Safely tie together 3 12V battery stacks?

Thread Starter

Kent McNaughton

Joined Oct 16, 2023
4
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.
 

Thread Starter

Kent McNaughton

Joined Oct 16, 2023
4
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.
 

KeithWalker

Joined Jul 10, 2017
3,030
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.
 

Irving

Joined Jan 30, 2016
3,795
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).
 

Thread Starter

Kent McNaughton

Joined Oct 16, 2023
4
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 ?
 

Thread Starter

Kent McNaughton

Joined Oct 16, 2023
4
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.
 

KeithWalker

Joined Jul 10, 2017
3,030
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.
 
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