BMS wiring length

[learning all the time]

Hey all. Due to the location of my batteries under my stairs I have very limited room for storage. I tried relocating my batteries but need serious reinforcement upgrades to do so and gave up that space for an inverter charger instead. I have a 560ah 8 cell battery and the length of the battery is the same as the wires. So I am leaving my batteries in the designed location but my balancing wires are stretched tight and leaves the BMS laying on top. Can I lengthen my BMS balancing wires and how far can I go?

 

[snoobler]

The sensing (as opposed to "balancing") wires don't pass any current beyond that needed to measure voltage, so their length isn't really important. Don't make them several times longer than they need to be, and make sure the means you use to extend them is of high quality and not susceptible to intermittent or permanent disconnects.

If you have a BMS with ACTIVE balancing where charge is passed from one cell to the other and measures in Amps instead of mA, as opposed to just bleeding of excess with a resistor, it's marginally more important. In that case, making all the leads the same length might be beneficial, but I would personally choose the lazy approach - only lengthen the ones I need.

More important is the length of cable passing the current from the battery, through the BMS and to the loads. That can be long too, but more length means you want heavier gauge.

 

[Factory400]

The maximum length of the wires depends on the current going through them and the gauge of the wires. Another consideration would be electrical noise that long wires can pickup and cause undesired behavior of the BMS. This interference is hard to predict as it is very dependent on what radiating sources are close by - like inverters, SCC's, etc.

If you go down a gauge number or so on the wiring, you could extend the length of the wires considerably. Consider putting ferrites on the ends close to the BMS to attenuate any noise that may freak it out. The noise can look like high voltages enough to trigger a BMS shutdown.

 

[learning all the time]

Excellent. Thank you guys. What gauge do you guys recommend? 14 or 16? I think the current leads are 18g as they are pretty small for both my Daly and my Overkill BMS's. I will probably use a terminal block to close the gap and shorten as much as possible.

 

[Factory400]

What is the balancing current, how long are you hoping the wires will be? 14-16AWG sounds really huge for most passively balancing BMS's

 

[Factory400]

Another consideration for long balance leads it FUSING. Since they are connected directly to the battery they present a hazard if they get pinched or shorted in some way. They would get exceptionally hot to the point of destruction and possibly start a fire along the way.

Small fuses close to the terminals will keep you safe.

 

[learning all the time]

You are correct. I believe the wires to be 26awg. Smaller than I thought. I will look into fusing them also as the pinching was also a concern but I had some design ideas to prevent that.

 

[HRTKD]

Instead of arranging your 8 cells in one column of 8, use two columns of 4. That requires shorter lengths of wire.

 

[Factory400]

Instead of arranging your 8 cells in one column of 8, use two columns of 4. That requires shorter lengths of wire.

It sounded like the arrangement is locked down because of the way the holder has been built.

 

[snoobler]

IMHO, thin gauge wires are their own fuses. Just duplicate what you have and lengthen as needed.

 

[Factory400]

If a BMS is balancing at 500mA with with a 26ga wire, that is roughly 20mV drop per foot.
A 3ft extension would add another 60mV of drop. Not horrible, but not great.

If the BMS only does 150mA balancing the same extension only adds about 18mV which is not much of a concern as long as all the leads are similar length.

If you have 20ga wire and a fairly low balancing current, your bigger limitation will be EMI where the wires become antennas and it is anyone's guess if the BMS is setup to filter that noise in a productive way. In that case, you can use shielded cable with a single ended termination with the battery side negative and/or add ferrites to each wire on the BMS side of the connection. That is a solid way to go about it.

 

[learning all the time]

Instead of arranging your 8 cells in one column of 8, use two columns of 4. That requires shorter lengths of wire.

Yes HRTKD that would be better but my arrangement just does not work. Battery holder is 10" tall x 8.5" wide x 23" long. I cannot even put a case on these as it is way too tight. As I mentioned I tried to relocate the batteries but my support options are limited also. My coach is beautiful just limited with power design capabitlities.

 

[learning all the time]

IMHO, thin gauge wires are their own fuses. Just duplicate what you have and lengthen as needed.

I have 3 bms's. 2 overkill i ran in parallel but had an issue with my microwave. oops. Both the bms's are still good but I got a Daly thats 250a. So I think I am going to use one of the overkill harness for an extension of maybe 12 inches. That should take the stress off my wiring. I want to keep the length minimized but not too short.