2 dead cells in LiFePo4 pack - advise needed

[pschmatz]

I've been running an LiFePo4 16s 48V 10ah kit for 2 season on my e-bike (approx 2200 miles) when my charger blew up. (I got a replacement from http://www.rechargeablelithiumpower.com/ quickly - excellent service). It charged up all the way to 53.5V, and everything seems to be fine. however I noticed that I was 2-3mph slower than usual.
I checked the cells and sure enough, 2 are dead.

My question is that I thought that the BMS should protect each cell individually, and shut it down all the way if there is one below the cut off voltage, so I wonder why the pack at all works with 2 dead cells?

Do I have a bad BMS? Is there an easy way to check?

BTW: My pack is using prismatic cells similar in size to the Foxx Lifepo4 10ah cells.

Thanks
Peter

 

[dnmun]

your charger should be adjusted to charge up to the full 58.4V but it is better to make your charger push almost if not right up to 60V.

your low cells were never able to get balanced because the charger was too low in voltage. nothing the BMS could do to protect it since there wasn't enough voltage to fill the low cells.

i wish a lot of the guys who think 3.4V/cell is enuff would pay attention to what happened to you.

 

[pschmatz]

Indeed, the 2 cells that are dead had been the ones that charged only to approx 3.35V. I manually balanced on occasion with a cell charger.
So, you are indicating that these cells died since they have never been fully charged?

My old charger was a "Li-Ion" with 58.4V 4amp, same as the new one (http://www.rechargeablelithiumpower.com/oscommerce2/catalog/48v16s-lifepo4-smart-charger-p-31.html)
Interesting though that my packed charged up to 53.5V with 2 cells being dead, so approx 3.85V for the remaining 14.

You recommend to rather charge up to 60v? Different charger?

Thanks Peter

 

[dnmun]

can you charge the low cells up with a single cell charger? there are always low ones, the BMS has some variation in the final shunt voltage from normal variation in the value of the resistors and the transistor. if you can get the voltage of the lowest cell up to 3.65V then it willbe fully charged, and the other cells above 3.65V don't take much more charge so the pack becomes more closely balanced. but the BMS has to stop the high cell from going over 3.9V. that's what it does for a living.

your low cells may not be dead, but they cause LVC because they aren't getting up to fully charged state.

 

[robb]

I too have 2 dead cells or rather 2 dead groups of cells in my lifepo4 caused by a short. I have opened up the pack (simpler than I expected ) and now wish to try to charge them individually hoping to bring them back to life. I have a lithium phone charger but the voltage is stated as 5v so would that damage the cells. I will check every few hours to make sure the voltage does not go up past 3.7 . Alternatively I have an old nicd charger that states it is 3v but I measured it as 4v output. Are the cells most likely to be permanently damaged after they have shorted and been standing at about 0.7v for a week.
Rob

 

[julesa]

Any lithium cell sitting at 0.7v likely has some permanent damage, but I'd still try to charge them up and see how much capacity they have, they might work OK for a while.

I woul NOT put one on a 5V charger and check 'every couple hours,' that's asking for trouble. Might try the 4v charger if you can watch it closely. It will sit at 3.3 or 3.4v for a while then when it's full the voltage will zoom up very quickly. Disconnect the charger immediately when it passes 3.6v.

Or you could just spend $15 on a single cell LiFePO4 charger, then you can set it and forget it.
http://www.all-battery.com/TenergyLiFePO4BatteryCharger01300.aspx

 

[dnmun]

I too have 2 dead cells or rather 2 dead groups of cells in my lifepo4 caused by a short. I have opened up the pack (simpler than I expected ) and now wish to try to charge them individually hoping to bring them back to life. I have a lithium phone charger but the voltage is stated as 5v so would that damage the cells. I will check every few hours to make sure the voltage does not go up past 3.7 . Alternatively I have an old nicd charger that states it is 3v but I measured it as 4v output. Are the cells most likely to be permanently damaged after they have shorted and been standing at about 0.7v for a week.
Rob

if you have an extra 5V cell phone charger then you can pump charge into the cells with that. just watch it and stop it before it goes over 3.9V and if you never discharged it below about 2V, you may be ok. no way to know but go ahead and charge them up with the cell phone charger.

it will climb rapidly above 3.65V.

 

[pschmatz]

My 2 cells seems to be completely dead, no voltage at all:
I tried charging them for 10min with my cell charger, but nothing - other than the charger getting hot - it seems there is no resistance in the cells.
In addition, one of the cells has a "belly"

I picked up tp cell on Ebay (Foxx power lifepo4 10ah - $14 each plus 10 shipping), which is really cheap (comapred to what I paid in 07). It seems that these cells are almost the same size than the rest of my cells.

