BMS LEADS

[ML3]

Gday all from down under
Installing 4 x CATL 310ah cells with Daly 200a smart BMS with bluetooth

Can I alter the lengths of the main power leads to tidy up and package the system neatly ?

 

[FilterGuy]

Can I alter the lengths of the main power leads to tidy up and package the system neatly ?

There should be no issue with altering the lengths of the main power leads.

There has been debate about the balancing harness leads. A lot of people say they must all be kept to the same length. I have never seen an argument that is backed up by the math to show why this is true.... but I keep them the same length anyway.

 

[Bob B]

I'm sure that FilterGuy has supplied the best answer ..... but, I personally don't worry about it. There is so little current flowing on the balance leads that voltage drop will be negligible.
I extended the leads on mine and cut them to allow for the harness to be easiest to manage. I cut the original wires pretty short and then spliced in a larger wire than the original.

 

[ML3]

Thaks fellers, I appreciate you all helping a learner.

 

[mikefitz]

then spliced in a larger wire than the original

The very light gauge wires have the advantage of acting as fuses if there is an accidental short.

Mike

 

[Bob B]

The very light gauge wires have the advantage of acting as fuses if there is an accidental short.

Mike

And the disadvantage of dropping voltage if you have any significant balance current .... and / or power for the BMS itself.

I still have a short section of the small wire .... I didn't completely eliminate it.

 

[rickst29]

I will be ending up the same BMS in the smaller 100A size, in just a few days, for use in 120Ah battery (cells arriving 10 days later, if I'm lucky). This is a very closely related question, so I ask it here instead of creating a new Thread: The Bus Bars and power lugs will be sized for the M6 bolts and the full width of the terminals.

Do I really want these tiny terminal balancing wire lugs on the same screw? Or should I instead use bigger lugs and connect the Daly balancing wires into bigger leads, to fit the and larger M6 copper lugs?

 

[John Frum]

I have 10 awg balance leads that are "spliced" with lever nuts to the trimmed 22 awg wiring harness that goes to my bms.
Its within 3mv of my meter and everything works as expected.
As @mikefitz said 22 awg is self fusing.
10 awg not so much.

 

[Just John]

I will be ending up the same BMS in the smaller 100A size, in just a few days, for use in 120Ah battery (cells arriving 10 days later, if I'm lucky). This is a very closely related question, so I ask it here instead of creating a new Thread: The Bus Bars and power lugs will be sized for the M6 bolts and the full width of the terminals.

Do I really want these tiny terminal balancing wire lugs on the same screw? Or should I instead use bigger lugs and connect the Daly balancing wires into bigger leads, to fit the and larger M6 copper lugs?

Large current carrying goes on first to make good connections with the terminals, I use 1/4 inch crimp connectors for the balance/sense leads (1/4 inch is easier to find than metric and works fine). That goes on after the current carrying connections, and aside from the very small balance current, really carries no current. Well top balanced cells really should very rarely need any balance current (if ever, to be honest ).

 

[curiouscarbon]

Has it been demonstrated that BMS measure cell voltage at same time as drawing balancing current? I assumed a smart BMS would momentarily stop the balancing current in order to ensure a better signal for cell voltage measurement.

for example measure for 0.01 second and draw balance current for 0.99 second

 

[FilterGuy]

Has it been demonstrated that BMS measure cell voltage at same time as drawing balancing current? I assumed a smart BMS would momentarily stop the balancing current in order to ensure a better signal for cell voltage measurement.

for example measure for 0.01 second and draw balance current for 0.99 second

Clearly that would depend on the BMS..... but I have no information about any of the BMSs. The level of the potential problem would also depend on the balance current. A 200mA ballance current would create a lot lower voltage drop than a 2A voltage drop.

 

[FilterGuy]

Has it been demonstrated that BMS measure cell voltage at same time as drawing balancing current? I assumed a smart BMS would momentarily stop the balancing current in order to ensure a better signal for cell voltage measurement.

for example measure for 0.01 second and draw balance current for 0.99 second

I can imagine a few different algorithims the BMS could use. As an example, the BMS could take a voltage reading immediately before and immediately after starting the ballance current and then assume the difference is the voltage drop on the lead. Then you can just leave it on till it hits the calculated target voltage or the other cells catch up. (I have to assume that the BMS would still stop every several seconds to check everything) Watching a couple of different BMS displays, they seem to turn the balance current on for a minimum of about .5 - 1 second, but It does not have to be very fast. Even if it only checked the voltage every 10 seconds it would probably be fine.