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BMS MOSFETs Explained

theshaww said:
I have 6 server rack style batteries. 2 of them have a weak cell and the 1 cell trips the charge MOSFET every day before it reaches full. My charge MOSFETs shut down due to that high cell, then it never turns back on while all other batteries fill and then work normally. I have to reset the BMS to get the battery to get the charge MOSFET to function again. Any help?
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Are they still under warranty? If so, contact the vendor.
So what happens if you start discharging - does the BMS ever get back in the state where it accepts a charge, or only after a reset?

If the latter, to me that sounds like a bug. Normally, you would have the charge MOSFET turn off, then balancing happens until the cell drops below a reconnect value, and then the battery takes in a charge again. What rack batteries are these?
 
upnorthandpersonal said:
Are they still under warranty? If so, contact the vendor.
So what happens if you start discharging - does the BMS ever get back in the state where it accepts a charge, or only after a reset?

If the latter, to me that sounds like a bug. Normally, you would have the charge MOSFET turn off, then balancing happens until the cell drops below a reconnect value, and then the battery takes in a charge again. What rack batteries are these?
Click to expand...
I think it's a bug. I was wondering if modifying the cell release voltage would help. Maybe if I make the release a bit higher?

They are Taicopower... They are garbage. I posted about them here a little while back. They lied about everything and the BMS is their own design, and it is also trash. They have worked for the most part, they just have cells that are weak in 2 of the 6 packs I bought.
 
theshaww said:
I was wondering if modifying the cell release voltage would help. Maybe if I make the release a bit higher?
Click to expand...

If you have access to the BMS parameters, this is worth a shot. Does the BMS have a balancer? If not, or it's too weak, you could add an active balancer that is voltage triggered to keep the cells in line.
 
upnorthandpersonal said:
If you have access to the BMS parameters, this is worth a shot. Does the BMS have a balancer? If not, or it's too weak, you could add an active balancer that is voltage triggered to keep the cells in line.
Click to expand...
It has passive balancing, but not strong... so I added an active balancer and I have it turn on at about 55v on my other packs, but this one is so weak, the whole thing triggers the 1 high cell and shuts down before it gets to 55v so it doesn't do anything. I set it to 54.5v, but it still doesn't do a whole lot, especially when charging with 20A total on the pack. I tried to up the release cell voltage, and I might up it some more if that doesn't help over the next couple of days. Thanks for the suggestions!
 
upnorthandpersonal said:
Are the cells welded at the terminals, or are they serviceable. If they are removable, you might want to see if you can find a replacement cell.
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TAICOPOWER
solder-png.140644
cells-png.140643
 
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upnorthandpersonal said:
Yes - you could do a calculation as follows (for e.g. a 200A Heltec BMS):

Rds_on = 3.2mR typical at 25C. We have 10 pairs on the top and 10 pairs at the bottom for this 200A Heltec version. These are configured back to back, gives 6.4mR per pair at 25C. These 20 back to back pairs give a resistance of 6.4/20 = 0.32mR. Heating occurring at 200A continuous, P = R I^2 = 12.8W.

So, instead of having to dissipate a huge amount of (wasted energy) heat with a large heat sink (which they technically could do), we only have to dissipate 12.8W or so at full 200A rated load. This means you typically don't need a huge heat sink, nor active cooling. In addition, it becomes very obvious with a temperature sensor when something is wrong, before it's too late.
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You need a very large heatsink when one of the two back-to-back fets is turned off. Current now flows through the body diode. Using your example, 200A, divided into 10 parallel fet pairs, 20A flows through each of the 10 body diodes. According to the CRST045N10N data sheet, at 25C, 0.8V diode drop when conducting 20A. Each diode dissipates 16W. Total dissipation is 160W! That requires a massive heatsink with active cooling.

What if the BMS is located in an enclosed battery box with no external cooling?
 
Cal said:
You need a very large heatsink when one of the two back-to-back fets is turned off. Current now flows through the body diode. Using your example, 200A, divided into 10 parallel fet pairs, 20A flows through each of the 10 body diodes. According to the CRST045N10N data sheet, at 25C, 0.8V diode drop when conducting 20A. Each diode dissipates 16W. Total dissipation is 160W! That requires a massive heatsink with active cooling.

What if the BMS is located in an enclosed battery box with no external cooling?
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When charging is disabled: Once the BMS starts drawing power, it automatically opens the charge MOSFET so the current doesn't flow through the body diode. When the current starts to drop, the charge MOSFET is disabled again so no charge can happen.

Same thing for when discharge is disabled. It's explained in the first post in this thread.
 
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Ampster said:
Use a car headlight or resistor to pull down the voltage of that cell before it trips the BMS.
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Thanks for replying. It runs off at the top and the bottom. It's just a weak cell. I've tried many things.

My main problem is that my BMS keeps freaking out and shutting OFF and locking out each time I have a cell over-voltage trip. If it stops charging a little early, I'm ok with that, but then it won't reset and keep working and start discharging once it's time.
 
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theshaww said:
My main problem is that my inverter is freaking out and shutting OFF each time I have a cell over-voltage trip.
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Technically it is the BMS that is shutting down the battery which shuts off the inverter. It is doing what it is programmed to do, nothing unusual about that behavior. Do you know what voltage the cell is at when that happens? If you can reduce the charging voltage to prevent that it should work, but at reduced capacity.
 
Ampster said:
Technically it is the BMS that is shutting down the battery which shuts off the inverter. It is doing what it is programmed to do, nothing unusual about that behavior. Do you know what voltage the cell is at when that happens? If you can reduce the charging voltage to prevent that it should work, but at reduced capacity.
Click to expand...
I have 7 batteries in parallel. 1 shuts down, it's ok with me, the other 6 keep charging. My problem is that the one that used to shut down early, used to pick right back up once they started to discharge and all was well. Now, all of a sudden, the BMS just locks out and it won't turn back on without a reset. I understand how the inverter and BMS are intended to work, but I use Growatt, and it won't charge well when charging at a voltage under 56v. Thanks for your reply.
 
Ampster said:
So your inverter is not freaking out anymore and you are running on six of seven batteries?
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I said inverter above when I meant to type BMS... The BMS just won't start back up after it shuts down the battery. I Fixed my post above. Meant to say:

My main problem is that my BMS keeps freaking out and shutting OFF and locking out each time I have a cell over-voltage trip. If it stops charging a little early, I'm ok with that, but then it won't reset and keep working and start discharging once it's time.
 
theshaww said:
I said inverter above when I meant to type BMS... The BMS just won't start back up after it shuts down the battery. I Fixed my post above. Meant to say:

My main problem is that my BMS keeps freaking out and shutting OFF and locking out each time I have a cell over-voltage trip. If it stops charging a little early, I'm ok with that, but then it won't reset and keep working and start discharging once it's time.
Click to expand...
I saw an earlier post that asked if that battery is still under warranty, but I did not see the answer?
 
Ampster said:
Then your choice is to remove it from the pack or. as others have suggested, rewire with an active balancer if that will get some benefit from that battery. Otherwise write it off to experience.
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Yep, I may just rewire it with an additional 20Ah battery in parallel with the weak cell...
 
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