BMS that interrupts the +ve lead & not the -ve lead?

[nate_syd]

Looking for a BMS that goes on the +ve lead, instead of the usual -ve lead.
Are there any?

Looking at trying an automotive LFP replacement, -ve is common, with 2 x +ve
1 x +ve = starter motor only, bypasses the BMS
1 x +ve = rest of the car, BMS, balanced, usual

 

[Samsonite801]

How many amps does the starter draw? Have you checked it with an amp clamp meter which can read inrush? There's probably a BMS large enough to handle starter amps, or you could parallel 2 batteries and use 2x BMSs to split the load...

Are you mounting the LFP battery in the engine compartment or in the trunk? Keep in mind about operating temperature extremes, freezing temps and charging and whatnot.

 

[nate_syd]

How many amps does the starter draw? Have you checked it with an amp clamp meter which can read inrush? There's probably a BMS large enough to handle starter amps, or you could parallel 2 batteries and use 2x BMSs to split the load...

Are you mounting the LFP battery in the engine compartment or in the trunk? Keep in mind about operating temperature extremes, freezing temps and charging and whatnot.

No i havent yet, but space & packaging is at a premium - so the less the better. Very tight packaging requirements etc.
if i can save the space & complexity by having a single BMS then i will - its also "unnecessary" adding a whole heap of componentry & connections/failure points for something that is used 0.001% of the time.
Trying to keep it simple & streamlined as much as possible.

Battery is in the front, engine the middle - also in Aus, so temps are no issues at all.

 

[Samsonite801]

I suggest to check the starter draw at peak inrush, because generally speaking, the average car starter should only draw up to maybe 250a. It seems like a 400a BMS should be able to supply enough current to start an engine.

If you really needed to bypass the BMS for the starter though, I wonder if you could use the style BMS which uses an external relay, and you could wire the external relay onto the positive side? Like a Chargery or something...


16s Example, but you get the idea:


Reference: https://chargery.com/doc/Chargery_BMS16T_specification_V3.0.pdf

 

[Rocketman]

To answer your original question- I am using the Batrium BMS (watchmon core) and using the bms to switch the positive line with a contractor (external relay). This is on my house batteries on the MotorHome. I went with this route because I wanted to maintain the “boost” ability to help start my MotorHome.
I haven’t tested it yet(trying to start the MotorHome off the house lithium battery. ) but I am pretty confident it will start the big diesel motor.