Melted wires between BMS and cells!

[robbob2112]

I’ve shared that the cells do seem to support it. See specs just posted.

The way I read it is they recommend 0.5C per cell for max longevity of the cells with it being allowable to run 1.0C if you need to... You have 2 cells in parallel to build it so you are actually running 0.75C per cell -- so no damage

Here’s some pix that were requested. Noticed that crimp sort of overlaps one another.

Getting the shrink wrap off the lug was a long process and it had melted badly.

YIKES - those are the fold over tab style ... they just press and hold but never really cold weld as enough force to do that would also deform the tabs and make them solid... with them lapped like the are just a bad job by Chins all around on the cables.

Here’s cables I’ve replaced with.

Notice that this brand recommended by many forum members for its flexibility has same melting temp as smaller ones that melted. Should be fine.

2 x 2/0 -- each 175a using a conservative chart --- 325amps using the windy nation 105c chart. --- either way 2 of them will more than carry the amps without getting warm.

Good job getting all your issues fixed... .

p.s. I did warn you to measure what length you needed on the grub screws --- 25mm is approx. 1in... --- the holes in the batteries are typically only 5mm deep

And we do appreciate you putting your problem out there for the world to see ...

And now I will go open my repacked wattcycle 300amp battery and re-do the cabling --- it didn't occur to me until this thread the wire lapping back on top of itself would be an issue.... I probably wouldn't run in to it until I had a failure.

 

[Glock24]

Someone commented on cells not supporting 1C. Here’s what I could find in the cell manual. Also attached cell manual.

There are 8 cells total for 12v 200Ah. 2P4S.

I believe that means and please correct me if I’m wrong but I can do 100A charge and 200A discharge, yes?

Document says charge and discharge current is 0.5C, maximum 1C.

You can charge or discharge at 1C for short periods of time. It says 120 seconds followed by a period of no current of the same duration. This is to allow cooldown.

Under normal use don't exceed 0.5C for continuois charge or discharge.

 

[Osmethne]

Please don't kick me while I'm already down, amigo.

Yeah, management really needs to do a better job communication that the purpose of this subforum is to learn how to avoid catastrophic failures through real life examples, not virtue signal by berating someone's possible mistakes.

Thank you for giving us this opportunity to learn.

 

[WorldwideDave]

For short durations. There are differences between max and continuous... I built the head in my GTI to rev to 9k, I don't run it there all the time, hardly ever since I'm out of boost that high...

if you have 2 cells in parallel, which keeps voltage the same but doubles amps (100Ah cells, FYI), doesn't that effectively change the C rating? Asked ChatGPT - is this wrong?

Individual Cell Specs:

• Capacity per cell: 100Ah
• C rating per cell: 0.5C
• This means each individual cell can safely discharge at a maximum current of 0.5 × 100Ah = 50A.

Parallel Configuration:
When you connect two cells in parallel, their capacities and discharge currents combine. So, if you have two 100Ah cells in parallel:

• Total capacity: 100Ah + 100Ah = 200Ah (the total capacity of the parallel setup).
• Maximum discharge current for the system: Each cell can discharge at 50A, so with two cells in parallel, the system can safely discharge at 50A + 50A = 100A.

C Rating in Parallel:

• C rating per cell still remains 0.5C for each individual cell.
• For the total parallel system, the C rating doesn't "increase" per se (because the cells are still rated at 0.5C each), but the system can handle a higher total current of 1C (because you're combining two cells).

In summary:

• Individual cell discharge rate: 50A (0.5C per cell)
• Total discharge rate for the parallel system: 100A (because the current capability of each cell adds together)

So, the C rating for each cell remains at 0.5C, but the total current capability of the parallel system is doubled to 100A, making it capable of handling 1C for the combined system. The C rating is per cell, but the system's ability to provide current increases with the parallel configuration.

Math looks right to me, but not an expert.

 

[WorldwideDave]

Pulling a high current for a long time is very different to pulling for a short time. I have oversized all my cables on this project, not because the right size can't handle the high current, but because it wont handle it for a long time without getting very hot.

I hear what you are saying. I followed some bad advice, and I've rectified it by replacing with matching conductors on both sides of the BMS. I've learned.

 

[42OhmsPA]

if you have 2 cells in parallel, which keeps voltage the same but doubles amps (100Ah cells, FYI), doesn't that effectively change the C rating? Asked ChatGPT - is this wrong?

