BMS to Battery Connection Order - Conflicting Information

[ztrain727]

Hello,

I have a heltec 200A BMS common port going on a 6S 5kw 21700 EV module as an off grid power wall build.

There is no spec sheet and a one page Chinese diagram included with this BMS.

The guy who sold me the battery builds power walls for a living. He advised I solder/heat shrink all leads to the BMS harness, then connect the B- lead to battery negative terminal, then connect the harness with all the balance wires.

My confusion is that everything I’ve seen on YouTube and other write ups, barring my misunderstanding, says to connect the B- to battery negative BEFORE soldering the balance wires/sense leads SEQUENTIALLY. This makes sense in a way re internal circuitry, except it seems sketchy to be soldering live connections, and goes against all other electrical work I’ve ever done.

Can I just hook up the B- to battery negative and then plug in the harness with all balance leads connected? I’m paranoid about frying the thing.

Thank you very much!

 

[Samsonite801]

Can I just hook up the B- to battery negative and then plug in the harness with all balance leads connected? I’m paranoid about frying the thing.

I always did it this way (connect B- first), because the BMS isn't actually powered on until the last step, which is to simply plug in the sense lead harness connector (it's where the power connection gets completed).

 

[ztrain727]

I always did it this way, because the BMS isn't actually powered on until the last step, which is to simply plug in the sense lead harness connector (it's where the power connection gets completed).

Right, I guess it’s a matter of whether each series needs to be energized sequentially, 0v, then 3.7v, then 7.4v etc… I considered trying to land the connector at a slight angle so they connect this way.

If you’ve had good luck just plugging it in, I think I’ll roll with that!

 

[Samsonite801]

Right, I guess it’s a matter of whether each series needs to be energized sequentially, 0v, then 3.7v, then 7.4v etc… I considered trying to land the connector at a slight angle so they connect this way.

If you’ve had good luck just plugging it in, I think I’ll roll with that!

Yeah, I string all the cells together as you connect all the balance leads (without BMS harness connector connected yet), connect in B-, then last step, plug in the BMS sense-lead harness connector.

Then Bluetooth comes online shortly after, log into it, check reported voltages, health up to par, no triggered alarms, etc...

 

[Ampster]

Can I just hook up the B- to battery negative and then plug in the harness with all balance leads connected? I’m paranoid about frying the thing.

Yes, that is my preferred way of doing it after sequentially checking the plug for the correct voltage. The voltage should increment from the first positive up to the pack voltage. Sometimes I have to use a paper clip or small wire to connect to the positive lead of my multimeter because the connector is so small. The negative lead can be connected to the pac negative. I also do not solder the leads but prefer ring terminals. I put the ring terminals on after the load carrying bus bars or lugs.

 

[ztrain727]

Ok amazing! Thank you so much for replying!

I think maybe those videos are just combining those steps of checking the voltage and then making the connections. Maybe some BMS’s don’t have a harness like that? Anyway, seems safer to solder/crimp with it disconnected…