48 volt lifepo4 battery cutting out prematurely

J

johnnyz383

100 W
Joined
Jul 28, 2013
Messages
212
Location
London
Having a problem with my 48 volt 20 amp battery. Its composed of 18650 cells. I had a problem when i took the battery out in may as one group of 16 cells were down to 0 volts. I traced the fault to the bms. I replaced the BMS and the 16 cells, and now at best im getting about 700 watts or about 14 amps before it cuts out. The battery when fully charged will read anywhere from 54-56 volts but that dissapates to about 53 volts once i start using it. When the battery cuts out, its only at about 51.5 volts. It performs great, just wondering why its cutting out so soon...i dont know if the cells are unbalanced...the kind of charger i have is one that will continue to run...green light when fully charged and yellow when charging...this will cycle on and off...wonder if i should charge it for a couple of days....any ideas??


John
 
Do you have a watt meter? How are you measuring individual cells? Do you have a multimeter?

Batteries don't cut out, you can drain them to zero volts, either the BMS, controller, or some other device may have high voltage cutoff or low voltage cutoff (LVC). Do you know what the LVC is anything that may have one?

Did 16 of the cells actually get to zero volts?

Theories that come to mind are voltage sag, this can be from bad cells, poorly balanced pack, or drawing more current than your pack can sustain. Or it may be a BMS issue (again?). It would be wise to keep an eye on cell balance with something like a cellog8 or use of a multimeter on individual cells. Some call a BMS a battery murdering system, some say BMS is great so long as you remember it isn't infallible, and some say BMS is a perfect blind no maintenance system. Any way you cut it, you should check cell balance at least on occasion with an accurate tool.
 
Bullshit.

Your battery is very likely out of balance. Some cell groups are not charging fully, and it will take days, or even weeks on that charger to fully balance. Once "full" and it goes into that cycling on and off thing, it's balancing. It's not going to be getting 5 amps of current from that charger anymore, so you can leave it on the charger 24-7 for years with no damage. It will only push 5 amps when the battery is discharged.

It WONT overcharge, unless you have a charger set to a too high voltage in the first place. 56v is not overcharged at all.

You can balance the pack manualy. The safest way would be to identify the highest charge cells, and discharge them with a car turn signal light bulb. Same thing your bms is doing, but a quicker discharge. Then back on the charger, when it goes green, look again for another cell or cells higher than the others.

But I'd just start leaving it on the charger every night for a week, and see if you get improvement. You might not get that much better though, since even fully charged and balanced, an old battery will never have the same capacity as when it was new. If you can get it back to 17-18 ah, you will be doing good.
 
Thanks guys...yes i have a watt meter. I pull as much as 60 amps, but the kelly controller will cycle down the amps as the load decreases, untill i pull the next gear ( see my trike Arctrike.com). It will sag to about 45 volts at 60 amps, from a nominal voltage of 53, but at 15 amps, cruising at 45kph it only goes down to about 50 volts. Im thinking that your right about being out of balance, when i take it off the charger as soon as the green light comes on the voltage goes from 60 to 54-53.5...but last night after charging all night it had lots of surface charge staying at 58 volts for a while...i have a bout 3000 kms on this battery..
 
Johnny who did you buy the pack from?

Demand a refund or replacement!
 
you need to put the battery on the charger and measure the cell voltages while charging and post up the voltages here. then we can see how far outa balance it is and if that is the cause of the cut out. but it sounds like your regular 3C discharges may have taken a toll on those little cells. since there are so many in parallel, some of them may be subject now to high rates of internal discharge and are pulling down the other cells in parallel with them.
 
...This time it cut out at 8 amps!!...getting worse....Oh believe me..i demanded..it took 2 and half months for them to send me 16 cells and a BMS. Then they send me the yellow 1400 mah lifepo4 batteries instead of what i had which was 1100 mah, so i soldered them in and have had nothing but problems...i guess these cells they sent me were not grade a quality...here is who sold me the shit on ebay.....http://www.ebay.com/itm/48V-40Ah-LiFePO4-Lithium-Battery-with-BMS-Fast-Charger-and-Bag-/171080036715?pt=US_Rechargeable_Batteries&hash=item27d52a316b

Ok, i took it apart after it cut out at 8 amps, sure enough the group of 16 they sent me were down to 2.7-2.8 volts and the rest were all 3.3. One of my solder jobs came off, so i dont know if thats what was doing it so i resoldered. As it was charging this group came up to the exact same voltage as all the others...

Ok fresh off the charger as soon as the light went green (total 60 volts)..total of 16 groups of 32 cells each....
1) 3.367 9) 3.361
2) 3.363 10) 3.356
3) 3.437...the yellow 1400 mah cells 11) 3.433 ...yellow 1400 mah cells
4) 3.370 12) 3.358
5) 3.364 13) 3.356
6) 3.360 14) 3.356
7) 3.359 15) 3,354
8) 3.359 16) 3,358

I dont know but they look all balanced to me....im going to put it back together and do a drive tonight and remeasure...this is driving me nuts....would i be better off with headway cells as the c rating is much better...

John
 
Ok...15 mins later almost all are now 3.34 volts including the 1400 mah cells....

So your saying that i would be exremely hard to determine which individual cells if at all, have a high discharge rate?

