Replacing shunt FET

[powersupply]

Hi all,
so I have this SmartBMS and learned that they use the single FET in the picture as a current shunt.


I would like to draw more power, but don't have a matching 100V/92 FET.
View attachment fb4410zpbf.pdf
I do have lower A FETS with the same internal resistance of 7mOhm.

I would like to swap the shunt FET with my lower Amp/Volt FET and put the shunt FET in parallel to the other two FETs onto the spare area pictured.

Question:
Would there be any drawbacks?

Are these SmartBMS tuned to the specific FET?
I will be getting the programmer from a kind member here so I could reprogram it if needed.

Thanks!

 

[dnmun]

why not just buy another 4110 and use it in unoccupied spot? what voltage does the pack run at?

 

[powersupply]

Mostly the matching of the FETs and also the cost.

These are not available at your local RS, and shipping just kills it.

I'm also very much into recycling.

 

[dnmun]

i don't think they have to be matched on a BMS but shipping is always expensive. did you look on ebay to see if you can get some of the surface mount? that would be cheapest but mouser might put it in an envelope for you too if you ask but i think they have a $5 minimum shipping small order arrangement.

maybe someone will sell you a 4110 surface mount but most of us use the regular tabs.

 

[powersupply]

After reading about fake FETs being sold on ebay, that is off limits for me.

But back to the issue, could the shunt FET be a 75A and 75V one with the same 7 mOhm?

It is a 13s4p (p=5Ah) pack with max. 30A draw.

 

[dnmun]

who knows. no idea which mosfet your comparing the 4110 to.

 

[powersupply]

I have many irfb4710 which 2 piggipacked would be 7mOhm.

Should I use them as shunt or put them in the third spot, parallel with the 4110?

I had bad experiences with unmatched/unrelated FETs in power supplies and UPS.

But don't know the dynamics of the BMS, should be much less critical, though.

I'd rather use the existing 4110 for the third power FET and 2 of mine as shunt.

 

[dnmun]

if you have a 100v battery then go ahead and put it in parallel with the other 4110s. they are either on or off so the gate charge should not matter.

 

[powersupply]

I'm still buggered by this part of the circuit.
MOSFETs in general have a very bad initial accuracy (around +/- 20%) of RDS.

The next factor is their variation with V-GS which can be eliminated here.

Then there is the heat induced unlinearity, on top of it's own heat generation the neighboring FETs contribute to that.

Why would they choose an expensive FET for a shunt?

 

[dnmun]

because it is not a shunt.

 

[powersupply]

Well, it is used as such it seems
Couldn't find the original discussion/unedited picture anymore, so just ignore the red stuff, it was from a mod.

When looking at the picture of the single FET
you'll see that its gate is shorted to its source.

I thought this has to do with the poor soldering job, but then I saw that others had documented this, too.

The PCB has provisions of driving the gate, but as mentioned, it is just shorted.

 

[dnmun]

i thought these were 4110 mosfets. did you test continuity between the gate and source? it will not charge if the gate is shorted to the source.

when you say shunt transistor that refers to the balancing transistor. the small mosfets tied to the shunt resistor used to balance the cells.

the mosfet tied to the charger negative is called the charging mosfet. i cannot believe it would have been shipped out with the gate shorted to the source. it would not pass inspection.

 

[powersupply]

Thanks for the reply.

Yes, these are the 4110 MOSFETs. Gate and source are shorted on both boards.
bmsbattery.com has no inspection or quality control.

when you say shunt transistor that refers to the balancing transistor. the small mosfets tied to the shunt resistor used to balance the cells.

Sorry, I was refering to the overall current (sense) shunt.

It seems that the 4110 is used as such.
I cannot see any resistor shunt (to sense the total current) on this board.

Will have to reverse the layout probably to understand what's going on.

Had hoped Methods would point me to his findings about this SmartBMS.

 

[dnmun]

BMS battery does not make the stuff it sells. why would they inspect it? did you actually test continuity? can you prove it is shorted?

 

[teklektik]

Couldn't find the original discussion/unedited picture anymore...

connections for? EcityP Smart S5-13 BMS

 

[powersupply]

teklektik, that is where I got my picture from, but there was another thread discussing this FET-as-a-shunt approach.
THAT thread has the unedited picture and THAT thread I cannot find anymore. Maybe it wasn't even on ES, exhausted my search.

dnmun, here is my affidavit

Affidavit of Upply Powers
The undersigned, Upply Powers, being duly sworn, hereby deposes and says:
I am 35 years old, am working as a researcher, and currently reside at 15 Verdad St, Santa Monica, California 90408.
I suffer no legal disabilities and have personal knowledge of the facts set forth below.
I declare that the gate of the discharge FET is shorted to the source of the discharge FET.
I further declare that I personally measured this phenomenon with a Fluke DM900 measuring device. Said measuring device was calibrated and checked prior to the measuerement. In addition I declare that both SmartBMS boards I posess exhibit said phenomenon.

Today is October 14, 2013
Upply Powers
(had to do that, sorry)

So yes, it not only is clearly visibly shorted, I measured it for you, too.

The short is prominent on both boards. I finally reversed some of the traces and it is impossible thay have functioned as is.
So the only other option is that my source from which I bought these (a member here) was not truthful about that he used one of the boards and that it worked.

But it stills leaves open the initial question on replacing the (shunt) FET.

Looking at the original schmeatic attachedView attachment Smart_BMS_Datasheet_o2m_oz890 9.pdf
I am talking about "MDSG1". A 4110 FET is used instead of the SUP85N10 on these boards.

Thanks!

 

[bigmoose]

The FET markings look counterfeit to me FWIW.

 

[powersupply]

Interesting!

Would make sense, considering shady BMSbattery.com (yes, yes, and their even more shady suppliers).

I read about fake FETs here for the first time. Never thought it would make sense comercially,
20 years ago: CPU's, yes, but FETs??

Only thing is that Crossbreak says he tests them with 2500W(!), and they seem ok.
So do these have more resistance or are they lower voltage/A??

I was thinking getting a bunch of FETs for my controller (just in case I want to fo go above their current limit).
Now I will get a few more for this BMS, too.

Thanks for the info!