Max amount of parallel batteries?

vonschott

WTF am I doing?
I just purchased 24 lion energy cells that are 3.2v and 50ah prismatic cells. My thought process was to make 4 cell packs, and then parallel 6 (4s6p) of them together with each pack having BMS for a total of 12v 300ah. But I just read that you shouldn't parallel more than 2 packs together. Is there any way around this? Even with BMS on each pack? I am just trying to get to 300ah for my sprinter van camper. Any way to have 3 separate 4s2p and have them all still charge and discharge in the same system somehow without subjecting myself to the potential risks in having a 4s6p system? What exactly are the risks of running 4s6p lifepo4 batteries? How does battle born or other retail companies suggest paralleling their batteries to achieve higher AH?

I appreciate the feedback!
 

electric

Solar Addict
Just parallel groups of 6 cells into 3.2v blocks of 300AH and then put 4 blocks in series with one BMS. This is how it's done. Don't listen to anyone saying otherwise, they have no idea what they are talking about.
 

vonschott

WTF am I doing?
holy crap. I didn't even think about doing it the other way around. Parallel blocks and then series vs series blocks then parallel. Mind blown haha. Thank you very much @electric ! how can I get away with one BMS for all 24 cells though? Is there a BMS that can handle that many connections? Each cell needs its own wire back to the BMS, right? Or does each block just need its own monitoring wire?
 

electric

Solar Addict
no, each cell does not need its own connection, only a parallel block needs a BMS connection. When cells are connected in parallel they essentially become one larger cell.
 

vonschott

WTF am I doing?
no, each cell does not need its own connection, only a parallel block needs a BMS connection. When cells are connected in parallel they essentially become one larger cell.
Ok that makes sense. If it were a series block, then each cell would need its own connection though, right? Because before building a series block, you would first parallel the cells to balance the voltage levels, but since these are parallel based blocks to begin with, no need to do so? I just got into this battery tech yesterday and had no knowledge of this field previously, so I appreciate the patience and responses for my novice questions.
 

Supervstech

Administrator
Staff member
Moderator
The only issue is the BMS you choose needs to have the amp rating to handle the intended load.
for this large a bank, I would source a BMS that active balances. Pulls watts from high cells and transfers to low cells...
 

vonschott

WTF am I doing?
The only issue is the BMS you choose needs to have the amp rating to handle the intended load.
for this large a bank, I would source a BMS that active balances. Pulls watts from high cells and transfers to low cells...
Do you have a recommended product that would do this? Price doesn't really matter. Would prefer something with bluetooth connectivity to apple iPhone if possible. or can set up an android device to be dedicated BMS remote.
 

SCClockDr

Solar Addict
Ok that makes sense. If it were a series block, then each cell would need its own connection though, right? Because before building a series block, you would first parallel the cells to balance the voltage levels, but since these are parallel based blocks to begin with, no need to do so? I just got into this battery tech yesterday and had no knowledge of this field previously, so I appreciate the patience and responses for my novice questions.
First you do need to voltage match all the cells. For doing that bolt the negatives together & apply a power resistor across the positives till current drops to zero.
Replace the resistor with a bus bar & add a cell till they all are in parallel. Let them set a day then hook them up in 6p4s & attach the BMS.
 

vonschott

WTF am I doing?
First you do need to voltage match all the cells. For doing that bolt the negatives together & apply a power resistor across the positives till current drops to zero.
Replace the resistor with a bus bar & add a cell till they all are in parallel. Let them set a day then hook them up in 6p4s & attach the BMS.
Thank you @SCClockDr, I will do so!
 

JAC

New Member
Do you have a recommended product that would do this? Price doesn't really matter. Would prefer something with bluetooth connectivity to apple iPhone if possible. or can set up an android device to be dedicated BMS remote.

Will has a good video on balancing LiFe batteries, here

There is also a vid that will give you an idea on how you can use a relay circuit (same type of relay used in your car) to augment your battery protection. This way, you’rw not limited by the BMS Ampere rating. I’m using the information to build my custom battery as well as a portable solar generator.

