izeman said:sure this will work fine. you could even use it as designed. what's the expected wattage of your powerwall?
with 5 genuine 3077 i can do 60A all day long without issues. if you fully equip the BMS with 10 genuine parts you may be able to double that current (even though double current means 4x the heat, but as i said my BMS runs cool at 60A, and you can still add a big heat sink w/ or w/o fan if needed).
just don't expect any noticeable balancing. with those super small bleeding resistors it will take probably years to balance an unbalanced pack.
i tried to do what you suggested, but it's a hard task, as there is very little space. you need to bend the resistor's pins and still it's a hassle. i would recommend against it.glyndwr said:Which are the balacing resistors, are they the very small ones between the fet legs or the R404 resistors that are clear in the pic in page 25?
If they are accessible, thinking outside the box, could a 1watt resistor be soldered off the board over the original resistors to create a larger drain?
glyndwr said:Which are the balacing resistors, are they the very small ones between the fet legs or the R404 resistors that are clear in the pic in page 25?
If they are accessible, thinking outside the box, could a 1watt resistor be soldered off the board over the original resistors to create a larger drain?
glyndwr said:Thanks Electricgod,
great tech explanation, understood, not worth the grief.
I`m not too bothered about which smart bluetooth bms i buy, whats your thoughts on this one
https://www.aliexpress.com/item/Smart-Display-300A-200A-150A-100A-70A-Lithium-Battery-Protection-Board-Balance-BMS-Coulomb-Meter-Li-ion-Lipo-lifepo4-8S-21S-24S/32826820690.html
In the desrcription it says a 200ma balance rate.
Price is basically the same but this has a built in very accurate current sensor (how accurate though who knows) the pcb does look well made though, whats your thoughts on it?
Thanks.
flippy said:even if you pushed 1A on each leg you would burn off 4W easy per balance lead. you need the resistors to stand up and have a nice loud fan on them to keep the whole thing from burning up.
glyndwr said:Thanks ElectricGod,
it seems you have alot of confidence in that type of bms, do you personally prefer this bms over the other, i`m more inclined to buy this one now, my max current is 50 amps, the lowest bms board they do is 70 amp, ive watched maybe all the youtube videos i could find on this bms, some really interesting ones covering the full install, which is very good to see.
Would you say the screen was a useful item to have? or just utilise the bluetooth and use a tablet or phone?
i did see a commet on youtube from one user whose bluetooth module had failed, something to do with the 3.3v regulator on the bluetooth part of the pcb, anyhowss, he then installed an external bluetooth receiver.
any help greatly appreciated.
Anthony.
glyndwr said:Thanks ElectricGod,
great advice, and much appreciated. Thank you.
Anthony
Inwo said:Balance current.
The main thing that slows testing unknown batteries is low balance current. A charger cant charge faster than the balance circuit bleeds current from high cells.
What I need is an add-on 5 amp balance board, driven by bms.
A stand alone board may also be ok. A couple years ago, I made some stand alone boards, but only good for about 2 amps.
I did not have enough gain, so turn on was slow. If I can connect to bms circuit, turn on should be sharp.
The thing is, when I need it, like now, I have no time to build it. When I'm finished testing my 320 LiFePo4 75ah cells, I'll have time, but no desire. Like fixing the roof.
lionman said:They should design the BMS with fewer, but more powerful discharge lanes.
Balancing performance would be a lot better in most situations if they had say 1/3 the number of lanes with 3x the discharge rating.
Just need some circuitry that ensures that 2 groups never connected to the same discharge lane, which some clever engineer could come up with.
ElectricGod said:I like that idea! Tap into the control lines for each cell and then pass those signals over to a parallel running board that can handle some significant amperage. There would need to be a custom made board for this that uses large resistors and larger SMT transistors, but it would not be hard to design. I think 2 resistors and a mosfet are all you need per channel on the add-on board. Snag the control signal from the gates for each mosfet on the BMS and send that over to the add-on board. The BMS should be none the wiser that it is balancing at 50mA or 5 amps.
ElectricGod said:lionman said:They should design the BMS with fewer, but more powerful discharge lanes.
Balancing performance would be a lot better in most situations if they had say 1/3 the number of lanes with 3x the discharge rating.
Just need some circuitry that ensures that 2 groups never connected to the same discharge lane, which some clever engineer could come up with.
Except that lots of people don't want an 8S BMS. 16S is pushing the lower limit for me. Most of my builds are 20S and I'm moving (slowly) forward on a 32S build.
What I wish is that BMS balancing had something to do with the current handling. 50mA balancing is pretty common despite the BMS being capable of 20 amps or 160 amps.
Balancing a 64 Ah pack at 50mA will take forever. It's fine for like 10 Ah, but for larger packs, it's just not enough balancing current. My band-aide is setting the balance start voltage low so that balancing gets a chance to do something on larger packs. The other option is to use multiple BMS's...which I also do.
themelon said:ElectricGod said:I like that idea! Tap into the control lines for each cell and then pass those signals over to a parallel running board that can handle some significant amperage. There would need to be a custom made board for this that uses large resistors and larger SMT transistors, but it would not be hard to design. I think 2 resistors and a mosfet are all you need per channel on the add-on board. Snag the control signal from the gates for each mosfet on the BMS and send that over to the add-on board. The BMS should be none the wiser that it is balancing at 50mA or 5 amps.
Not cheap and not quite what you are talking about but I just happend across these:
https://www.ebay.com/itm/323194962541
ElectricGod said:themelon said:ElectricGod said:I like that idea! Tap into the control lines for each cell and then pass those signals over to a parallel running board that can handle some significant amperage. There would need to be a custom made board for this that uses large resistors and larger SMT transistors, but it would not be hard to design. I think 2 resistors and a mosfet are all you need per channel on the add-on board. Snag the control signal from the gates for each mosfet on the BMS and send that over to the add-on board. The BMS should be none the wiser that it is balancing at 50mA or 5 amps.
Not cheap and not quite what you are talking about but I just happend across these:
https://www.ebay.com/itm/323194962541
I have run across active balancers before...found a couple on aliexpress...can't remember where now. What I remember is they are a good bit cheaper than this auction.