When lifepo4 are positioned apart

[maxwell652000]

i have a 48v/20ah pack which is split in 2 parts (8 cells on each side of my rear wheel)there is one 6gauge wire completing the series connection which was 36inches long,,the problem i was having is that the cell on the negative side of the 36" long wire would drain faster then all the rest of the cells ,,,the lvc would cut out the current (under load) and upon checking the readings of the cells i had 3.03v on the cell in question while all the others registered 3.24v,,,is it the length of that wire causing that cell to lose its volts faster?,,,i managed to cut down the lenght to 15inches but will not know if this will remedy the problem until my next ride,,,any comments or suggestions? when i recharge the cells they all come back to 3.4v

 

[Pete]

What are you using for the connection at each end ?

Have you extended all of the BMS sense wires or just the ones on the opposite side ? And how have you done the extenstion ?

6 AWG should be plenty, all I can think of is there is that there is a slight resistance difference causing the power to take the least path of resistance.

How are the separate packs configured ? 48V10Ah each and in parallel or 24v20Ah in series? If you wire them in series you force the 6AWG to take the full current and this should stop the imbalace problems you are getting

 

[maxwell652000]

hi pete
each cell is 20ah so they are in series only, all sensor wires are extended as well ,the 6awg is now 15" instead of 36", there are 8 cells on each side,the cell that loses the volts faster then the rest is located at the end of the pack where the 6awg wire is connected to the negative post leading to the other 8 cells connected of course to the positive post to continue the series connection ,,,,the cell that is connected to the 6awg on the positive side stays balanced with the rest of the pack. im hoping that now that the 6awg that completes the series connection between the 2 packs (8 cells each at 20ah each)is only 15" as opposed to the 36" that this will solve the issue,still i would appreciate any input.,,, just to let you know im using a 40amp controller on a crystalyte 5303 ,,,just the other day as an experiment i rode the bike pretty much full throttle reaching speeds on average 40km topping out sometimes to 50km on the straightaways also went 4 or 5 times on over passes on the highways throttling full pin up the hills,,,no peddling whatsoever the lvc cut out when i reached a range of 30km,,,at that point i checked all the cells voltages ,,,all were at 3.24v while the cell that was connected to the negative post of the 6awg was only at 3.03v (at that time 36")now that i shortened the 6awg to 15" i will do the same experiment and will post the results.any comments will be appreciated.

 

[maxwell652000]

hi pete
one pertinent question you asked ,,,"What are you using for the connection at each end ? " i did not answer in my last post,,the 6awg is properly soldered to a copper post which is then screwed to the posts of the batteries.

 

[Pete]

So it is only one cell dropping voltage, not the whole sub pack ? Sorry, re-read your first post and you already mentioned that.
I think you simply have a weak cell. Just to prove it, try swapping the cell over to the other sub pack. I am pretty sure you will find the low voltage moves with the cell.
How old is the pack, number of cycles, cell manufacturer?
Are you able to monitor individual cell voltages under load ? Don't go having an accident trying to do this, please

I don't know enough about the discharge curve and voltage characteristics to be able to tell you how far gone the weak cell is compared to the rest.

 

[maxwell652000]

im going on a ride today and will swap the cell if i see the same problem,,,i recd the pack sept 2007 and have gone through apx 13 cycles (400km on my speedometer divided by 30km each ride = 13times apx )i thought about swapping the cells but i hope and pray that maybe the 6awg was just too long at 36",,,if i must swap i will swap,,,but what do you think about the fact that when i charge the pack all the cells register 3.34v including the one that im having the problem with,if it is in fact weaker wouldn't it register lower at the end of charging? (the readings are when the cells are rested overnight ie:charge is complete 10pm then i check all cells the following morning. thanks for your input its always appreciated

 

[jmygann]

Any Pics ? Where did the packs come from ? Manu.? How mant cells total ??

 

[GGoodrum]

I agree with Pete. You've most likely got a cell tha has been stressed, and now has a reducecd capacity. No way a 36" length of 6 gauge wire is going to cause any problem like this. The good news is that the LVC function in the BMS is doing its job. It could be worse In any case, this weak cell now defines the pack's capacity.

-- Gary

 

[maxwell652000]

yep u guys are right,,,the same cell dropped to 3.03 (not loaded)when the lvc cut while all other 15 cells registered 3.25 now that the wire is only 15" instead of 36",,,as a last resort ive swapped the cell at 3.03 with a 3.25v so now that cell is not on the 6awg,,,buuut my spidy sense (along with pete and gary) tells me its a weak cell ,,,,i think i will have a new cell shipped in but just to be sure i'll try again tomorrow now that the cells are swapped any one want in on this order? i can bring them in at apx$1.65 per ah + shipping (lower if we can bring in a strong order)at 1.65 it comes out to $528.00 for a 48v/20ah pack or as another example $198.00 for a 36v/10ah these of course are wholesale prices.as always thanks for the input,,,i need to experiment as i will be building an electric car soon,,,

 

[Lenk42602]

maxwell,
should the need present itself, please post group purchase proposal requests in the online market section under a new topic heading.

peace,

len

 

[jmygann]

so do I assume it was a bad cell or is there a problem with splitting the packs as that is what I want to do ???

 

[Pete]

Bad cell.

No worries splitting the packs as long as you wire the two halves in series. From a safety perspective have a think about how your system will react if you manage to damage the insulation of the heavy gauge wire that bridges the sub packs. You should have a high current protection device (fuse or breaker) close to the positive termial of each sub pack.

I personally think best practice would be to extend all of the BMS sense wires so they are all the same length. I realise that the difference in voltage drop over the two different length groups of sense wires will be minimal. But you need to extend / replace half of them, so you may as well do them all. With the dollars involved in these packs I personally want to have everything going in my favour