5 cell bms

[andrew.box]

Hi, I got some 5 cell li-ion BMS pcb's on ebay and am wondering how they're supposed to perform. It doesn't seem that they are preventing the pack from discharging as I would expect. I'm connecting them as shown in the diagrams and taking power from the P+ and P- terminals. Using a hobby charger set to the NiMH discharge setting I am able to discharge down to 12.5V, or presumably lower but I don't want to, however I expect the pcb to shut off the pack at around 13.2V (2.6V per cell). I'm looking at the voltage at B- and B+ (so the actual battery voltage, not the pcb output).

Anyone used these before and know anything about them? Not sure if it's defective or I'm misunderstanding something. Here's a picture of the pcb.

 

[andrew.box]

Also, over current protection doesn't seem to work. If I plug it into my 18.5V drill, and hold the bit to give some resistance, the FETs get blistering hot almost immediately.

Perhaps this PCB is just a P.O.S.?

 

[dnmun]

why do you think it is a bms? you have no pictures of your actual device and how it connects to the batteries or do you mention what kinda batteries you are using or how the batteries are charged or how they are discharged or how those power mosfets are controlled and what they do when they are switched.

so what is the question? no info to work from.

 

[andrew.box]

Ok, maybe it's a pcb not a true BMS, I'm pretty sure it doesn't balance. Can you not see the picture of the device in my post?

I wish I had a schematic, but I don't. I just thought it was likely someone on here was familiar with this device, and maybe had a schematic or experience with it. It looks like the negative terminal of the battery goes through a 0.05 ohm resistor, then through 2 FETs in series (8 pin devices) with the gate of the 1st FET meant to control or cutoff the current. Not completely sure though, there is also a small, multipin IC on the chip that has the numbers scratched off that must look at all the voltages and compare them, then do something.

I'm using low cost 18650 li -ons. Probably not the best choice in hindsight, I would get turnigy lipos if I did it again, but these seem to have a solid 2000mAh capacity but probably lacking in high current delivery.

The project is rebuilding 19.2V nicad battery packs as 5S li-ion, the charger I'll use is a accucel 6 hobby charger (does up to 5S lipo/li-ion and max power of 50W or 6A, whichever comes first).

The question is, how is this supposed to respond when battery (Cell or pack) voltage drops below the cutoff level? What is the norm for this type of device (this is probably from something like a laptop or camcorder battery OEM application)? Are the power FETs simply turned on/off to increase resistance between the battery and load under low voltage or short circuit conditions?

 

[dnmun]

the labels indicate it should have 6 cells though. since we cannot see yours, there is no way to know what you have done.

what are the traces on the back and there are 6 caps in a row on top so i would guess first off that you just don't have the correct wiring of the batteries, but i agree it is likely for li ion, but doesn't look like the BMS in the laptop packs i have opened.

 

[andrew.box]

Indeed, it does have provisions for 6 cells. I've attaches images of mine and the picture of how to connect it using 5 cells (from the seller). For the connection of the + on B5, I used the B+ terminal (the diagrams says B5+ B6- or B6+/B+ terminal is OK).

 

[andrew.box]

the other picture

 

[parabellum]

Are you discharging on P+ P- ?

 

[andrew.box]

Ya, discharging on P+ P-.

 

[amberwolf]

Based on less than a minute of googling, I assume one of the boards here:
http://stores.ebay.com/Recycle-Powers-Store/_i.html?_nkw=6+cell+pcb&submit=Search&_sid=188671482
is the one you're using?

If so, they list these specs for a 6-cell version:

Li-Ion and Li-polymer Battery packs must have a protection circuit module to keep Li-Ion/Polymer Battery from over charge, over discharge, and to prevent accidental battery explosion due to its extra high energy density. This protection circuit is specially designed for 22.2V Li-Ion Battery packs made of Li-ion and Li-Polymer cells.

100% Brand New

Keeps 22.2V Li-Ion Battery packs from overcharge and over discharge
Limit the Max discharge Current to 7.0 A.
Designed for 6 Cells Li-ion /Li-polymer Battery Packs

Dimension: 108mm x 40mm x 10mm

designed for: 16340 17500 17650 18500 18650 or any other 6pcs 22.2V battery.

It's possible that it will not actually work with anything other than 6 cells, for 22.2V. I don't suppose you've tried a different one with 6 cells just to see if it makes a difference?



There is also this listing:
http://stores.ebay.com/Recycle-Powers-Store/_i.html?_nkw=5+cell+pcb&submit=Search&_sid=188671482
for 5-cell versions. Specs and photos appear the same, other than saying they're for 5-cells at 18.5V.


Maybe the seller mixed them up if there are actually two different kinds? Or more likely there is something else going on, or more info needed to actually get them to work as expected.

 

[andrew.box]

Yes, that's the exact one and the exact seller. I will try it with 6 cells, it might be the case that they'll only work with 6. Not sure whey the 5 and 6 cell versions would be identical. Too bad with these types of products you almost never get any good documentation.

 

[Ricky_nz]

On the sellers ebay page if you look at the photos carefully there are a couple of smd resistors in different positions between the 5 and 6 cell versions so you might be able to identify which one you actually have.

 

[parabellum]

I had very bad results with smart BMS (micro controlled) from BMSbattery.com. Almost any failure lets the BMS to kind of shutdown (maybe to protect it self ) and mostly open instead of closed , so you have unprotected battery without knowing that. They got funny reset capability so the brain gets on again, I finally was tired to flash it and trow it into trash bin.
Notice, even if it is 6s BMS and it works fine, do you really want this crappy thing to protect your batteries? If something is wrong it shouldn't permit you to discharge at all in first place.

 

[andrew.box]

I was looking at bmsbattery.com stuff a bit, too bad to hear you had some not so good results with one of those units. If this one is anything like that then I probably don't want it protecting my batteries. I'm half tempted to just get a low voltage alarm from hobby king and stick it in there, when it beeps just go charge em up.

 

[parabellum]

Unfortunately have not heard about such thing as cheap, reliable, digital BMS. For cheap, analog is still the better way to go, human BMS works best for me.

 

[amberwolf]

I'm half tempted to just get a low voltage alarm from hobby king and stick it in there, when it beeps just go charge em up.

Have you looked at the Methods' HVC/LVC boards? They dont' have a beeper on htem, but you could easily add one for the system, and they would take a lot less power to run them than the HK stuff (less drain off the pack). As an example, Auraslip recently posted that his battery monitors were actually causing uneven drain on his packs, because of the amount of power they use. Over 500 miles it was becoming problematic.

 

[andrew.box]

I haven't looked at those, I will do a search and see what I can find. Thanks for the tip! It's true that the beeper probably wouldn't work for my drill packs since they have an LED that's always on etc, probably way too much drain.

 

[bigmoose]

Andrew, is there a number on U2 the larger surface mount IC?

 

[andrew.box]

It's been a while I know, but I ended up using 4S 1300 mAH lipo packs inside my 19.2V craftsman drill packs. Instead of a BMS, I just used HK low voltage alarms with a momentary switch. Not elegant, but every few minutes I can hold the switch and make sure the low blue LED is still on solid, when it flashes I know I'm getting low and it's time to recharge. It's been working well for a while now. Lipo is way better than the old nicad cells were. I can run my 19.2V string trimmer for 20 minutes plus and still have battery to spare.