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Two 12V 280AH LiFePo4 batteries each with their own BMS in parallel

Phero

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Apr 13, 2021
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Hi!

I'm a noob who's been following Will for some time now before taking the plunge and making my own setup. (Thanks, Will, for your excellent videos!)

I followed one of the beginner setup tutorials as you can see from the attached picture. It works great with one battery but I would like to add another to the setup.
So now, basically I have two 12V LiFePo4 280AH batteries each with their own 4S 12V Overkill Solar BMS. I thought I'd ask for your opinion before proceeding and I do something potentially stupid.

I know I can put two batteries in parallel and basically make a 4S out of 8 batteries if the are balanced well to begin with. But I'd prefer to have two separate boxes with each 4 batteries if possible.

I have read conflicting statements on the net with some saying it's not a problem to others saying that the BMSs could go into a state of repeatedly switching on and off.

Is this a good idea or will it destroy BMS and/or batteries?

Thank you.
 

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I have 2, 24v with that same type BMS, no problem. I can monitor and control both via Bluetooth.

Someone needs to provide details if they say something bad will happen. The only way the BMS will switch on or off is within the parameters set. Ideally a BMS will never have to take any switching action. If it does, your charge parameters need to be adjusted on your charger or your cutout setting on your inverter needs adjusted, or maybe your load is too great. The BMS should only switch as a safety after design and planning have missed the mark.
 
I'm doing the exact same thing. I have my two 12v 280Ah 4S battery packs, each with their own Overkill 4S 120A BMS, connected to a Lynx distributor (essentially 1000A bus bars) for battery input. Each battery is separately fused and has its own cutoff switch. At max draw, my inverter will pull about 200A at 12v. So each battery will see a max of 100A through the BMS. I plan to add two more 280Ah battery packs later this year for a total of four.

Just make sure your BMS's all configured the same. Set your parameters like you want them on one, save the config file, and then load the config into the other BMS. See the Overkill manual for more info on that.

Good luck!
 
with common ports seems ok, having seperate charging ports, that I could see would be "bad" hehe
parallel BMS units have no idea there is another bms; when any over/undervoltage situation occurs they would just do what they are suppose to do to protect their cells of responsibility. When one BMS disconnects (charging or the discharging fets); the other BMS would happily continue along, do make sure the "voltages" are the same before you parallel them just to avoid large currents between the two when connected. If there is a large voltage difference, then the higher pack will "charge" the lower pack until their voltages equalize.
This situation could happen depending on how each pack turns it charging fets back on after turning them off, there is some hysteresis.

if the two packs have different capacities, lets say A is 100Ah, and B is 80Ah...
you pull 80amps (so EACH pack provides approx half the load so 40amps, resistances and other things will always make the two packs share the load differently), after 2hr B will hit 2.5V/cell and shutdown; however, A will now provide 80amps until it hits its "dead" limit.
Seems like no issue there...

when charging each pack will take current until it hits its charge voltage limit and then turn off its charging fets.
Its charging fets will stay off until it hits it "charge fet on" state.
If there is a voltage difference when the charging fets have been turned off then there will be a current rush depending onthe difference.

I think you could completely(well mostly) avoid the charging issue by never allowing the "charging fets" to shutoff.
Basically you want to avoid any cell hitting the 3.65 limit; which you should probably be doing any way.
So set your max charging voltage for a cell voltage of 3.3-3.4(4S == 13.2-13.6V charging voltage)
 