 

electric

Solar Addict
First you do need to voltage match all the cells. For doing that bolt the negatives together & apply a power resistor across the positives till current drops to zero.
Replace the resistor with a bus bar & add a cell till they all are in parallel. Let them set a day then hook them up in 6p4s & attach the BMS.
While using resistors to limit the current is a solid engineering approach, in practice it's not really needed as dV between cells is low enough not to create large currents. In practice you can just parallel all cells with bus bars in one step, no need to stage it in multiple steps.
Also, this entire exercise has questionable value, depending on how long you allow cells to sit in parallel, because it does not balance cells to a degree needed to avoid BMS intervention at the end of charge. I.e. this will roughly balance the pack, but even tiny difference will be magnified at the end of charge when dV goes up exponentially. So, don't expect to get a perfect balance by doing this, you'd still need to watch dV at the end of first charge cycle to see how large the spread is.
 

electric

Solar Addict
for this large a bank, I would source a BMS that active balances. Pulls watts from high cells and transfers to low cells...
Active balancing is a snake oil of DIY battery world. No one needs active balancing and the way it's done by those cheap Chinese boards is more dangerous than helpful. Risk of failure is too high because they cut corners in circuit design and when it fails it will likely short out across the cell and either burn out ( best case ) or drain your cell to zero.
 

Supervstech

Administrator
Staff member
Moderator
Active balancing is a snake oil of DIY battery world. No one needs active balancing and the way it's done by those cheap Chinese boards is more dangerous than helpful. Risk of failure is too high because they cut corners in circuit design and when it fails it will likely short out across the cell and either burn out ( best case ) or drain your cell to zero.
Eew! That sucks...

I was thinking along the lines of the Kickstarter BMS... something quality and customizable.
 

Supervstech

Administrator
Staff member
Moderator
You can add a very high quality 5th wheel to a car, but WHY???
I simply hate the idea my watts are being wasted as heat.

Plus, a quality active BMS can transfer at the C-rate of the cell... instead of a simple resistive load pulling down the high cells.
 

electric

Solar Addict
I simply hate the idea my watts are being wasted as heat.

Plus, a quality active BMS can transfer at the C-rate of the cell... instead of a simple resistive load pulling down the high cells.
Watts are not wasted on a healthy battery, maybe a few milliWatts on average.
There is no active BMS in the world that matches C-rate of the cell, the inductors and switches would cost more than the battery and take more space than the battery.
I think you have some misunderstanding of how much balancing is actually needed, which seems to be a very common problem on this forum and online in general.
 

Supervstech

Administrator
Staff member
Moderator
Watts are not wasted on a healthy battery, maybe a few milliWatts on average.
There is no active BMS in the world that matches C-rate of the cell, the inductors and switches would cost more than the battery and take more space than the battery.
I think you have some misunderstanding of how much balancing is actually needed, which seems to be a very common problem on this forum and online in general.
You could be right.

But if we are going to be dealing more and more with used cells, and large ones at that...

More research is needed.
 

electric

Solar Addict
I have been dealing with all kinds of LFP cells for over 10 years, very large cells and packs, anything you can imagine I have seen in real life. Never seen a need for active balancing. Blue elephant gun must be working as I can't see any blue elephants anywhere.
Sorry for sarcasm, this subject is a pet peeve of mine. People pay way too much attention to balancing, making it into some mysterious thing. It's just not there. Not even with old cells or very big ones.
One thing is true, if your cell has enough self discharge to require a lot of ongoing balancing, you need a solution. And the best solution by far is to replace the cell, not to spend futile efforts in balancing it. Don't throw good money after the bad.
 

Supervstech

Administrator
Staff member
Moderator
I have been dealing with all kinds of LFP cells for over 10 years, very large cells and packs, anything you can imagine I have seen in real life. Never seen a need for active balancing. Blue elephant gun must be working as I can't see any blue elephants anywhere.
Sorry for sarcasm, this subject is a pet peeve of mine. People pay way too much attention to balancing, making it into some mysterious thing. It's just not there. Not even with old cells or very big ones.
One thing is true, if your cell has enough self discharge to require a lot of ongoing balancing, you need a solution. And the best solution by far is to replace the cell, not to spend futile efforts in balancing it. Don't throw good money after the bad.
Replacing a cell may indeed be the wise and best option... but what is a diy enthusiast to do when they cant afford more cells?
 

April

Solar Enthusiast
Active balancing is a snake oil of DIY battery world. No one needs active balancing and the way it's done by those cheap Chinese boards is more dangerous than helpful. Risk of failure is too high because they cut corners in circuit design and when it fails it will likely short out across the cell and either burn out ( best case ) or drain your cell to zero.
I saw this and I thought "Doomsayer"
I'm hoping you can justify your statement above because I have been using them for 6 months and have shorted a couple out because the one line of solder points where the harness plugs in is not protected like the rest of the board .
I have burnt out a transistor and a resistor but that hasn't "shorted out my cells or destroyed them".

The part of the circuit that I have shorted just does not work any more.

So there are two types ,one with an inductor and another with capacitor. Can you describe exactly how they " cut corners in circuit design" please . I hope you will pass me some detailed information that I can learn from and not just generalised non specifics . Thanks in advance!
 
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