BMS boards not current protecting popping FET's  - Page 1
Forum section: electronics / projects

author: davisdesigns
author_rank: Contributor
timestamp: February 05, 2022, 02:50:20 am
content: What am I doing wrong here am I misunderstanding the term current protection on thes BMS boards? sure when the mosfets explode I guess it's technically protecting the battery pack that is if they don't short like they did on one of the other brand BMS's I had.They shorted on the charging side which was my fault using too high voltage but it's in no way protecting the batteries if the FET's short right.. I noticed something was wrong when I saw charge voltage at 4.45v and the BMS is supposed to cut off at 4.2v that was a balance board.I'm going to leave a lot of information out because it would just be too long i'm not going 18650 anymore.This is the current set-up (pic's and data) that I ran lipo packs BMS etc.I benched tested set up and seem to work fine how to clamp meter on it cut out at 75 amp stall, supposed to be cutting out at 60 amp the FET's just got barely warm.So today I started packaging the whole battery pack soldering on better wire and heavier gauge on B- B+ P- P+ ...tested it again everything seemed to work. Wrap it all up with capstan tape made a slot for the balance charge port in the case modified the terminals... tested it again...I was then ready to put the pack in the lower half of the case when I decided to retest and use my hand as friction to do a stall test again I heard a pop pop then no power and noticed that a FET's is blown out?? No shorts from my soldering..What am I doing wrong here are these BMS boards not really overcurrent protecting as stated and just junk or am I misunderstanding the terminology being used "current protection" ?Any ideas on how I should proceed better way to protect the batteries from over discharging? that's all I care about well if I could have a temp cutoff that would be nice but not necessary.I have seen these adjustable voltage cutouts for cheap but they are big, require a press of a button when tripped and can't handle the current of cordless tools under load. The only thing I can think of is find a small self resetting circuit breaker and use that with the BMS?and yes I'm sticking with the Lipo packs 18650 40c are way to expensive.I'm using 120c 4s 1500mAh liposKIA100no3ahttps://radioremont.com/wa-data/public/shop/products/14/40/4014/attachments/KIA50N03A.pdfBM3451 tndc t20b not correct data sheet but close "tndc t20b"http://www.chinesechip.com/files/2015-07/216f8baf-ac82-4f52-8f55-539fee4c97ef.pdfwell for get the images not going to jump threw the hoops of finding a place to upload then figure out what is the correct html to get them to show here.. sorry try to help people help me as much as I can but if I can upload a image or control V it in...so here is a amazon link on what I havehttps://www.amazon.com/dp/B08PTWGD7H?ref=ppx_yo2_dt_b_product_details&th=1

author: HumbleDeer
author_rank: Contributor
timestamp: February 05, 2022, 03:56:27 am
content: The MOSFETs switch the load resistors (or load devices) that are used to drop excess current during balancing, as well as switching the load you attach on the output. If you short the MOSFETs, you short your protection too. Some boards have different sets of MOSFETs for the two purposes, but it's not common - it's cheaper not to.Please show pictures of what you are physically hooking up. If the MOSFETs keep blowing, that means the module can't sense the overcurrent. So you might be wiring something up that goes through the mosfets, but not through current sense. 60A is not the rated current for it to sustain. It's probably about four. 60A is where it cuts out if you were to suddenly draw a peak that high - on the outputs.try PostImage for uploading images. Then copy the 'Forums' or 'BBCode' option.

author: oPossum
author_rank: Super Contributor
timestamp: February 05, 2022, 04:09:50 am
content: Suddenly disconnecting a load can create a voltage spike that can destroy MOSFETs due to max Vgs being exceeded. Your motor controller should limit current draw to below the battery protect cutout. You really don't want the battery protect to ever trip during normal operation.

author: davisdesigns
author_rank: Contributor
timestamp: February 05, 2022, 04:18:42 am
content: You are mixing two separate BMS issues.. Not concerned with the over charge issue.. I'm balance charging with balance charger now less to go wrong with these BMS's I covered the photo uploading issue at the bottom of the post.. to many hoops to jump through to get a photo here

author: davisdesigns
author_rank: Contributor
timestamp: February 05, 2022, 04:20:44 am
content: my cordless tools where Nickel-metal hydride battery, no protection for lipo

author: davisdesigns
author_rank: Contributor
timestamp: February 05, 2022, 04:27:21 am
content: and no sudden disconnect. plugged it in ran it then stall tested it.. POP.. FET gone.It is wired correctly I am 100% sure of that. I designed and assembled PCB's for a decade, Just don't fully comprehend how a circuit works..

author: uer166
author_rank: Super Contributor
timestamp: February 05, 2022, 04:32:12 am
content: If you want people to diagnose your issues, don't be lazy and actually provide all the info: full spec of your pack (voltage, current, what kind of BMS), provide schematics of the full pack, as well the type of load you're trying to power.

author: oPossum
author_rank: Super Contributor
timestamp: February 05, 2022, 04:35:11 am
content: Quote from: davisdesigns on February 05, 2022, 04:27:21 amThe sudden disconnect would be when the overcurrent protect trips. The MOSFET opens, voltage spikes, MOSFET is destroyed due to Vgs max being exceeded.

author: davisdesigns
author_rank: Contributor
timestamp: February 05, 2022, 07:15:37 am
content: ahh did you not read the post I did supply all that including data sheets on the FET's and controller on the BMS and the BMS that has all the parameters . I can't post picture here because the setup is archaic and i'm not a web designer so I don't know html or what ever you need to have a picture show up.

author: davisdesigns
author_rank: Contributor
timestamp: February 05, 2022, 07:27:47 am
content: Like an ignition coil when the points break and the gap in the FET was equivalent to the spark plug gap. ? So ultimately one need to match the current draw to the BMS ? I'm sure the drill stalled hits a 100A on a fresh pack but only draws less then 10A just running.The data on the FET says 200A peak pulse but they are Chines so probably half that. think i might try and find quality replacement FET's or do you think that would still have the same issue?

author: Siwastaja
author_rank: Super Contributor
timestamp: February 05, 2022, 08:03:14 am
content: I'd say, the only thing you are doing wrong is to use broken-by-design crap.Very usual trap. Designing an operational BMS which actually works is non-trivial. Cheap Ali modules are hit-or-miss, and more likely miss. Also DIYing your own following appnotes by TI & similar companies also likely results a dangerous and nonfunctional product.Which is why I laugh at people who lecture others about the importance ofhavinga BMS, but with no emphasis at all which kind and which brand of BMS, and how it is designed.Worst, MOSFETs tend to fail short, so they stop protecting and let the cells overcharge or overdischarge.There is no other way but to design the MOSFET stage properly. Means actual desaturation protection, SOA protection, snubbing voltage peaks of fast switching, etc.I'm sorry I also can't recommend a working product. I design my own. I'd guess you need to pay ridiculously high prices and still do a lot of homework if you want to buy a pre-engineered solution. So hobbyists just practically need to rely on crappy products and hope they have luck.

author: davisdesigns
author_rank: Contributor
timestamp: February 06, 2022, 07:54:10 pm
content: That's all that's available in a small package, cheap junk. Daly is far too big and expensive for a cordless tool battery pack.Do you know of a schematic that shows how the FET's should be designed, maybe I can modify these boards?