number of cells in parallel I need for the installation. If a cell becomes weak, I'll still be able to catch it as the entire set of parallel cells will show the performance drop.

Similar to this but different cells and balancers.

Thanks for sharing that list of BMS units, I have looked at or are familiar with most. I couple I'm not though so I will look at them. Currently my top picks are:

Elithumate Vinci LV
Emus (need to speak with them yet on few things)

REC is my current go-to. They are very knowledgeable and very helpful. They are lacking in some features but they solve my immediate issue and that is CANBUS comms with a SI cluster. The other issues I can deal with. Something I'm really looking forward to is a BMS with remote management. I currently design my off-grid systems with a separate UPS backup for the internet router and comm box units such as the Venus GX on Victron systems. This way if they call me about something being down, I can at least see if I can solve it remotely before having to go on site. Unfortunately no SI compatible BMS that I currently know of allows for LAN/WiFi access and management. So I can see the rest of the system (inverters/chargers etc) but just not the BMS in case it is a cell issue for example.

For my BMS + comm interface adapter application, I'm considering Nuvation or Orion although I may consider Chargery as it's much cheaper. I would need to look further into thier charging algorithm before considering them. Emus looks like a good alternative but I also need to review a few things with them first.

For your reference, I had considered Batrium but after hearing thier support was lacking and personally not having great luck with thier support, I went with REC. If it wasn't for these 2 things, I'm very interested in their units. They are quite advanced on paper.

I'm currently speaking with a few active cell balancer suppliers. The QNBBM balancers are great but also expensive. Looking to see if there is a cheaper 2A-3A alternative for smaller banks I'm working on. I find the cell balancers in most of the BMS units to be quite lacking. If you have any suggestions on active balancers I would be happy to hear them

I'm probably going to be operating in a SOC window of 10%-90%. Just as you mentioned, temp rise vs resistance are cycle life killers (both connected) along with absorbing without BMS charge current control. Floating is not great either but it's an interesting issue in an off-grid system. It's tempting to use an LTO or supercapacitor bank as a buffer to let the LiFoPo4 bank rest after charging and only reconnect at the end of the day as the LTO nears 30% SOC. Not sure if I'll do this but it's an interesting concept. Quite a few issues to engineer out before that concept would be usable though.

Again thanks for the feedback!!


A note on Chargery BMS'

They are certainly no Orion or REC but they are not a bad BMS.  The "Current Flavour" is just about to go through a major firmware update and some new hardware bits are coming for them as well.  A few  of us have been working with Chargery Engineering and they are one of the few companies out there who not only listen to their clients but hear what they are saying too.  Due to NDA I cannot disclose too much but it is a leap forward in several ways.  I believe the new revisions will be available in June.  I can say that they are working on a Smart Solid State relay / contactor control system to work in combination with the BMS' and they should handle Big Amps (600+).

I appreciate your direction and it is most logical and reasonable but there are a couple of "gotcha's" lurking there too.   I'm sure you are aware of the pro's & con's of paralleling cells within a Pack.  I travelled that path and realized it wasn't for me or my residential application, there I only implement Series Packs, as I prefer "fine management" of each cell.

Balancing is a tricky beast indeed  but there is one gotcha there too....  Take an 8S pack for example, your down to 60% SOC and the sun comes up and the SCC starts doing it's charging, everything is peachy & great... but not...  The first cell (+) will take all the juice 1st and the rest passes into the pack as it should BUT #1 cell will hit 3.65V and stay there while it's neighbours catch up to that cell.  Balancing while charging can help with that while balancing during discharge wouldn't.  This is a universal issue as you already know.   This get's to be an issue as more cells are paralleled because your seeing the collective voltage of the cells in the set, which can hide a failing / weak cell because it's neighbours are supporting it, unfortunately, what if one cell has lower resistance and charges faster than the others within the set, next you can have individual cells within a set exceed 3.65V (which is nasty stuff).  I know people argue & debate on this but my own experiences and pack builds, + rebuilds and retesting has shown it to be the case, as all cells "vary / wander" a bit on their own, as it were.  NOTE that during the charge cycle, cells will reach 3.65V during charge but once charging is done the cells will settle over an hour or so (pending on temps & cell capacity of course) and will usually drop down in voltage ad balance out.  A good Pack with Good Cells that are well balanced to start with should not need more than Passive Balancing, especially in an active system (meaning not a weekend only use trailer battery used only for a few outings). Remember that passive balancing can work during charge / discharge or when sitting idle in "storage mode".

As far as the BMS is concerned it is the last stop for control over the battery pack and subsequently the entire bank.  The cut-offs as handled by the BMS is the last safety stop IMO.  The SCC & Inverters / Chargers should all manage their own cut-off points independent of the BMS.  For example, having the Inverter Low Volt Cut-off kick off @ 2.6 VDC per cell (20.8V for 24V pack) but having the BMS set to cut-off @ 2.50V per cell.  Also there is the problem that IF one cell within the pack goes below the LVD threshold the BMS will cut-off the pack, (it happens). 

There is no right / wrong with different use cases and how we apply solutions.
Depending on the equipment used, Some form of intermediary is needed and the Raspberry Pi with Node-Red and assorted tools can certainly handle doing it.  I am sure that the Arduino and similar solutions can do it as well with whatever apps & programming systems they have available to them.  If your running with ONE Pack and One BMS life is much simpler but once you have more than one pack in a bank then things start to get "interesting" with the whole BMS game.

TBH:  IMO, Midnite could certainly do a bit more when it comes to having the SCC's interact with Lithium Based Battery packs and banks.  Even simple things like having an LFP (LiFePo4) battery config / profile which is CC/CV only...  Right now it's a PITA to furtle around with the settings and adjustments.  They could certainly use a firmware update but I asked about that a couple of years ago and got zilch answer.  Lead is Dead time to stay current.