Does LiFePo4 18650 risk exploding if used while wasted?

vodk1c3

vodk1c3

100 W
Joined
Jul 27, 2012
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Québec, Canada
Couple of days ago my bike fell on the ground. 48V lifepo4 vpower.
One 16s string is now wasted charge at 2.6v and idle at 1.1v. Cells are made of metal 18650 strings.

The amount of glue of this pack is terrible; I wish to avoid repair.
Would it be ok if I just doesn't care about those cell; or will they explode and ruin the whole pack if used regularly?
Thinking of bypassing the BMS for discharging or faking a good voltage for this string on the bms.

Even if it's my fault that the pack is not working; would not recommend anyone now; too much hassle to repair.
 
we have tried to warn people to avoid them for this very reason.

yes the tape is horrible, and you have to be so gentle in pulling it off where it is stuck onto the metal parallel straps on top and bottom.

BUT you have to do it. it may be simple or it may be bad.

do you know which channel is dead? can you take pictures of the entire pack with a marker or tape where that channel is and where the central line between the two sections is?
 
Yes I know witch channel but cannot identify in the pack yet; need to get a hair dryer or something to get off some glue.
Is it soooooo a bad idea to use it "as-is"!? Those 18650 metal case look pretty hard... not like pouch stuff.
 
since there are no pictures we have no idea what you mean by glue. i assumed you meant duct tape. if you use it at all while that cell is at less the 2V then the pack is ruined anyway so the decision may have already been made.
 
Thanks, Yeah will post picture soon.

Could you explain to me why would using this would ruin the pack??? Cell exploding/things shorting??? Or something else???

Edit : By glue I meant something like "hot glue"...
 
For reference picture : http://www.ypedal.com/cammycc/Index.htm

I'm not really asking how to repair right now; I just want to know the reason why I shouldn't bypass BMS and let this string die and move on with it...

If it's not too dangerous.
 
i don't understand why it is so bad for the cell to be discharged while carrying a large load (current). it may be that there is some kind of damage to the electrolyte when there is need to carry so much current and there are no longer any lithium ions available to carry the current. so the other metals inside the cell like the aluminum electrode or the copper could corrode and form an impervious layer. 2.1 volt is considered the cut off point from the literature for lifepo4, and there are some special thundersky cells that use yittrium along with the iron in the cathodic phosphate material which is higher, and the lipo is considered totally discharged at 2.7V but most manufacturers use 3V.

when a cell self discharges down to low voltage, they don't seem to lose a lot of capacity but the internal resistance most probably changes, gets worse.
 
Thanks again,
So... you seem way more informed than me on this subject... Still does not answer the question directly
By using it I know that I will ruin 1s of my pack;
But is it dangerous for the other 15S....
The idea would be to discharge without BMS (and taking care that the other 15s stay within LiFePo4 range) and charging with the bms.
Would this string of cell explode or something?

Just for fun; Luke, a chainsaw, and a headway :)
https://www.youtube.com/watch?v=8IBapfB0Imo
 
Willing to bet that there are a few cells you could salvage from that single damaged series chain. Instead, you wish to let them waste away. Not my idea of conservation, but to each their own bliss. I'll drink a few brews and follow in your experiences... :)
 
Exactly what I would do... I was beginning to think that my english was too bad or that I was not clear enough...
I really wish it would be as simple as ignoring that one string... repair could take me 10+hrs and even give me more problems in the process...
Will open a beer myself and wait to see any more comment...
 
Do NOT use this pack until you remove the bad cells..

no, you canot just keep using the pack .. the bad cell block will get really hot, possibly start a fire, both while charging and discharging.... you canot use the pack with bad cells . you must open it up and remove the bad section.

