MOSFET for bypassing BMS while discharging (common charge discharge port for a UPS)

[imranmahsud]

Please help me how to wire the circuit with MOSFET (100 amp mosfet as per requirement of load) so that charging of the battery is done through the BMS but discharging is direct by bypassing the BMS since it is rated for 15 amps. Please see the attached image
https://www.dropbox.com/s/hkssio21sf6zgdu/Screenshot_20190302-210407.jpg?dl=0

 

[d123]

Hi,

I think you need to provide (a lot) more information for anyone to sensibly address the question: Is it a 5V or 500V battery and 100A MOSFET? That makes a big difference as to how the solution can be implemented and what - if any - MOSFET gate protection may be needed. Or, if the controller can charge while the battery is discharging or if it charges then turns off until the battery reaches x level...

You could use a comparator or standard logic or an op amp or just use the MOSFET gate as the signal input that decides when to turn on/off the discharge path. But it depends on a lot of unknowns, for me at least.

 

[FvM]

Bypassing the BMS thwarts it's ability to monitor the battery charge state. Safety functions like deep discharge shut off are also disabled. A bad idea.

 

[imranmahsud]

The rated current for the bms is 15 amps while during discharge i am anticipating 100 amps since the hybrid inverter will cut of battery at a preset lower voltage, I want only the balance function of the bms during charging and want to skip the bms during discharge. Do you think this
https://www.evselectro.com/irf3207-mosfet-2220?search=Mosfet&page=3
Is appropriate mosfet for the task and how to wire it is the real question or other appropriate solution
This is the bms
https://www.dropbox.com/s/4q7li7saibj14qa/Screenshot_20190302-173455.png?dl=0
https://www.dropbox.com/s/1nvdzjqwwiennzr/Screenshot_20190206-085421.png?dl=0
The inverter i am using will cutoff at preset lower voltage, i only want the balancing feature of the bms during charging and direct discharge when there is load shedding /blackout since the rating of the bms is 15 amps

 

[KlausST]

Hi,

Is appropriate mosfet for the task and how to wire it is the real question or other appropriate solution
This is the bms

A seller´s internet page is no datasheet. --> You have to read the datasheet. I recommend to download it directly from the manufacturer´s internet site.

Klaus

 

[betwixt]

Observe the warnings already given. However, if I'm understanding your requirement, you want to take battery power FROM the charger connector as well as the power out connector, and you are aware of the risks, a diode seems to be all you need.

Something like this maybe: https://uk.farnell.com/vishay/vs-100bgq100/schottky-rectifier-100a-100v-powertab/dp/2690127

Brian.

 

[imranmahsud]

Thank you but diodes have high voltage drop thats why

 

[KlausST]

Hi,

Thank you but diodes have high voltage drop thats why

Did you give the battery voltage rating?
How much voltage drop are you talking about?
--> Without values from your side it's impossible to give exact answers.

Thus my vague answer: The voltage drop of the diode will be negligible compared to the battery voltage variation.

Klaus

 

[betwixt]

...and the diode I mentioned has around 1V drop at 100A forward current, even a good MOSFET would struggle to do much better.

Brian.

 

[wwfeldman]

What does the BMS documentation say about using the battery while it is being charged?
I am guessing that its okay, the BMS limits its own current. Then the 100A comes 85 from the battery and 15 from the BMS. Which should not be a problem.

There must be something I'm missing, as the charge circuits and the load circuits are usually independent, and one can charge the battery while it provided power to a load.

When you write bypass, so you mean disconnect?
If you need to disconnect the recharge when the battery is in use, look for a control input to the BMS that will do this for you.
Otherwise, put the MOSFET switch in the charging system low voltage return. Turn OFF the MOSFET when the current out of the battery is high enough.

You can put the MOSFET in the high voltage side, but it may be easier and safer to do it on the low voltage side.

 

[betwixt]

According to the text, the idea is to use the charging socket to draw current FROM the battery. So more voltage in will use the BMS but when the voltage source is removed, the same socket feeds battery power out. That excludes any possibility of the source providing some of the load current. I think it a risky tactic, primarily because the charge socket only has to be rated for the 15A maximum charge current, using it as an output could make if pass 100A or more.

Brian.