John
 
johnnyz383 said:
...This time it cut out at 8 amps!!...getting worse....Oh believe me..i demanded..it took 2 and half months for them to send me 16 cells and a BMS. Then they send me the yellow 1400 mah lifepo4 batteries instead of what i had which was 1100 mah, so i soldered them in and have had nothing but problems...i guess these cells they sent me were not grade a quality...here is who sold me the shit on ebay.....http://www.ebay.com/itm/48V-40Ah-LiFePO4-Lithium-Battery-with-BMS-Fast-Charger-and-Bag-/171080036715?pt=US_Rechargeable_Batteries&hash=item27d52a316b

Ok, i took it apart after it cut out at 8 amps, sure enough the group of 16 they sent me were down to 2.7-2.8 volts and the rest were all 3.3. One of my solder jobs came off, so i dont know if thats what was doing it so i resoldered. As it was charging this group came up to the exact same voltage as all the others...

Ok fresh off the charger as soon as the light went green (total 60 volts)..total of 16 groups of 32 cells each....
1) 3.367 9) 3.361
2) 3.363 10) 3.356
3) 3.437...the yellow 1400 mah cells 11) 3.433 ...yellow 1400 mah cells
4) 3.370 12) 3.358
5) 3.364 13) 3.356
6) 3.360 14) 3.356
7) 3.359 15) 3,354
8) 3.359 16) 3,358

I dont know but they look all balanced to me....im going to put it back together and do a drive tonight and remeasure...this is driving me nuts....would i be better off with headway cells as the c rating is much better...

John
Click to expand...

you need to measure the cell voltages while it is on the charger. measuring them after you remove it from the charger doesn't help. we need to see if one of the cells has stopped the charging by climbing to HVC while on the charger.
 
Well after a short run using 4.5 amps, the groups 3 and 11 are at 3.26 volts and all others are at 3.314. Im suspecting that theres something wrong with these cells?...they should be able to hold their voltage better since they are more amperage no?...i measured the voltage of all cells while they were charging most were at 3.42 volts and total of 55.5 volts, once the voltage reaches 56 volts is a very short period of time before it gets to 60 volts in which case the light turns green....


John
 
Im not sure what you are asking but if you want to know which cell group caused the the charger to stop charging , it was the new yellow cells that are rated at 1400 mah that rose to 4.25 volts, and then the charger's light went green.

John
 
ok, so once the bad connections are fixed, you need to use a dummy load to drain down those yellow cells while the other cells continue charging up. i use some big cement power resistors connected to the cell with alligator clips on both ends of the cell to drain that specific cell while the others charge up.

i use about 5 ohms 5 watt power resistor. it goes slow, about 800mA drains off so the cell will remain high until the others catch up, and then you have to remove it. don't leave it on too long so that the cell drains down below the voltage of all the others.

the low cells are not filling with charge because the pack is unbalanced and the high cells cut off the charge. when the low cells get to LVC early because they are not charged up then that is what you see for total capacity.

once you fill up the other cells to full charge then you can see how much capacity is left then. it may be close to what you need and once the new yellow cells are balanced to the others then 'normal' capacity may return and the battery should give full performance again.
 
+1 for cement resistors. Those always make a pretty good dummy load if you don't exceed their ratings too far.
 
Thanks for the reply and i will do this but, i would have to drain the entire group of higher ah cells. Also, this would not answer the logical question as to why the new cells which are initially higher, become drained faster than the cells which are lower to start with and all of the remaining cells have the same amount of voltage. Its actually the new higher amperage cells that are tripping the LVC first !

Would this be indicative of either bad cells or perhaps one of the cells being bad...

John
 
Thanks Dnmum

But re-read the post the higher cells (new ones) are falling faster in voltage than the original cells and they are tripping the LVC.


John
 
if the new cells you added do not have enuff capacity to match the others then you need to add some more in parallel to them to get the pack to function normally.

balanced means in terms of the amount of charge stored. the BMS can only work from the voltage presented to the comparators.

it is always best to use identical cells when building a pack in series and when using replacements too, but you can add some more to the channels that are short in capacity and then maybe it will be able to function normally. getting the cells to 4.25V doesn't help them either.

you may wanna add some more cells now before hassling with this more. you will gain a lot of capacity for the price of just a few cells.
 
Ok i never considered this. Are you saying that 4.25 volts is too much? I replaced the original cells that dropped to zero with 16 of the yellow cells, how many would you suggest to add?

Thanks

John
 
4.25 is pretty high. I personally dislike exceeding 4.1v per cell and usually stop between 4 and 4.1 and never go below 3.5v if possible with lithium batteries.
 
add as many as it takes to match the capacity of the other cells. your BMS is defective if it allowed the cells to charge above 3.9V and allowed one cell to drain down to 0V.
 
Well, after letting my 48 volt lifepo4 battery sit for a week, i took it out and noticed that almost right away the voltage began dropping quicker than normal, sitting at about 48 volts instead of 51. I stopped the ride, came back and checked each string of 16 cells. Each string was at 3.3 except one which was only at 1/2 volt. I charged it up, and when the green light came on indicating a full charge, the string that was at 1/2 volt is only at 3.0 volts and the others around 3.35-3.6 volts....are the cells recoverable and what caused this in the first place?

John
 
Its composed of 18650 cyl cells...i kept it on the charger and those 16 cells went from 3.0 volts now down to 2.5 volts?!?? what??...is my BMS faulty or what??
 
well, that's what you need to figure out.

All it takes is for one cell in the group to have an internal short and self-dicharge, taking all it's brothers along for the ride.

I would start by removing the BMS, tapping into that low voltage group and giving it a 2 amp charge, monitor the group for any signs of heat, and time it so you know how many amp/hours are going back in to bring it back to full charge voltage.

report back.
 
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