Daddy Tanuki
Solar Wizard
you can also blow a BMS as I found out the hard way. roasted two of them due to inrush current/spark on a 48 volt system. expensive mistake.
Do LFP produce heat with discharge?
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Roswell Bob
Solar Enthusiast
Take a look at this thing. https://www.ametherm.com/datasheetspdf/SL320R230.pdf
Roswell Bob
Solar Enthusiast
What kind of a BMS was it that you smoked? There has been a lot of chatter recently concerning BMS capability as far as surviving direct short circuits and powering into a large cap bank. My feeling is that a BMS should be able to handle any type of short or power up into any size cap bank. It isn't difficult to design this into BMS. I am curious as to how you solved problem.
Roswell Bob
Solar Enthusiast
Yes, but there are far worse places to live than Wyoming
Yes, that can do, but it's not necessary to use a thermistor. You can just use a power resistor (say, 20 Ohm/50W) instead, something like this.
Roswell Bob
Solar Enthusiast
Yes, a resistor works fine if you bypass it when caps charged up. I've used these before without a contactor. Not very efficient though.
Are you familiar with Stromberg or Vaasa Controls? They make industrial gear up your way. I worked with them on a few projects many years ago.
But precharge is only a couple of seconds, so not a big deal. I have one that is just a push button with a resistor wired across the main inverter cut-off switch.
Yes, very familiar. Same as some others in the area such as Vacon (now Danfoss), The Switch, Arcteq, ABB, etc.
Roswell Bob
Solar Enthusiast
Yes. precharge is only a short time. I have heard stories of fuse clearing and blown up BMS from inrush. I am curious as to how BMS are designed to account for inrush. I have said before that a BMS should be able to protect itself from starting into a dead short or a dead short applied while it is up and running.
I may put a few of these inrush limiters in parallel when i switch over to LIFePO4 on the large inverter.
I worked for ASEA in the 1980s. When Brown Boveri and ASEA merged Stromberg was aquired. The engineers we worked with from Finland were unhappy because Stromberg was closing a facility in Vaasa. That is when a few of them spun off Vaasa Controls (Vacon). It was an interesting time.
I've tested that on mine. Several times on purpose even
The JK BMS (100A version) handles it fine in combination with a Class T fuse. The Fuse blows and the BMS is undamaged. This is when coupled to a 6kW inverter on a 48V system. The fuse was 225A rated.
I don't think a BMS in and of itself can handle it. It's well out of spec for any of those FETs.
As I mentioned, Vacon is since a few years acquired by Danfoss. I believe some of the engineers have started a new business because of that. History repeating itself.
Roswell Bob
Solar Enthusiast
I am using the Overkill solar 100A BMS on 48v system with a 6kW Growatt inverter. I think that the Overkill BMS is very much like JK BMS. I did a spice model and thought the 200A Class T fuse had a good chance of surviving power up with some wiring impedance. (Signature Solar told me 6800uF x 4 capacitors.) I am considering the inrush limiter without a bypass contactor to get it connected to LiFePO4 cells. Maybe do a small PC board to run a contactor with short delay. I am also considering omitting bus bars and going with #8 wire between caps. A couple of milliohms of impedance is enough to manage the current and keep fuse from blowing. Do you know what your bus cap arrangement is? I can run quick spice model and see what current looks like.I've tested that on mine. Several times on purpose even
The JK BMS (100A version) handles it fine in combination with a Class T fuse. The Fuse blows and the BMS is undamaged. This is when coupled to a 6kW inverter on a 48V system. The fuse was 225A rated.
I don't think a BMS in and of itself can handle it. It's well out of spec for any of those FETs.
Yes, BMS may nit handle it but, I recently did a 600A/1000V breaker for a client. Breaker had to charge a large bank of capacitors softly. I measured heatsink temp by fet along with the voltage across fet. I was able to control drain current so that the product of Vds and drain current gave me a power such that the junction temperature rise over the jheatsink kept Tj<125c. The key is controlling the drain current using Vgs and the part transconductance. The breaker had nearly 100 SiC fets, but only one fet to charge the bus. I recall we had to be able to turn off 10,000A during a fault.
Same on mine. I ran a Spice model on inverters years ago, thanks for reminding me.
This is the power board on my inverter (I have spares for both power and control board, just in case):
This is an example of the BMS breaking because of the inrush current:
Heltec (JK) 200A Smart BMS with 2A Active Balance
GXMnow: I had thought about pulling one cell out of each of these, and going with 7s, even if the intent is to end up in series at 14s. It is a better voltage range for the majority of inverters out there. These are bolted together Samsungs, so a cell will just pop out.
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Roswell Bob
Solar Enthusiast
No doubt that there are BMS which cannot deal with inrush.This is an example of the BMS breaking because of the inrush current:
Heltec (JK) 200A Smart BMS with 2A Active Balance
GXMnow: I had thought about pulling one cell out of each of these, and going with 7s, even if the intent is to end up in series at 14s. It is a better voltage range for the majority of inverters out there. These are bolted together Samsungs, so a cell will just pop out.
Daddy Tanuki
Solar Wizard
They were Daly 16s 200 amp. toasted two of them in a row before my calcified brain realized the problem and took corrective measures.
Since then no problems.
Roswell Bob
Solar Enthusiast
Can you tell us what you did to fix problem? I am looking at doing some kind of inrush limiter and I would like to know options. I prefer not to design and lay out a board and so forth. Would rather buy off the shelf if something decent available.
Daddy Tanuki
Solar Wizard
OK
so simply put I used a 15 watt resistor to manually set the BMS. it was the initial inrush that fried the unit. Basically I started with one BMS, and then went to convert the system to 3 (three) bms's. i used one to fire up the system initially, and then tried to substitute the second unit when I noticed that I could not shut off the discharge side of the the BMS. not sure why as even now that same BMS will not shut off discharge. instead I tried to swap out two of them and roasted both of them when I swapped them out. I then started using a resistor to fire them up and that issue was solved. I installed two more when I changed the pack from 3p16S to 16s3P. and had no issues at that time, other than getting the packs to play nice with each other.
Roswell Bob
Solar Enthusiast
Yes. I think when you short one fet you lose control of the other function. There is a short blurb here that explains it in detail. Basically two back to back fets so you can control current in either direction.
Link:
BMS MOSFETs Explained
I'm writing this post since it seems there is a lot of confusion (especially in the comment section of a certain youtuber in Australia) on why both MOSFETs in a typical single port BMS are controlled individually and why/how this works when they're in the same series circuit. It will hopefully...
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