SOLVED: Extend/Lengthen BMS Sense/Balance Wires

[TorC]

Conclusions, original post follows:

There's no particular limit, except if it gets long enough for wire resistance to matter to balance current, and there aren't sufficiently strong EMF fields to couple onto the sense wires in your installation.

If adding wire:

1. Up to about four to five feet tends to be officially supported by BMS manufacturers.
2. It's prudent to use a larger gauge if you need to go past this, especially for BMSs that include active balancing.
3. Most installations won't have problematic EMF in the area, but shielded cable can be used for runs greater than about four feet if you expect problems that can't be otherwise corrected and/or are paranoid about this.
4. If you already need to extend the wires, and are going up in gauge and/or using shielded wire, it's preferable to make the connection as close as reasonably possible to the BMS.
5. DIN rail terminal blocks are useful for making extension connections that can be tested and fixed or changed if needed.

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Going to set up my batteries, and each 16s string is one long row. Narrow available cabinet space (12"x5' per shelf) means a square 2x8 is not practical. The JK-B2A24S20PC BMS I have doesn't have long enough balance/sense wires for this, so I need to extend them somehow. I've seen conflicting reports of max length to extend to - a very marginal 130cm/51" from @Nami here, and someone who reports Hankzor store said 150cm/59", which would work. AIUI, the primary issue with extending is the wire acting as an antenna.

I have some reclaimed 18/4 stranded, foil-shielded security wire lying around that might reduce that effect, and no planned use for it. I figure I'll use plain crimps and marine HS tube to seal them, but could use another method. Currently I see three basic options:

1. Extend only the wires I need to extend from the existing end.
2. Extend the wires I need to extend from near the BMS.
3. Extend all the wires to the same length, from either point mentioned above.

Anyone have opinions, ideally with some reasoning, as to which is better?

 

[Lt.Dan]

I have some balance leads easily in excess of 8ft, and in varying lengths, and standard 18ga stranded wire. No noticeable problems here.

 

[Skypower]

I’d probably just go up one wire size and splice into the existing wire a few inches from the connector. The longer wires I’d run tight and close to the cells along the run and away from the others to lessen its ability to act like an antenna in free air and reduce coupling to noise from the other leads.

 

[TorC]

Thanks. I'll take the final answer as, "There's no particular limit, except if it gets long enough for wire resistance to matter to balance current, and there aren't sufficiently strong EMF fields to couple onto the sense wires in your installation."

Therefore: If adding wire:

1. Up to about four to five feet tends to be officially supported by BMS manufacturers.
2. It's prudent to use a larger gauge if you need to go past this, especially for BMSs that include active balancing.
3. Most installations won't have problematic EMF in the area, but shielded cable can be used for runs greater than about four feet if you expect problems that can't be otherwise corrected and/or are paranoid about this.
4. If you already need to extend the wires, and are going up in gauge and/or using shielded wire, it's preferable to make the connection as close as reasonably possible to the BMS.

I'm editing the thread title to add a keyword so hopefully this will come up easily in the future when people search for it. If you have comments, I'll consider them and edit as appropriate.

 

[rhino]

Terminal blocks are also useful in that situation. They also provide easy means of measuring cell voltages by being able to measure at the terminal block.

 

[Lt.Dan]

Terminal blocks are also useful in that situation. They also provide easy means of measuring cell voltages by being able to measure at the terminal block.

This is exactly what I used. A 16 terminal block.