Big Battery 24V inconsistencies

Well I received the replacement unit from BB on Saturday and this one was supposedly a brand new unit but the case looks pretty rough with scrat he's and such. However I tried charging it the past few days but I can only get it to charge a max of 27.2v and then it just stops charging. The power switch on this does work but even after turning it off all night, by the next day it loses the.2v and is on 27.0.

I'm wondering of this unit has bad cells in it or something. Maybe someone can tell me what they think.

Atomic Ed said:

Maybe someone can tell me what they think.

Click to expand...

I think we did, don't deal with BB....

It sounds like they sent Bad Battery as replacement

NwCali said:

I think we did, don't deal with BB....

Click to expand...

Very funny. I meant with the battery not going beyond 27.2v

curiouscarbon said:

It sounds like they sent Bad Battery as replacement

Click to expand...

Is that what a bad battery would do? I wonder if they sent me someone else's return

Try cycling the battery 5-10 times, fully charged, fully discharged, one cell might just need to get in line. Also try and load test it see if you’re missing some Amp hours.

Another option is just let it bake at +27v and see if the passive balancer of the BMS will bring the low cell up.

Last ditch effort crack open the case find the low cell and see if you can top balance it.

I looked at some of the Big Battery stuff. Glad I read this thread.

without more info it’s difficult to differentiate between bad cells or other factors from what i gather.

may i ask, do they continue to take in ampere hours while staying stuck at 3.4vpc?

perhaps the high cell voltage cutoff activate and charge is blocked by bms? waiting for passive balancing to catch up might help if cells are simply not balanced, as mentioned by another

may i ask, what voltage charger is set to?

if the battery cell terminals cannot reach at least ~3.6 volt per cell during an attempted charge to 3.6 volts per cell, then i personally would consider them defective. but i’m an amateur, there are many ways to quantify he performance. usually i only configure charger to go up to 3.55 volt per cell for headroom of imbalance. but everyone has different situation.

hope this helps somewhat. sorry it’s wonky ?

740GLE said:

Try cycling the battery 5-10 times, fully charged, fully discharged, one cell might just need to get in line. Also try and load test it see if you’re missing some Amp hours.

Another option is just let it bake at +27v and see if the passive balancer of the BMS will bring the low cell up.

Last ditch effort crack open the case find the low cell and see if you can top balance it.

Click to expand...

Thanks but I haven't cycled the battery yet as I could never get it to fully charge. I have been charging it each day and it never goes beyond 27.2v and I turn the battery off at the end of the day only to find it ba k at 27.0v the next time I turn it back on.

I have contacted my dealer again and they are looking into it and will get back to me soon I hope. As far as opening the battery to check cells, I will not open it as it is supposedly a new battery and I do not want to mess with something that is already likely defective. I paid for a new battery, waited months and now on my second unit from them with problems and still have no working system.

thank you for the update

?sorry to hear that your system completion is being delayed

curiouscarbon said:

without more info it’s difficult to differentiate between bad cells or other factors from what i gather.

may i ask, do they continue to take in ampere hours while staying stuck at 3.4vpc?

perhaps the high cell voltage cutoff activate and charge is blocked by bms? waiting for passive balancing to catch up might help if cells are simply not balanced, as mentioned by another

may i ask, what voltage charger is set to?

if the battery cell terminals cannot reach at least ~3.6 volt per cell during an attempted charge to 3.6 volts per cell, then i personally would consider them defective. but i’m an amateur, there are many ways to quantify he performance. usually i only configure charger to go up to 3.55 volt per cell for headroom of imbalance. but everyone has different situation.

hope this helps somewhat. sorry it’s wonky ?

Click to expand...

Hi. I do not have access to check the individual cells as this is the pre-built hawk unit from BB and I will not open it up as its supposed to be new and will not want to oid the warranty. Basically this is the replacement they sent which seems to be defective, for the original unit I received from them defective from the start with bad power button and losing charge whith no loads.

The charge settings are 29.2v as stated in the literature from BB as thier recommended setting. However this battery will not charge past 27.2v even for days on charge. I think at this point they just sent me a defective returned battery from someone else's return. I also noticed the case looks rough compared to the first one and the rope handles on this one are red where the first one was white. Maybe likely its an older unit and the fact it won't charge seems to be telling.

