Battery Management System BMS: Pros and Cons for using Contactors or MOSFETs

[tnn85]

I am currently building up a DIY Energy Storage System for my home (surprise! surprise!)
For the inverter/charger and for the battery cells I have already chosen the parts I want to use:
1) Inverter / Charger: 1 x Victron Energy MultiPlus-II 48/5000/70-50
2) Battery Cells: 16 x EVE Enegy LF280K 280Ah LiFePO4 3.2V  in a 16S1P configuration

I am still not sure which Battery Managment System to choose.
Reseaching the web I have stumbled on these brands /types:

• 1. REC Q BATTERY MANAGEMENT SYSTEM 16S (https://www.rec-bms.com/rec-q-battery-management-system-16s/):
     - very expensive
     - contactor style BMS (external contactor necessary, pre charge unit necessary
     - Passive Balancer 0.9 A
     - RS485/CAN
     - can directly talk to Victron GX
     - European manufacturer ?
  
• 2. Batrium BMS WatchMon-CORE with CellMate-K9 (https://www.batrium.com/collections/kits/products/watchmon-core-k9)
     - even more expensive due to tax and shipping
     - MOSFET style BMS
     - Passive Balancer 0.5A ?
     - RS485/CAN
     - can directly talk to Victron GX
     - Australian manufacturer
  
• 3. Daly BMS (https://dalyelec.en.made-in-china.com/)
     - not very expensive
     - MOSFET style BMS
     - Passive Balancer with only 0.03A
     - RS485
     - cannot directly talk to Victron GX
     - Chinese manufacturer
  
• 4. Jiabaida or JBD BMS (JIABAIDA Smart BMS-AP20S006)  (https://www.jkbms.com/products/active-balancer-bms/)
     - not very expensive
     - Contactor style BMS
     - Passive Balancer with 0.260A
     - RS485 and CAN (optional). For RS485 adapter necessary
     - cannot directly talk to Victron GX
     - Chinese manufacturer
  
• 5. JK or Jikong BMS (JK-B2A24S15P or JK-B2A24S20P)
     - not very expensive
     - Mosfet style BMS
     - Active Balancer with 2.0A
     - RS485 and CAN (optional). For RS485 adapter necessary
     - cannot directly talk to Victron GX
     - Chinese manufacturer
  

I am still not sure which Balancing current i need. I assume if have only a small voltage difference between the cells on arrival and i connect them all in parallel, they will equalize and i won't have the need to balance them with higher currents anymore.

For the switching element (MOSFET or Contactor). What are the Pros and Cons?
It seems that contactors are used in the more expensive BMS setups but there are quirks with using relays as well.
For example: The BMS which use contactors only have one contactor for Charge and Discharge. You cannot separately aktivate charging and discharging with seems to be possible with MOSFT-style BMS.

Does anyone here have more experience with BMSs or Contactors and MOSFETS as swithcing element than me.

Thank you in advance

[Faringdon]

You shouldnt try to equalise cells by putting in parallel.
Have an individual small charger on each one to gradually bring them to the same voltage.
Any balancer is good as it kind of  reduces the speed with which the cells may become well unbalanced.....but  to really handle unbalanced cells in the same stack, then the balancer needs a current capability of [C/10] * 1/time  ("* 1/time" so it comes out as amps)...it probably isnt worth implementing such a big balancer though...just monitor the cell voltages and fish out the bad one.......the cell voltages will vary significantly in light load or no load....its the cell voltage during high charge/discharge currents that counts.

Best to just monitor the cell voltages at all times, especially when they are  in high discharge or charge rate, and check that the voltages are pretty equal....if one goes unbalanced, shut the pack down and fish the bad one out.
Are the non Chinese made ones just using Chinese hardware and re-branding it? Ive worked in plenty of companies where the hardware was really from China, even though the co swore that it was not.
Maybe a small token amount of certain products were "British (wherever) made", to try and fool people into believing that the co had full Design/manuf capability when it did not.

If your charger "Pumps" up its output cap to the same voltage as the battery stack before getting switched to the stack, then there will be limited inrush, and fet or relay will be fine.

[Siwastaja]

You shouldnt try to equalise cells by putting in parallel.

Of course you should. That's the easiest and most reliable way to force them to the same voltage, and being able to use a single charger reduces cost, as you don't need to buy many chargers that would end up unused.

It's of course worth noting that some chemistries (many LFP cells) have quite flat curves in the middle, so trying to equalize the cells at 50% will be iffy. But you would balance the cells "at top", normally, anyway, so basically just connect all the cells in parallel and use a charger to fully charge them, all at once.

Just note that any time you connect li-ion cells in parallel - MEASURE VOLTAGES FIRST. They need to match within maybe 0.05V - 0.1V. With flat curve chemistries, I would prefer 0.05V difference absolute max. Individual charging may be needed if the cells shipped at different SoC (should not happen, would be a red flag for poor quality) or were individually played with. Failure to follow this advice will cause excessive currents during the first moments after being connected in parallel, likely enough to destroy the cells, specifically the ones with lower voltages which receive massive charging currents.