Individual Cell Specs:

• Capacity per cell: 100Ah
• C rating per cell: 0.5C
• This means each individual cell can safely discharge at a maximum current of 0.5 × 100Ah = 50A.

Parallel Configuration:
When you connect two cells in parallel, their capacities and discharge currents combine. So, if you have two 100Ah cells in parallel:

• Total capacity: 100Ah + 100Ah = 200Ah (the total capacity of the parallel setup).
• Maximum discharge current for the system: Each cell can discharge at 50A, so with two cells in parallel, the system can safely discharge at 50A + 50A = 100A.

C Rating in Parallel:

• C rating per cell still remains 0.5C for each individual cell.
• For the total parallel system, the C rating doesn't "increase" per se (because the cells are still rated at 0.5C each), but the system can handle a higher total current of 1C (because you're combining two cells).

In summary:

• Individual cell discharge rate: 50A (0.5C per cell)
• Total discharge rate for the parallel system: 100A (because the current capability of each cell adds together)

So, the C rating for each cell remains at 0.5C, but the total current capability of the parallel system is doubled to 100A, making it capable of handling 1C for the combined system. The C rating is per cell, but the system's ability to provide current increases with the parallel configuration.

Math looks right to me, but not an expert.

Yea, current would add in parallel.

 

[robbob2112]

In parallel capacity adds voltage is the same, in series voltage adds and capacity is the same...

So putting two cells in parallel means each will provide 1/2 the current when you draw on them....

One note - with 2 cells in parallel you have no idea the state of each cell --- they will have slightly different internal resistances which means one will provide more current than the other .... if the cells are well matched that means 50/50 or 49/51 or similar... if they are poorly matched it will me 60/40 or worse

and if one fails the other will try to hold up the voltage --- if the failure is as an open then your capacity is cut in half and if it fails as a partial short it will have a lower overall voltage and will suck down the voltage of that cell and you will be constantly trying to balance the others.... This generates heat and a premature failure of the BMS with a burned board as the result -- there are several examples in this section of that...

The most likely cell failure is a partial short -- dendrites growing and shorting two plates together across the electrolyte...

Total short only happens during a vent or cell physical damage ...

or if the robot assembling it was having a bad day... I hope these all get rejected at the factory --- but IMO that is what (insert random letter name here) is making their batteries out of ...

 

[WorldwideDave]

The way I read it is they recommend 0.5C per cell for max longevity of the cells with it being allowable to run 1.0C if you need to... You have 2 cells in parallel to build it so you are actually running 0.75C per cell -- so no damage

You are suggesting that in parallel, it would go to 75A rating, and not 100A?
See my response to the other person on this - looks like 2 in parallel would be a full 1C across both, but not a scientist. Asked chatGPT. Reply to that if you think it is incorrect information. Looks like the individual cell stays at .5C, or 50A, even in parallel, but the 2 together goes to 1C, or 100A.

Reminder: 2 in parallel, 4 of those in series, for 200Ah 12V. Each are 100Ah cells.

 

[robbob2112]

No, I am saying in parallel the max is 1.0C (0.5C + 0.5C) and you are using 0.75C ....

 

[WorldwideDave]

Yeah, management really needs to do a better job communication that the purpose of this subforum is to learn how to avoid catastrophic failures through real life examples, not virtue signal by berating someone's possible mistakes.

Thank you for giving us this opportunity to learn.

while there are moderators - and they are some pretty cool cats - they definitely don't 'moderate' comments other than to move them to chit chat zone, where you can truly say whatever you want about anything and anybody. Cyberbullying hangout. Seen some truly racist, hurtful things over there from some very toxic individuals who clearly need to step away from the screen for a while.

 

[WorldwideDave]

Yea, current would add in parallel.

So then are you agreeing with ChatGPT that if you have two .5C cells in parallel that they become 1C when together? Asking because that is just what I believed.

 

[42OhmsPA]

So then are you agreeing with ChatGPT that if you have two .5C cells in parallel that they become 1C when together? Asking because that is just what I believed.

Yes, assuming the cells are perfectly matched, resistance for all connections identical, etc.

I also believe that just because something sounds good in theory doesn't mean it's good in real world operations. I also think best practice would be to parallel packs with separate BMS not parallel cells with a single BMS.

 

[WorldwideDave]

In parallel capacity adds voltage is the same, in series voltage adds and capacity is the same...

Agreed

So putting two cells in parallel means each will provide 1/2 the current when you draw on them....

so yes or no...you agreeing with ChatGPT that if you have two .5C cells in parallel that they become 1C when together?