The first unit would charge all the way to 29.2v this one will not. I am at my wits end with this whole battery fiasco and BB, only my dealer has been decent to me throught this whole mess. I will see what they tell me and I am ready to wash my hands of this nightmare, get a refund and move on to something else now that months of my time have been wasted.

curiouscarbon said:

thank you for the update

?sorry to hear that your system completion is being delayed

Click to expand...

Thank you for the kind words. I needed that as this has been a tough thing to deal with for months now and unfortunately concurrent with a serious family medical emergency with my Dad.

Its really amazing how difficult it is to obtain a new working battery.

thx for the charger voltage parameter! that helps my understanding of what might be goin on.

good call to not open it up. don’t want refund to be denied because of “end user modification”

29.2/8=3.65v per cell. that’s right at the max allowable voltage for LFP. so the pack should absolutely make it past 27.2V if left for (pack kwh capacity) / (charger power) hours while delivering power.
27.2/8=3.4v per cell

coincidentally 3.4 volt per cell is the general threshold where cell voltage imbalances really scale up. for a given unit of watt hour going into the cell, the voltage will change much faster for that unit at 3.4 than at 3.2.

current personal theory is that one or more cells are reaching 3.65 and others are lower like 3.5 or less or something. basically like if a few are high and some are low then it can be at 27.2V pack but not 3.400 volt on each cell at the end of day.

one aspect that i think i’m misunderstanding is whether the charger is delivering full power to the pack up until the end of day. if it is, and the voltage is not rising, the power has gotta go somewhere so i was thinking that the charger is sitting at 29.2V but the BMS cuts off charging at 27.2V because one or more cell reached 3.65V early while others are much lower like 3.4V.

sorry if this just confuses things

I’d say load test the unit, that’s really the best way to see what’s up with the battery, if it tests under their claimed capacity you have a valid claim to send it back and find a new battery for your project.

Also when you’re charging it does the BMS go into high voltage disconnect?

Yes when its charging it will disconnect at 27.2v then after a few minutes it will resume charging and then repeat the whole process over and over.

ok; that behavior of cycling between charging up to 27.2V, disconnecting for a few minutes, reconnect, repeat, does sound like there is a chance of cell voltage imbalance leading to cell high voltage disconnect being origin of behavior

i cannot confidently claim it is certainly the case due to unable to access cells without potentially endangering refund process. but i feel relatively confident that there is a chance that one or more cells are reaching 3.65v and the BMS sees that and disconnects, maybe even enables resistive balancing to pull voltage down, and when it goes back down one way or another, reconnect occurs and the cycle begins again. is theory

if a resistive balance function is enabled and using 30 milliamp then i would assume that means the low cells are effectively able to be charged at most 30 milliamps faster than the high cells. so if it is doing resistive balancing, the cycling may eventually bring it into working state. is theory

curiouscarbon said:

if a resistive balance function is enabled and using 30 milliamp then i would assume that means the low cells are effectively able to be charged at most 30 milliamps faster than the high cells. so if it is doing resistive balancing, the cycling may eventually bring it into working state. is theory

Click to expand...

Since this unit appears to be used and not brand new is it possible the cells are just weak? I have no idea what this unit had been through before they sent it to me. Besides on new units, wouldnt they test the cells prior to assembling the pack and not mix in weak ones?

As a bystander to it all, from the consistent reports of inconsistencies with the behavior of the cells from the org lead me to a sad conclusion that mixing in weak cells and hoping client either never needs specified max current or does not notice or gives up during RMA. Cynical interpretation admittedly and I am actively trying to reason myself out of this position. I even PMed the ceo one time on good terms to help with an error on the site and they were kinda responsive and that’s cool and stuff but now they are blocked because the psychological distress of combining these powerful electrochemical cells with unclear messaging during time of need is way too much for me in general but also during this time of public health focus (you are not the first person whose unfortunate account i have read). There was one case where a member reported that their pack was leaking electrolyte and was smelly which is a hazard. They uploaded a photo of a clearly corroded BMS connector and the ceo logged on and tried to brush it off as “white grease”. So now white grease is now a meme.

Anyways to recap this random stream of consciousness, Im really sorry that it’s dragged out this long for you. One thing is for sure. There are plenty of people here who will want to help you figure things out going forward. ? may your system builds go smoothly in the future.

Atomic Ed said:

Since this unit appears to be used and not brand new is it possible the cells are just weak? I have no idea what this unit had been through before they sent it to me. Besides on new units, wouldnt they test the cells prior to assembling the pack and not mix in weak ones?

Click to expand...

weak and/or imbalanced cells is the inference i ascribe to