[Faringdon]

....yes i agree with what you say, but as you say, the cells may be well different in charge fullness at first...if so, then a very small charger , as you say, on each one can bring them all up to fullness......only needs to be small charger because presumably you can wait  a few days for them to get to the same level , in the first  instance.

If you are re-purposing ex used EV batteries,...stand by....these have been shown to go unbalanced rather more quickly than one woudl like. I once worked in a co that touted itself as a green - goddess because they used used ex EV batts...but they soon stopped doing it...too many unbalanced cells.

[Siwastaja]

Yes, but if you buy 16 brand new cells, they will be at the same voltage - but of course, when it comes to safety, never assume, measure.

If you see large enough voltage differences preventing parallel connection, I would take that as a big red flag that indicates either poor manufacturing control (end of conditioning criteria), or even worse, increased self-discharge which could indicate internal contamination or damage during manufacture.

I have assembled thousands of Samsung 18650 cells into packs and they always come within 10mV of each other. Still, I always measure every single cell.

But at some point, they changed the shipping voltage to something much lower (went from 50% to 25% SoC  IIRC), so if you mix different batches, you can get different voltages without it being any sign of a problem.

[tnn85]

Well thanks for your replies guys.

From what I read already elsewhere and also from what you say is I have learned that I should measure all cells on arrivals. Check voltage differences. If voltage difference is in the 50mV range I can connect them all in parallel (LF280K internal resistance according spec: 0.25mOhm -> 50mV/2*0.25mOhm ->  100A ?).
Then charge them up to 3.65V. Connect BMS and let them get top balanced?

@Faringdon: You mean if my cells in theory differ really bad i would need 280Ah/10 * 1/1h -> 28A balancing current? Thats pretty much.

I know I won't need a high balancing current since the cells are sold as brand new directly from the manufacturer EVE (not some Alibaba reseller of old / used cells). 

However, regarding the MOSFET / relay question: Don't you see any problems using any of those technologies? I know there are problems with contacts wear out for relays with high direct currents and so on. MOSFETS have other flaws.

If you are re-purposing ex used EV batteries,...stand by....these have been shown to go unbalanced rather more quickly than one woudl like. I once worked in a co that touted itself as a green - goddess because they used used ex EV batts...but they soon stopped doing it...too many unbalanced cells.

But you are talking about Tesla 18650 or 21700 cells? They are easier to handle from the form factor point of view, but connecting them (welding/solder) them is a hassle for the amount of capacity they have. Bigger cells (LFP) or pouch cells used from other manufacturer could be easier to handle? 
 

[Faringdon]

@Faringdon: You mean if my cells in theory differ really bad i would need 280Ah/10 * 1/1h -> 28A balancing current? Thats pretty much.

Thats what you tend to need to balance those rogue cells, and as you correctly calculate, its not worth it, just fish the bad cell out and replace it.
A balancing circuit to bring a rogue cell "back into line"  isnt generally worth it.......just have a bit of balancing to keep the cells pretty well similar in voltage, so that  the formation of a rogue cell is delayed as long as possible.
As you know, a bit of balancing is also good, because,  otherwise , once the baddest cell has gotten full, then the entire stack  stops getting charged up....so you loose capacity.

Your 100A calculation is interesting...and correct.........would  it be too much trouble to simply charge all the cells till full in the first place.....then parallel them.....then you definetely avoid those high currents.

I dont know what currents youre switching, but a properly rated contactor might be less hassle, whereas mosfets might need to be paralleled and  heatsunk. Plus a hi side drive circuit made for them...plus the fets might have to be back to back, so that their internal diode doesnt conduct when its not wanted to. But you are not doing high frequency switching, so either contactor or fets seems fine.

[tnn85]

Your 100A calculation is interesting...and correct.........would  it be too much trouble to simply charge all the cells till full in the first place.....then parallel them.....then you definetely avoid those high currents.

No, it would not. I was just thinking: Connecting "empty or half empty" cells is less dangerous than connecting full cells. As far as I understood the measured voltage on the battery poles especially for LFP cells stay almost flat up until you reach the end/full SOC when charging.  If so, then there can be a huge difference in SOC for different cells with almost the same voltage. What happens if you connect a battery with 30% SOC and a battery with 70% SOC with the same voltage measured across their poles? In theory I would expect 0A current flow. That is correct right?.

[NiHaoMike]

If you are re-purposing ex used EV batteries,...stand by....these have been shown to go unbalanced rather more quickly than one woudl like. I once worked in a co that touted itself as a green - goddess because they used used ex EV batts...but they soon stopped doing it...too many unbalanced cells.

It helps to get more modules than you are planning to use for the main pack and swap out the modules that do not match well. It would also help to make entire strings using only the modules from one pack rather than try to mix and match them. And an active balancer (as part of a BMS or separate) would help a lot to resolve any remaining mismatch.