Please review my battery pack so I don't make a 🔥

[deucesdown]

@thisguyhere

some pictures

I’ll post more details later when I have time but some photos for now.

[lazarus]

is less than 0.01A

Less than 0.1A, so it’ll take about 10 days to do a full discharge. If we assume balancing will only need to drain 10% then maybe 2 days to balance.

[thisguyhere]

it depends how out of balance it is.

so…let’s say cell 5 is like 0.05v below all others, bms will drain all cells except cell 5 down by 0.05v while continuing to charge cell 5 up.

hope that made sense.

[pookybear]

I rarely get my BMS hot. Just maybe warm ish. Sometimes, not at all.

[thisguyhere]

If all cells are in balance, bms doesnt go into balance charge, so no heat from resistors.

Also, check your charger voltage, it should be 50.4v, then when in CV, it should go up to 50.5v - extra voltage needed for balancing.

[pookybear]

CV?

I’ll check charger when I get home

Edit: Google helped me figure out what CV means.

[Ian-mountainboard]

is that for a 12s?

[Bataleon]

Would you recommend a smart BMS @deucesdown? I like the option it gives to lower the balance voltage, so you don’t continually need to recharge to 4.2v per cell and therefore increase cycle life.

[thisguyhere]

Yea 50.4v is 12s, 42v for 10s

[pookybear]

@thisguyhere

50.3v

Do I need to adjust it to 50.5v?

[Pedrodemio]

@thisguyhere

i’m finding these small bms ineffective for large packs - i mean, balance current on the d140 (and most bms for that matter) is less than 0.01A - so at that rate to balance a, let’s say 6p cell or 18Ah cell, that’ll take forever.

If your battery is becoming unbalanced you have other problems, not the BMS

Quality cells won’t go out of balance if your pack is built properly

[thisguyhere]

correct, but bms should correct even a variance of 0.01v.

when i measure drift in my own packs, all with hundreds of miles on them, they’re nearly perfectly balanced.

there’s a reason why people running large packs only even mess with a bms.

[thisguyhere]

you actually have to measure voltage with a load, ie when battery’s fully charged.

because no load, my chargers show 50.3v too, but when battery’s fully charge and it’s CV, it goes up to 50.5.v

[LEE]

Is the connector wiring correct? Incorrect wiring may cause abnormal heat generation. In my experience, BMS had two patterns due to connector miswiring. Unusual fever. Not charged.These are 2 patterns . Just in case, unplug the balance connector and measure the voltage per parallel. I misordered the balance line in the past and had an unusual fever in BMS. At that time, 7V was shown during a certain parallel. Some BMSs detect an abnormal voltage between parallels and do not start charging. Cheap BMS has no detection function and will charge even if the balance wiring is not correct.

[pookybear]

How do you check this if charger is plugged in the battery?

[Ian-mountainboard]

I dont know how cheap a cheap bms is. Its a bestech D140 charge only. but i checked over my voltages and balande wires over 10 times literally. https://medium.com/@esk8life.orders/checking-and-correcting-voltage-drift-in-a-battery-pack-616e139d2e3b i also followed this digram that was given to me from @thisguyhere who i bought it off.

[deucesdown]

@Bataleon

Would you recommend a smart BMS @deucesdown? I like the option it gives to lower the balance voltage, so you don’t continually need to recharge to 4.2v per cell and therefore increase cycle life.

So I think I’m in my own world of pain lol, but here’s my opinion. If we’re using charge only BMS, we’re only using 2 functions. Balancing when charging, which it does a piss poor job of, and cell overvoltage protection during charge, which can give you some indication your pack is out of whack. As everyone is saying, good packs tend to stay in balance. Cheap bms I really don’t think they can measure voltage accurately and precisely enough to hit 4.20v on each cell. So whatever calibration drift there is within the BMS is how tightly your pack will be balanced. The smartBMS indicates cell voltage to 3 digits…

Given all this, I think we’re better off with a battery monitor. Something that shows you per cell voltage and alarms on over or under voltage and if there’s a big gap between cells. Charge to 4.1v per cell, don’t balance charge unless warranted.

Or, go full monty bms with discharge, and actually get full protection.

I think diebieMS is the class of the bunch, especialy with METR integration, but only goes to 12s and is large, expensive, and can be hard to get ahold of. The smartbms to me feel cheap, but at least you can see what’s going on. They have versions up to like 120a if you have space. The downsides are, they’re a bit big, and in some versions, the bluetooth module drain the battery even in standby. Upsides are, bluetooth per cell voltage, can control balancing very well, has a remote LCD available, and @janpom daVEGA can show per cell voltage. Oh and the small one goes to 15s, great for 13s or lifepo4 15s. If you use it for discharge as well, the app has some logging, and I think you can use it as e-switch, and maybe even password lockout your board via app.

If you get a smartBMS, get at least 40a. The lesser ones have fish paper on bottom rather than another aluminum plate.

PS if you’re charging to 4.1v per cell consistently the cells do not necessarily need to be balanced. Top balancing (balancing when pack is full) is to prevent any cell from exceeding max charge voltage. At 4.1v you’re not in danger. Your pack may be slightly out of balance at 4.1v per cell, but may balance out when it gets to 4.2v. In any case it’s good to be able to see per cell voltage so you can manage the pack.

[thisguyhere]

open it up, read the main terminals as it’s charging…

[thisguyhere]

or, i use a watt meter, i goes between source and load - but it’s yet another thing to buy.

https://www.amazon.com/gp/product/B07PKHPC1T

[pookybear]

Yea, I already have one of those. I use it when I charge. It seemed like it shows the battery voltage not the charger.

I’ll check again.