One note - with 2 cells in parallel you have no idea the state of each cell --- they will have slightly different internal resistances which means one will provide more current than the other .... if the cells are well matched that means 50/50 or 49/51 or similar... if they are poorly matched it will me 60/40 or worse

good point. Does the BMS software show any of this by looking at the voltage of each cell? Asking because other than seeing how 'balanced' the cells are with one another, I'm not truly sure of the value of having that information.

and if one fails the other will try to hold up the voltage --- if the failure is as an open then your capacity is cut in half and if it fails as a partial short it will have a lower overall voltage and will suck down the voltage of that cell and you will be constantly trying to balance the others.... This generates heat and a premature failure of the BMS with a burned board as the result -- there are several examples in this section of that...

That would suck. Doesn't the BMS sense when one of the 4 cells in a 4S pack are bad and do something like turn off charge/discharge?

The most likely cell failure is a partial short -- dendrites growing and shorting two plates together across the electrolyte...

not good

Total short only happens during a vent or cell physical damage ...

glad I have not experienced this

or if the robot assembling it was having a bad day... I hope these all get rejected at the factory --- but IMO that is what (insert random letter name here) is making their batteries out of ...

the names are mind blowing. Like DUMFUME and REDODO come to mind. CHINS also bad

 

[fnnwizard]

Battery did not have lid on it during this issue

That would help, but with the fr4 only slightly higher heat conductivity than abs, sitting on top of the connection, can't be cobsidered free air.

Also, those calls are rated 1C continuously with a spec for both 2c and 3c pulses, so I don't see a problem with a 1c continuous discharge, as the 1c is where capacity specs were garnered.

 

[WorldwideDave]

No, I am saying in parallel the max is 1.0C (0.5C + 0.5C) and you are using 0.75C ....

Correct...I think we are then in agreeance that at 150A, I am within the rated capacity, and although I can do 150A, which is what my BMS is and why I bought it to match the cell rating, for the health and longevity of the cells and to avoid issues, I should keep it closer to 100A. Do you agree with both of these points? Asking because many have not understood that there are 2 cells in parallel here, and the individual cell rating, while important, isn't the only contributing factor to whether or not my system can support 150A discharge.

We all agree the wires from cells to bms sucked. That's been solved.

 

[WorldwideDave]

Yes, assuming the cells are perfectly matched, resistance for all connections identical, etc.

I also believe that just because something sounds good in theory doesn't mean it's good in real world operations. I also think best practice would be to parallel packs with separate BMS not parallel cells with a single BMS.

I'm learning this. Just tried to replicate what the OEM battery was on the first attempt. I can buy another BMS that matches, split the 2s4p pack into two separate 4s packs, but will most likely either built a new 4s 314ah pack, and get another 16s bms for my 48V setup next. Wanted to learn with 12V first and super glad I did. Plus, with all these 2/0 conductors I've been using on my setup, with 48V I don't have to change the conductors. Was my plan all along.

 

[WorldwideDave]

That would help, but with the fr4 only slightly higher heat conductivity than abs, sitting on top of the connection, can't be cobsidered free air.

Yes I have learned that. I am leaving it off the top now.

Also, those calls are rated 1C continously with a spec for both 2c and 3c pulses, so I don't see a problem with a 1c continous discharge, as the 1c is where capacity specs were garnered.

There have been many, many discussions about this - scroll up or jump back to end of page 2 where it starts.

 

[fnnwizard]

Yes I have learned that. I am leaving it off the top now.

There have been many, many discussions about this - scroll up or jump back to end of page 2 where it starts.

Yes, I did. I am saying you are well within spec operating at 150-175 A for those 2p cells. I've never like cells parallel together for the reasons a few have written, but as long as you also allow time to run at much lower power, they do tend to self balance to within a couple percent, provided the connections are good.

 

[robbob2112]

Correct...I think we are then in agreeance that at 150A, I am within the rated capacity, and although I can do 150A, which is what my BMS is and why I bought it to match the cell rating, for the health and longevity of the cells and to avoid issues, I should keep it closer to 100A. Do you agree with both of these points? Asking because many have not understood that there are 2 cells in parallel here, and the individual cell rating, while important, isn't the only contributing factor to whether or not my system can support 150A discharge.