Then, you can use the pack as a 15s, instead of a 16s, but you will need a charger with lower voltage and/or a BMS for 15 cells... or.. replace the bad chunk with good cells to make it 16 again.
 
you can hack the BMS to 15S and use it on the remaining channels. it would function normally and you could adjust the charger voltage down by one cell.

i have hacked a pack down to 14S because the replacement cells were so expensive.i even adjusted a little 36V3A ping charger brick up to 51V to charge it. and tweaked it up to get 3.8A from a charger brick.

the v2.5 signalab is easy to hack to 15. it can be done on th eVpower BMS too if you can reach the top transistor on the top channel with the soldering iron to remove the resistor that connects it to the LVC signal line. after you remove that resistor then the LVC signal on #16 is not transmitted to the mosfet drivers. but all the other channels still work normally.
 
vodk1c3 said:
or will they explode and ruin the whole pack if used regularly?
Click to expand...
They probably wont' "explode", but the pack is not going to operate properly unless you remove or at least disconnect the damaged cells. They will also continue to drain the good cells paralleled with them, whcih will probably then be damaged as well, and then you'll have to do even more repair work. :(

If you take the BMS off, there will be no cutoff when the bad cells are out of power. Then you will end up reverse-charging them as you continue to drain the other more-full cells in series with them, and the results of that are probably overheating, possibly fire.


For some examples of repair methods, check out my Vpower and Volgood pack repair threads.
 
Thank you guys, you probably saved my pack and even more...! It's greatly appreciated.
Ypedal you made it really clear ;)

Guess I have some work to do then!! Not sure if I will try to replace the cells or remove and hack BMS/charger.

Will share the experience afterward... let's hope I don't short anything.
 
I plan to do it slow; cause in my experience rushing thing is not really the answer.
And will pedal to work happily, it will be just good for my body.

Will share picture and ask questions if I still need.
Couple of days; maybe a week or more you will have comeback of the experience for sure :)
 
Did start to work; amount of glue is insane... real pain in the a&&
Still donno witch string it is from the two in red. Will probably have to open the whole pack anyway to replace or remove anything.
Positive is #2 from the balance connector; Negative is #3
The pack interconnect tab will also give me some trouble probably. Need not to stress the tab weld while splitting pack...
 

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you are counting backwards. the numbers will count from the the bottom of the pack B- to the top of the pack B+, you said 2-3, so that is channel #15. you should be able to charge that channel up with a single cell charger.

if you can find a cell phone charger brick you should charge up just that row now before cutting anything else apart. inspect the connections of the sense wire to the straps to be sure they are continuous.

i don't see any physical damage to that row. you should charge it up with the cell phone charger to verify it will not hole the charge. don't cut it open yet.

you need to take a close up picture of the BMS showing the shunt resistors for that channel so we can see if the shunt resistors have overheated.

look at each end of those straps running the length of the row and see if one has cut into the insulation on top of the cell at one end. the positive end of #15. take pictures.
 
yeah, from what i am able to see in that one picture, the cells appear ok, physically, so it may be as simple as a crushed or nearly broken ballance wire from the cell group to the BMS..

check all cell groups directly with a volt meter at the cell tabs... then compare voltage thru the ballance wires.


edit, and since you have gone this far, may as well split the pack in 2, seperate them to fit in side panniers instead of that one big brick.
 
Exactly what I did !!! I started to charge with my IMAX B6 yesterday just after the picture.
Charging really slow at 0.5A (small wires I guess) but I did put 5A+ in the string now and voltage is holding at 3.3v.

So far so good...!!! I'm pretty exited. Possible BMS issue will be investigated.
After charging I might try a discharge test with the IMAX to know if the string still have some nice capacity and to cycle slowly for the first time since the low voltage issue.

Splitting the pack would be a good idea for the bike aesthetic indeed; but I'm still not sure about it because I fear breaking something.

Thanks alot!
 
you need to provide a picture of the BMS with a closeup of the shunt resistors for that channel so we can estimate if it has been running hot for a long time.

you can measure each of the shunt resistors too in order to see what their value is and compare that to the channel suspected.

splitting the pack is the best way. you gotta get all that weight down low so panniers are the solution. you will need to connect the two sections with large wire, at least 10AWG or even two 10AWG. you can solder to the straps that run across the ends of the cells.

but first get them charged up and once they get above 3.65-3.7V then let them sit and watch and measure the rate at which the cell voltage drops.
 
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