Overvoltage protection via BMS

[BradCagle]

Would be handy to have a list of BMSs, and what max voltage they can protect the battery from. Although this may be impractical as I see these BMSs change overtime, even ordering the same spec JBD BMS you might get one that is clearly different from a previous.

Do we just have to tear them apart, and look up the specs for the MOSFETs?

I'm becoming more, and more concerned with MPPT controllers shorting, and sending full PV voltage as I just had one do this.

 

[squowse]

Would be handy to have a list of BMSs, and what max voltage they can protect the battery from. Although this may be impractical as I see these BMSs change overtime, even ordering the same spec JBD BMS you might get one that is clearly different from a previous.

Do we just have to tear them apart, and look up the specs for the MOSFETs?

I'm becoming more, and more concerned with MPPT controllers shorting, and sending full PV voltage as I just had one do this.

Which MPPT?

 

[BradCagle]

Which MPPT?

The one here:

 

[740GLE]

I’d invest in a better SCC

Also I don’t think any BMS could withstand voltages of a PV system if VOC is high enough.

 

[RCinFLA]

If the Voc of panels is greater than BMS MOSFET breakdown voltage, then you are vulnerable if SSC shorts input to output and doesn't matter what BMS cell overvoltage cutout is. Only a separate secondary SSC output overvoltage supervisor protection circuit will save you. Some quality SSC's include this circuit.

It is unlikely PV Voc will be less than maximum allowed battery voltage since it would inhibit SSC output charging voltage and with MPPT charge controllers it is often much greater than battery voltage.

Since SSC's PV inputs are likely to get subjected to voltage spikes due to nearby lightning strikes it is very important to have good voltage surge suppression on both PV positive and negative lines to ground and make sure PV SSC case is well grounded.

When MOSFET is turned off and the MOSFET breakdown voltage is exceeded, it has a voltage avalanche (similar to zener diode), has a significant avalanche voltage drop between drain to source and passes some current which overheats MOSFET causing its failure. It can fail open or shorted. Shorted is the condition that will destroy LFP battery.

8S BMS usually have 40v MOSFET's and 16S BMS are usually 80v or 100v. Some BMS put higher voltage MOSFET's in lower voltage model BMS. I have seen an 8S BMS with only 30v breakdown MOSFET's. It is common to have some ringing overshoot voltage on battery lines due to their series inductance so you need some margin on BMS breakdown voltage on their MOSFET's. (Taping battery pos./neg lines together reduces the voltage overshoot ringing).

Chinese have a reputation of changing parts from manufacturing run to manufacturing run depending on what parts are available or getting a bit lower cost for parts.

I have seen different manufacturing lots of JK 16S/20S active balancer BMS's built with HYG035N10NS2P MOSFET's and JMSH100 MOSFET's. Both MOSFET's are 100v breakdown although the JMSH100 MOSFET's have slightly greater Rds_ON series resistance.

 

[BradCagle]

8S BMS usually have 40v MOSFET's and 16S BMS are usually 80v or 100v. Some BMS put higher voltage MOSFET's in lower voltage model BMS. I have seen an 8S BMS with only 30v breakdown MOSFET's. It is common to have some ringing overshoot voltage on battery lines due to their series inductance so you need some margin on BMS breakdown voltage on their MOSFET's. (Taping battery pos./neg lines together reduces the voltage overshoot ringing).

Chinese have a reputation of changing parts from manufacturing run to manufacturing run depending on what parts are available or getting a bit lower cost for parts.

I have seen different manufacturing lots of JK 16S/20S active balancer BMS's built with HYG035N10NS2P MOSFET's and JMSH100 MOSFET's. Both MOSFET's are 100v breakdown although the JMSH100 MOSFET's have slightly greater Rds_ON series resistance.

Pretty much what I was thinking, we wont know what the BMS MOSFET breakdown voltage is unless we take them apart. Even then will have to run PV VOC under what the BMS breakdown voltage, which will probably be much lower than what we would like.

Would be nice to design a safety device that inserts immediately after the charge controller battery output that interrupts the overvoltage fault.

 

[BradCagle]

I’d invest in a better SCC

Also I don’t think any BMS could withstand voltages of a PV system if VOC is high enough.

Agree, I do have better controllers. However I cant help but think this can happen even on high end controllers

 

[toms]

My BMS opens a contactor that sits between the PV and the SCC as a secondary protection in the event of a SCC malfunction.

 

[BradCagle]

My BMS opens a contactor that sits between the PV and the SCC as a secondary protection in the event of a SCC malfunction.

That's a good one.

 

[curiouscarbon]

contactor safety disconnect between photovoltaic and solar charge controller seems like a very reliable approach!

Normally Open type contactor would outperform almost all MOSFET.. perhaps

some even contain inert gas sealed between the two internal contacts ??

 

[squowse]

If the Voc of panels is greater than BMS MOSFET breakdown voltage, then you are vulnerable if SSC shorts input to output and doesn't matter what BMS cell overvoltage cutout is. Only a separate secondary SSC output overvoltage supervisor protection circuit will save you. Some quality SSC's include this circuit

Do you happen to know which ones do and don't have this circuit? Victron does I believe.

 

[RCinFLA]

Contactors have their issues also. Their series resistance can be greater than MOSFET's. Biggest issue is coil constant hold current. Some back down coil drive current just to maintain contact closure but that subjects contacts to bounce from mechanical shock, and inverter ripple and surge current magnetic vibration. Its risk is welded together contacts.

 

[Ampster]

its risk is welded together contacts.

I welded a Contactor trying to activate it with just putting two wires together instead of using a switch. I inadvertently gave it too many bounces.