We all agree the wires from cells to bms sucked. That's been solved.

we agree

The reason to use 1 BMS for 4 cells in this case would be so you can see all the different individual cell conditions... With 8 cells strung as 4s2p you loose visibility to all the cells and can only see the pair.... Otherwise folks would just do 16s16p with one BMS and call it good ... Bad things can happen when you can't isolate a bad string --- The house burned down thread was 7 strings of 16S and they were all in parallel and fused separately ... but they only had one shunt trip and no way to take a string out of service when the BMS detected an issue...

So, best practice -- one series string and BMS per battery then parallel whole batteries..... the exception being Batrium and the similar type BMS but that is a whole other discussion.

 

[WorldwideDave]

Yes, I did. I am saying you are well within spec operating at 150-175 A for those 2p cells. I've never like cells parallel together for the reasons a few have written, but as long as you also allow time to run at much lower power, they do tend to self balance to within a couple percent, provided the connections are good.

To those wondering why people don't like 2 cells in parallel together, it is mainly because with any BMS you can't see the state of any of the individual cells - you are looking at the voltage of the cells in parallel. The BMS has 4 leads - 4s BMS - but when you have 8 cells (2p4s), you just can't see what is going on. I'm not complaining - good experiment/test environment - but now I better understand.

 

[WorldwideDave]

we agree

The reason to use 1 BMS for 4 cells in this case would be so you can see all the different individual cell conditions... With 8 cells strung as 4s2p you loose visibility to all the cells and can only see the pair.... Otherwise folks would just do 16s16p with one BMS and call it good ... Bad things can happen when you can't isolate a bad string --- The house burned down thread was 7 strings of 16S and they were all in parallel and fused separately ... but they only had one shunt trip and no way to take a string out of service when the BMS detected an issue...

So, best practice -- one series string and BMS per battery then parallel whole batteries..... the exception being Batrium and the similar type BMS but that is a whole other discussion.

Yes thank you I have learned this very well. I like the idea of getting four 314Ah cells and repurposing my BMS, lugs, and conductors on a much more accurate and simple setup.

 

[fnnwizard]

To those wondering why people don't like 2 cells in parallel together, it is mainly because with any BMS you can't see the state of any of the individual cells - you are looking at the voltage of the cells in parallel. The BMS has 4 leads - 4s BMS - but when you have 8 cells (2p4s), you just can't see what is going on. I'm not complaining - good experiment/test environment - but now I better understand.

In addition to this, there are a couple other more "problematic" issues.
1. Connections between the parallel cells are very hard to get exact, = resistance between each P cell will not be exact = imbalance is easier to manifest at some of our current draws.

2. Imbalance cells in parallel never really balances out 50/50.
They may be at the same V when connected, but if disconnected , the higher cell will continue to be the higher cell. The soc delta depends on how long they sit at no/low current levels. It takes a few mV differential to move the Li ions, but as unequal cells are joined, there's both an over V bump (for lower V cell) and under V slump ( for higher V cell) so the migration of li-ion stops just short of true equalization.

 

[WorldwideDave]

They may be at the same V when connected, but if disconnected , the higher cell will continue to be the higher cell. The soc delta depends on how long they sit at no/low current levels. It takes a few mV differential to move the Li ions, but as unequal cells are joined, there's both an over V bump (for lower V cell) and under V slump ( for higher V cell) so the migration of li-ion stops just short of true equalization.

that was some geeky stuff. All I did was replace the bolts with studs in an hour long project while cells were at about a 18% SOC charge last night. Cause for alarm?

 

[fnnwizard]

that was some geeky stuff. All I did was replace the bolts with studs in an hour long project while cells were at about a 18% SOC charge last night. Cause for alarm?

My comment was a general one for parallel connected cells in a pack. For you specifically, Just replacing the bolt should have no consequences as long as, torqued properly. You want same pressure between the cell pad and buss bar, make sure they are clean and oil free.
For the studs, just be careful it is not bottoming out against cell so that when you tighten the nut, it digs into it more. Other than that, I see no issues.

 

[WorldwideDave]

Did a 100A charge and 100A discharge today while sitting by the pool under the cover of solar panels while it rained. BMS and cells appear to be fine.



didn't fully go into 'storage' or 'float' mode as the sun went down at 100% SOC - think there are about 5Ah left to go in charge cycle to get it there 100%.
The load is just idle standby with inverter and whatever victron gear is consuming.
battery never got hot according to both temp sensors, me touching conductors and bms. Never got more than warm. However, didn't run at 150A for 4 hours like I do on sunny days to test, either. Plus MPPTs were on while running at 100A discharge, but sun went behind rainclouds often today.