You can have multiple packs in Parallel without issue. When setting packs in Series that were things start to go bad fast, many BMS' will not cope with that.
  1. Make sure that all the BATT wires from the battery terminals to the common DC busbars are the same length.
  2. make sure each battery pack is fused properly for the "total" Max amp potential your system will draw. (each battery should be able to operate "standalone", meaning being able to handle the full Charge/Discharge potential of your system.
  3. Both Packs should be equally charged when put online together. +/- 0.500V difference is fine.
  4. Once both packs are set in place then you can charge the pair so they balance up & even out.
  5. If ALL battery packs are of identical capacity such as 280AH, their matching up within a parallel bank is pretty easy and they will stay very close together through their operation unless there is a fault. If you install different capacity packs, ie: 100AH & 200AH into the same parallel bank things get tricky once the packs are outside of their common voltage ranges as the lower capacity cells will discharge & charge slightly quicker, this can result in issues if the difference is too much and not accounted for.
Points of Note:
  • During charge/discharge you will notice the two packs will take a similar Amperage but it will fluctuate a little so you may see one pack getting 12A while the other takes 10A and then flip back & forth a bit during charge / discharge. This is NORMAL
  • The power curve for LFP is 3.00-3.450vpc, charging cells above 3.525 will gain you little to nothing but wear on the cells.
  • Unmatched Bulk Commodity ESS cells will deviate and can go as much as 1mv per AH of storage capacity. So 280mv on a 280AH cell. Passive Balancing cannot correct such a large deviation. Active Balancing can take the edge off of this issue. CAUTION, There are Good & Bad Balancing systems out there. This is a consideration item IF required.
!!! WARNING !!!
Inverters require a Pre-Charge circuit to precharge the Capacitors when it starts up which is a "huge" surge which can cause damage to your gear. The JBD BMS' do not have such built-in, therefore it's advisable to make a precharge circuit. Will has talked on this point in his video's and it's all over this forum. A few people have made simple switched Pre-Charge circuits with a momentary switch to use it prior to turning on the Inverter. Link to a good thread on it below.

My settings for 24V so divide by 1/2 for 12V system.
Using a Midnite Classic-200 SCC
Absorb: 28.2 for 15 minutes (3.525vpc) (boost for some brands ?)
Equalize: OFF
Float 27.9V (3.4875vpc)
MIn Volts: 22.0 Max Volts: 28.7
Rebulk Voltage: 27.7
End Amps: 14A

This get's the bank charged to full with high amps (Constant Current) and then float (Constant Voltage) tops off so the cells are on average between 3.475-3.500. I am running 7/24/365 so float is used up by the Inverter + provides whatever the packs will take to top off.

NOTE: I also use a QNBBM-8S Active Balancer on each of my battery packs which really makes a positive difference in keeping all the cells up & level. Here is a link to their QNBBM-4S models below. There are also other brands BUT Caution is warranted ! Heltec is another Known Good Active Balancer maker with various options (some are quite pricey) and we have many people using an assortment of them as well.

QNBBM-4S: https://deligreen.en.made-in-china....on-Rechargeable-Battery-Active-Balancing.html

Hope it helps, Good Luck
PS: There are a few links in my Signature which will likely be very helpful to you.
Steve
 
Someone needs to provide details if they say something bad will happen.
I just read this on Reddit
Ouch!
Don't.
https://us.reddit.com/r/AskElectronics/wiki/batteries#wiki_parallel
"Batteries (each with its own BMS) just the 2 power terminals: very bad: as soon as the protector BMS of one of them turns on, excessive current will flow from the higher voltage battery to the lower voltage battery, resulting in damage"
 
Inverters require a Pre-Charge circuit to precharge the Capacitors when it starts up which is a "huge" surge which can cause damage to your gear.

Thank you for your long and detailed answer!
I'm currently using my Laboratory Power supply to charge each battery so they're within equal charge when connected.

Regarding the pre-charger. Thank you for warning me. I actually thought this was to used only once but it makes sense that is has to be used every time the batteries have been disconnected.

I have a resistor and a button like in the attached image. Would that do?
I'm also wondering if there is a way to measure if the pre-charge has been done and make a diode light up. I'm ok with doing an Arduino setup if that's what it takes.

Again, thank you.
 

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Each battery is separately fused and has its own cutoff switch.
Thank you for the advice! Is there a way for you to show me how that is set up? Don't you need some pretty heavy duty switches to carry the amperage?
 
