FlexiBMS Lite - Flexible Configuration BMS w/ CAN-bus

@janpom

https://lcsc.com/product-detail/TVS_AnBon-SMF51A_C435454.html

I’d recommend this TVS for the charger side protection.

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With the breakdown voltage at about the Vds-max rating for the FETs and the clamping voltage up over 80V you feel that TVS would still protect the FET?

Has anyone scoped out the charger voltage transient to estimate its power? That might be important for the max power spec for the TVS.

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Im really trying to think here. Assuming it’s not the charger (just for the sake of the argument), is there something else that may have affected these mosfets? Perhaps something I soldered?

Depends who arguing the point

A PSU company would state never switch it on with the load connected. As the charger would have to deal with the spikes

A BMS would say never connect with a live connection. As the BMS would have to deal with the spikes

It’s a game of not my issue to deal with blame some one else.

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Considering the charger side doesn’t really have any capacitance apart from the stray one, there isn’t really a lot of energy in the current rush that might happen, so even if we are not in the voltage region of peak clamping capability, it’ll be able shunt that short and sharp pulse transient, but at the moment there isn’t anything absorbing the voltage spikes.

Only realistic thing I could come up with is a static electricity zap from the human body (now even better with synthetic material clothing!) to the C+ pad, that could be in the thousands of volts and absolutely be capable of causing the punch-through event for the Mosfet.

I’m gonna improve the state printout logging, so that for the battery and charger voltages, it’ll print out lowest-latest-highest values, because there could be something drastic going on, but due to the printout currently only printing the latest value once every second, there is quite a lot things that might be missed based on it. In your case the worrying thing was that the charger side sense circuit had measured over 60V at one point.

State:3472:3474:3475:3474:3459:3468:3466:3474:3474:3472:3472:3471:41240:18934:0:295:297:194:chargingEnd:
State:3471:3474:3475:3474:3459:3468:3466:3475:3474:3472:3472:3471:41181:34262:0:295:297:0:chargingStarting:
State:3471:3474:3474:3474:3459:3468:3466:3475:3474:3472:3472:3471:45547:18934:88:306:297:279:startingCurrent:
State:3471:3475:3475:3474:3459:3468:3466:3474:3474:3472:3472:3471:45567:18934:84:306:297:279:startingCurrent:
State:3471:3474:3474:3474:3459:3468:3466:3475:3474:3472:3472:3471:41181:18934:0:295:297:0:chargingEnd:
State:3472:3474:3475:3474:3459:3468:3466:3475:3474:3472:3472:3471:41181: 61408 :0:295:297:0:notCharging:
State:3471:3474:3474:3474:3459:3468:3466:3475:3474:3472:3472:3471:41181:43441:0:295:297:0:chargingStarting:
State:3472:3474:3474:3474:3459:3468:3466:3474:3474:3472:3472:3471:41181:30240:0:296:297:0:startingCurrent:
State:3471:3475:3474:3474:3459:3468:3466:3475:3474:3472:3472:3471:45547:18934:88:307:297:279:startingCurrent:
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Sorry if this has been answered already but is the battery voltage measurable from the charging port with the FlexiBMS?

Something like this:

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No, there is a fully blocking charging Mosfet setup that doesn’t allow battery voltage to be measured on the charger side by default, unless the Mosfets are commanded open. This is meant to protect the battery and BMS from any kind of short-circuits on the charger side.

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Is there a charge+discharge version yet? Flexibms pro?

Not yet.

I would almost prefer more to make a E-switch module with CAN-bus that can be added into a system (with a Lite already for the charging path control for example) and connected to the existing CAN-bus network or connect it with CAN to just the BMS. Add also an opto-isolated input, for power switch.

Discharge path control just usually means having to deal with more current then the charging path and that means paralleling Mosfets for higher current handling + bigger space need. A simpler module, would also be easier to design a simple heatsink for, but that’s just my current, slightly sleepy evening head thoughts.

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I like this, and on top of that add a shunt for precise battery current measurement and you can either just leave at that or start playing with SOC and SOH calculations

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Maybe I’m being ignorant but I can’t imagine a use case for that.

The BMS can command it to shut down in case of any fault situation that would required the power to be cut, instead of having a BMS that can cut the discharge plus an e-switch, unnecessary redundancy that would take more space

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Don’t most people turn their boards off when charging? Well, now that I think of it, in the near future where everyone has a robogotchi uploading their rides when they get home and giving them a notification when the board is charged, maybe not… :thinking:

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You can still have it off and charge, the FlexiBMS would connect as it is to the battery, and the discharge wires would go through this new e-switch before going into the rest of the board

Ye ye, I was just thinking “no point in a smart e-switch protecting the ESC when charging if I just turn it off before charging” but I now see the point; thanks! :+1:

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You can also add more accurate current measuring for the discharge path.

One of the key design decision/question that needs to be decided is in what configuration should the Mosfets be configured in. Aka, fully blocking or one-way blocking towards the battery.

One-way blocking would have the pro of having only half of the Rds(on) resistance compared to the fully blocking setup, as it only has one Mosfet inline instead of two and it would always allow current to flow from the ESC/Load towards the battery through the fet’s body diode, aka braking should always work.

The only pro I can think at the moment for the fully blocking setup is that if the battery is getting overcharged, then it could stop the current flow from the ESC, but in EV use case that would mean that the ESC side is generating, aka braking. I’d see that more as a safety hazard.

Thoughts?

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I had a blown FET from a battery disconnecting mid-ride (bad solder joint)
Wouldn’t this cause the same every time?

I’m guessing/assuming the blown Mosfet was on an E-switch? Was it the battery side + terminal that got disconnected? Was the E-switch a high-side connecting/disconnecting type?

I would hope that the ESC would be fast enough to catch the fast rising battery side voltage and be able to lower the braking regen, so that the battery side voltage doesn’t rise high enough to damage the E-switch Mosfets.

Nope, it was on the VESC. That board has no e-switch at all.

So the +Bat wire got disconnected from your VESC during a ride and the VESC blew a Mosfet?