How do common port FET BMS work?

[kuranaga]

hi

maybe this is a stupid question... but... if charger and load are on the same BMS port and the BMS disconnects the charger due to over voltage, does this automatically mean that the load is also disconnected? I hardly see how it would not be the case?

If load and charger are disconnected at the same time, how does the pack return to normal voltage if it can't be discharged? or even worse: if over discharged how does the charger recharge it if it stays disconnected? In the first case self-discharge could eventually bring the voltage down back, but in the second case... u are dead!

ps Im not talking about separate ports or BMS with DIY relay connections like the chargery 16T.

any comments?

 

[sunshine_eggo]

hi

maybe this is a stupid question... but... if charger and load are on the same BMS port and the BMS disconnects the charger due to over voltage, does this automatically mean that the load is also disconnected? I hardly see how it would not be the case?

And you are correct.

If load and charger are disconnected at the same time, how does the pack return to normal voltage if it can't be discharged? or even worse: if over discharged how does the charger recharge it if it stays disconnected? In the first case self-discharge could eventually bring the voltage down back, but in the second case... u are dead!

Loads and charges cause voltage drops and increases, respectively (V = I * R and whatnot). Furthermore, completely unloaded cells will drift even farther than the initial change over time.

Example: Discharge to 2.5V, BMS cut-off. There will be an initial notable bounce. If left disconnected for some time, you'll see the voltage climb to near 3.0V

ps Im not talking about separate ports or BMS with DIY relay connections like the chargery 16T.

any comments?

Yes. See above.

 

[mvas]

maybe this is a stupid question... but... if charger and load are on the same BMS port and the BMS disconnects the charger due to over voltage, does this automatically mean that the load is also disconnected? I hardly see how it would not be the case?

That is not always the case. Many Common Port BMS units will have two MOSFETS, one will enable charging and one will enable discharging via the common port.

 

[kuranaga]

That is not always the case. Many Common Port BMS units will have two MOSFETS, one will enable charging and one will enable discharging via the common port.

but you are aware that mosfets are bidirectional devices?

draw me a schematics of two mosfets in parallel, so that one lets the current in one direction and the other mosfet in the other direction

 

[mikefitz]

all is explained here, thanks to

BMS MOSFETs Explained

I'm writing this post since it seems there is a lot of confusion (especially in the comment section of a certain youtuber in Australia) on why both MOSFETs in a typical single port BMS are controlled individually and why/how this works when they're in the same series circuit. It will hopefully...

diysolarforum.com

Mike

 

[kuranaga]

yeah you can do that relying on the body diodes, then still you have either very high voltage drop (and deadly heat dissipation) when one of the mosfets disconnects and you have lot of charge/discharge current and/or your devices can't detect the battery voltage properly.

There is just no good solution for common port BMS. the right thing is to use 2 ports.