THAT is someone who is confused. That is SERIES connection and NOT Parallel.
Series increases Voltage, not Capacity.
Parallel increases Capacity NOT Voltage.
Always engage in Critical Thinking, if in doubt (even the slightest) ASK !
There is NO "Stupid Question" except for the one NOT ASKED !

That resistor is likely OK.
Momentary Switch only needs to be held down for approx. 2 seconds for caps to charge. (bigger inverter = more time)
LED/Diode indicator ? I'm not sure how that would be setup, may have to crack the inverter and wire something in... Practical ? I dunno.

If you go and look at the "About my System" link in my signature, you'll get several answers to your current & pending questions. Including switch to disconnect battery bank etc...
 
I have two of the same cells and BMS, paralleled. Works fine.

My inverter is only 1000 watts. I have no problem charging the capacitors. My inverter is shut off when not in use (using the inverter's on/off switch), but I don't cut power to it.
 
That resistor is likely OK.
Momentary Switch only needs to be held down for approx. 2 seconds for caps to charge. (bigger inverter = more time)
LED/Diode indicator ? I'm not sure how that would be setup, may have to crack the inverter and wire something in... Practical ? I dunno.

If you go and look at the "About my System" link in my signature, you'll get several answers to your current & pending questions. Including switch to disconnect battery bank etc...
Thanks for the input. Looks like it'll take some time to go through your About page but what an amazing write-up. I'll bookmark this for later when I'm doing something similar for my house (once I have one).
 
Thank you for the advice! Is there a way for you to show me how that is set up? Don't you need some pretty heavy duty switches to carry the amperage?
Yes, I used Blue Sea Systems battery disconnect switches. They aren't cheap, but you get what you pay for. Each switch can handle 200A, which is the maximum DC current my system is currently able to draw anyway. The only thing I can't do off a single battery is run my inverter at full load through a single BMS. But any practical load (i.e. no air conditioner!) will be fine. I ultimately plan to go to a 4 battery bank in which case I'll have N+2 redundancy for full load through the BMS's even if two batteries are offline.
 
I'm doing the exact same thing. I have my two 12v 280Ah 4S battery packs, each with their own Overkill 4S 120A BMS, connected to a Lynx distributor (essentially 1000A bus bars) for battery input. Each battery is separately fused and has its own cutoff switch. At max draw, my inverter will pull about 200A at 12v. So each battery will see a max of 100A through the BMS. I plan to add two more 280Ah battery packs later this year for a total of four.

Just make sure your BMS's all configured the same. Set your parameters like you want them on one, save the config file, and then load the config into the other BMS. See the Overkill manual for more info on that.

Good luck!
This is exactly what I'm wanting to do. Can I ask how you set it up to the Lynx? Did you stack the positive lugs from pack #1 and pack #2 and the negative lugs from pack #1 and pack #2 on top of each other on their respective pos. and neg. battery input site (left side of the Lynx)? Or did you first bus bar them? Or some other way???
 
This is exactly what I'm wanting to do. Can I ask how you set it up to the Lynx? Did you stack the positive lugs from pack #1 and pack #2 and the negative lugs from pack #1 and pack #2 on top of each other on their respective pos. and neg. battery input site (left side of the Lynx)? Or did you first bus bar them? Or some other way???
I used a Lynx distributor for the battery input and fuses inside the Lynx as the main battery fuses. Each battery (up to four) connects to its own associated positive and negative input. I have no other bus bars outside of the two Lynx distributors and a small 6-way fuse box on the far right side of the electric panel for small loads like the tongue jack, break away braking system, etc. I hope that makes sense.
 
make sure each battery pack is fused properly for the "total" Max amp potential your system will draw. (each battery should be able to operate "standalone", meaning being able to handle the full Charge/Discharge potential of your system.
Hey Steve,

Why would this be necessary? In this case, won't the 100A BMS trip well before it could get to the max amp potential of the system? (and wouldn't you want it to?)
 
It's for safety. I wouldn't rely on the BMS to be your primary protection against a fire caused by overheating due to a short circuit or overcurrent draw. The BMS's only job should be to protect the cells in the battery and the condition of the pack.
 
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