I'm putting together a 12V 4S3P battery system with (3) HXYP_4S_3828_BMS battery management system modules, each managing one of the three stacks. Can I safely tie the three stacks together at the 12V terminal? My concern is that there may be a slight voltage difference between the stacks. The batteries are 32700 LFP.

 

Thats not how itsdone...

Parallel 3 cells and put 4 such groups in series. Only 1 BMS required..

 

I'm putting together a 12V 4S3P battery system with (3) HXYP_4S_3828_BMS battery management system modules, each managing one of the three stacks. Can I safely tie the three stacks together at the 12V terminal? My concern is that there may be a slight voltage difference between the stacks. The batteries are 32700 LFP.

Your suggestion saves me parts, but more importantly, space. Thank you, Irving.

 

You're welcome, and welcome to AAC...

 

I'm putting together a 12V 4S3P battery system with (3) HXYP_4S_3828_BMS battery management system modules, each managing one of the three stacks. Can I safely tie the three stacks together at the 12V terminal? My concern is that there may be a slight voltage difference between the stacks. The batteries are 32700 LFP.

The current flow between the batteries could be very high when you initially connect the three batteries in parallel if there is a difference between their terminal voltages. Connect them together with a low value (10 Ohms) resistor in each battery + lead until the batteries all read the same voltage. Then it will be safe to connect them directly together.

 

The current flow between the batteries could be very high when you initially connect the three batteries in parallel if there is a difference between their terminal voltages. Connect them together with a low value (10 Ohms) resistor in each battery + lead until the batteries all read the same voltage. Then it will be safe to connect them directly together.

Another, longer solution, but one that enhances battery longevity, is to charge all the cells individually to 100% charge them to the same terminal voltage, then put them away for a period of time - ideally a month. then measure the lightly loaded terminal voltage and group cell into triples of similar voltage (ie having similar self-discharge rates).

 

The current flow between the batteries could be very high when you initially connect the three batteries in parallel if there is a difference between their terminal voltages. Connect them together with a low value (10 Ohms) resistor in each battery + lead until the batteries all read the same voltage. Then it will be safe to connect them directly together.

I read somewhere that something below 50uV difference between stacks is "same voltage." Though I don't remember if this was specifically LFP batteries. Would you agree that <50uV is a safe place to directly connect the stacks ?

 

Another, longer solution, but one that enhances battery longevity, is to charge all the cells individually to 100% charge them to the same terminal voltage, then put them away for a period of time - ideally a month. then measure the lightly loaded terminal voltage and group cell into triples of similar voltage (ie having similar self-discharge rates).

I do have some time before completing the design & prototype fabrication. I can use that idea. Thanks for the suggestion. Battery longevity (# of charging cycles) was one of the key facts leading me to choose LFPs.

 

I read somewhere that something below 50uV difference between stacks is "same voltage." Though I don't remember if this was specifically LFP batteries. Would you agree that <50uV is a safe place to directly connect the stacks ?

The internal resistance of a typical 18650 cell is 150 to 250 milliohms so very little current will flow between the cells with that low voltage difference.