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Designing a MOSFET switching PCB for diyBMS, low temp, low and high voltage shutoff?

I know there has been talk about relays for under and over voltage control, I know Stuart is designing a shunt to measure current, I don’t know to much about mosfet switching, I have had limited success with mosfets on a solar HWS directly connected to solar panels, BMS like Daly use mosfets in there design for low temp, under and over voltage control, it would make the diyBMS complete to be able to add a mosfet PCB to deal with high current DC switching

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Daly also uses a monster of a heatsink to dissipate the heat generated by the mosfets.

I agree it would make a more complete package but it would also likely be more expensive than an external contactor or shunt trip.

There are lowbattery protection units on the market from Victron that can act as a switch these can switch high DC current. Downside Victron says these can only operate up to 36v and their highest current rated unit till 63V I actually can’t believe these MOSFET units can’t handle like 90v DC just for switching. Anyone who has experience? A other option I was thinking about is using DC solid state relais but also on that I have no experience. Maybe someone has and can share knowledge?

I have tried DC SSR, but they are very poor, as they heat up very quickly, there resistance pass through must be very poor, there rating values are no where near there stated rating

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I’ve been using two “battery protectors” from for my system as a last line of defense. You can program all the parameters with an app.

Stuart is designing a current measuring board at the moment, maybe it is an idea to replace the shunt resistor with a x amount of mosfets. These mosfets have a reasonable low internal resistance the more in parallel the lower the resistance. It is still possible to measure current by taking the voltage over the mosfets. If the mosfet resistance is not linear then calculating current might be a bit more difficult.

OK guys I started to work on a mosfet switch with current measuring capabilities for charge and discharge. Specs so far are and a lot tbd:

  • Max operating voltage 100v DC
  • Max current 60A (used mosfets can handle triple the current but don’t know how much heat is produced)
  • common charge discharge port (do we need separate ports?)
  • current measuring by shunt or clamp what is preferred?
  • Daisy chain comunication or I/O ports (isolated)?
  • do we need a circuit to precharge capacitors in load and charger? (soft switch)
  • anything else I forgot?
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It’s a great starting point there Albert

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Hi, did you finish this Project? I am looking for similar switch for my battery pack also. If you can, please share your build and any special things to consider.
Thank you

@SLP sorry having to say but due to the postponed new law on delivering energy to the grid (we can now 1 year longer at a 1:1 rate put and pull energy from the grid) I have paused my battery storage project, this also to save charge and discharge cycle on my cells. For me there is no need to start working on it till mid 2022. In the meantime I follow the diyBMS project of Stewart from the side and sometimes do some experiments on available HW but no real built activities. If you’re looking for a battery disconnected switch and your system is 48v I can advise you to look at the Victron battery protect products they can be used very easy as mosfet switch and have a low internal resistance.