Basic BMS question: When disconnect BMS from LIFEPO4 Cells, does it matter to remove Negtive connect first or Positive one?

[hondaman82]

Hi Everyone,
I am building my very first battery pack from 4x280AH LIFEPO4 cells and Daly BMS so I want to make sure no obvious mistake occur. I will need to remove and reconnect the BMS to the cells several times for top balancing series / parallel and other purposed. My question is when remove the BMS, the first step is disconnect the cable sleeve from the main board, then remove the battery connectors. Is there any order of removal? can i remove Negative connector first, then Positve 1, then postive 2... or is it Positive first then negative? thank you !!!!

 

[boondox]

Well, in an automotive setting when many metal objects nearby are connected to negative one removes the negative first. This is to reduce the chance of shorting the positive to ground with a wrench or whatever. Electrically it makes no difference.

 

[cinergi]

As long as you have removed the connector from the BMS first, it doesn't matter.

 

[ChuckNorris]

What if you don't remove the connector first tho!?
Does removing the negative first disable the bms safely? @cinergi

 

[cinergi]

What if you don't remove the connector first tho!?
Does removing the negative first disable the bms safely? @cinergi

That might shut down the BMS but it doesn't remove the danger of disconnecting individual sense leads. Always disconnect the sense lead connector(s) before servicing the battery.

 

[ArthurEld]

The way I do it is to turn off charging/discharging in the BMS settings.
Turn off balancing.
Turn off the on/off switch to my inverter.
Pull the plugs for the balance leads out of the BMS.
Disconnect negative.
Disconnect positive.
Disconnect the rest of the balance leads.

I'm not sure that is the best way but I'd like to hear if it's not.

 

[ChuckNorris]

i understand the best way to do it. But i'm looking for an alternative way because how i want to mount everything i can't get access to the bms connector without first disconnecting each of the 17 bms leads on my 16s bms. So i figured removing the black first cell negative first would disable the bms and make it safe to then remove the 16 balance leads 1 by 1. Then when i get to the bms. THEN i disconnect the 17pin connector. Does this work. Or as soon as i start unplugging the black or red balance leads that will then damage the BMS?

 

[ArthurEld]

I am no expert but from what I gather, you can burn up your BMS if you accidentally touch the BMS balance wire to the wrong terminal.
So, maybe if you are very careful it would be ok. Hopefully someone with experience will reply.
Might be worth the effort to move the BMS at some point so you can access the balance wire plug or plugs

 

[cinergi]

i understand the best way to do it. But i'm looking for an alternative way because how i want to mount everything i can't get access to the bms connector without first disconnecting each of the 17 bms leads on my 16s bms. So i figured removing the black first cell negative first would disable the bms and make it safe to then remove the 16 balance leads 1 by 1. Then when i get to the bms. THEN i disconnect the 17pin connector. Does this work. Or as soon as i start unplugging the black or red balance leads that will then damage the BMS?

No, that's why I said you must disconnect the connector at the BMS. I general, you cannot have some of the sense leads connected and not others. The problem has to do with the voltages between the sense leads damaging the circuitry because you've got incorrect voltages heading into each channel of the circuitry due to partially-connected leads even with the negative disconnected (there's still voltage/potential even with that disconnected - measure pins 2 and 3 at the disconnected connector with the negative lead disconnected). Some BMS can handle this due to the way they're designed, but you'd want to see that in writing unless you want to risk blowing up your BMS and voiding your warranty.