I still wonder what happend: Did the charger blow up and take out the battery, or vice versa? What I can tell is that my BMS must have gotten really hot (some foam insulation melted on the heat sink). In normal operation (20A max) it usually only gets warmish, but never hot.

Peter

 

[Ypedal]

imo, most likely is that the cells went bad, no fault of the BMS or charger... cells fail sometimes, when they do they cause the BMS to prevent you from using the pack by triggering LVC or HVC.

The charger not going up to peak voltage would only prevent the pack from getting a full charge, but trying to ballance a pack that has bad cells will cause the BMS to get hot for extended periods of time ( bleeding power from the good cells to try and make the low one's come up ) .

Time to get hacking !!

 

[AndyH]

I've been running an LiFePo4 16s 48V 10ah kit for 2 season on my e-bike (approx 2200 miles) when my charger blew up. (I got a replacement from http://www.rechargeablelithiumpower.com/ quickly - excellent service). It charged up all the way to 53.5V, and everything seems to be fine. however I noticed that I was 2-3mph slower than usual.
I checked the cells and sure enough, 2 are dead.

My question is that I thought that the BMS should protect each cell individually, and shut it down all the way if there is one below the cut off voltage, so I wonder why the pack at all works with 2 dead cells?

Do I have a bad BMS? Is there an easy way to check?

BTW: My pack is using prismatic cells similar in size to the Foxx Lifepo4 10ah cells.

Thanks
Peter

Hi Peter,

As you alaready know, your charger is set for 58.4V by default - 3.65v per cell. If you need to increase to voltage - to allow some extra power for the BMS for example, you can adjust your charger up thru 60V. I wouldn't do that unless you're certain your BMS needs the extra voltage, though, as 60V is 3.75V per cell. If your BMS shunts, and it's set for 3.65V per cell, the BMS is going to work hard - and make a lot more heat - trying to get rid of the of the extra voltage.

When you say your pack charged to to 53.5 - I assume that's pack voltage after charging? I'm not surprised by that - especially with two dead cells. The BMS might have let you down, but the charger is doing its job just fine.

The belly is an indication of a bad cell. If you can, compress the two bad cells with flat boards then try to recharge them. Cells get really unhappy when the electrolyte is allowed to move away from the 'plates' and into a swollen case. You might be able to revive the cells if you compress them for a couple of days. You might not if they've been starved too long.

What BMS are you using? How is the LVC connected to the bike?

Andy

 

[ejonesss]

actually the bms has an lvc that should trip the brakes and tell you that it is dead.

you will just hit the lvc sooner.

i suspect that you blew 2 of the tc54's so the 2 cells was never protected and allowed to drop so low that you are destroying them.

it is easy to blow the tc54's if you wire the taps wrong.

your charger should be adjusted to charge up to the full 58.4V but it is better to make your charger push almost if not right up to 60V.

your low cells were never able to get balanced because the charger was too low in voltage. nothing the BMS could do to protect it since there wasn't enough voltage to fill the low cells.

i wish a lot of the guys who think 3.4V/cell is enuff would pay attention to what happened to you.

 

[AndyH]

Good thoughts, Eric. Unfortunately, we don't know what type of BMS Peter is using.

edit: nope - not a G/F BMS.

 

[pschmatz]

Let me get you guys a picture tonight of the battery and BMS.

The battery was from ebike.ca from their "experimental" bin, it is not pretty to look at for sure :wink: but it did work fine for 2 years

Thanks Peter

 

[dnmun]

i agree with andy on this, sounds like the charger is there but you have some dead cells.

pull the sense wire plug off and measure each one, and write it down, #1-#16

 

[pschmatz]

Ok - here we go http://picasaweb.google.com/pschmatz/Lifepo4?authkey=Gv1sRgCLnUndKy3_XApAE&feat=directlink

Now there are a some burn marks on the board, but it seems that this is due to the "expert" soldering job

Thanks Peter

The Voltage is as follows:

#1: 3.36
#2: 3.66
#3: 3.66
#4: 3.66
#5: 3.67
#6: 3.66
#7: 3.65
#8: 3.67
#9: 3.54
#10: 3.65
#11: 3.68
#12: 3.66
#13: 3.34
#14: dead
#15: dead
#16: 3.49

 

[pschmatz]

Ok, I checked with Justin at Renaissance Bicycle, and he suggested not to run with an BMS at all for "a lot less troubles overall".
Assuming there is no overcharging, and that I have a LVC set on the Cycle Analyst, and that one checks the balance frequently.

Thanks Peter

 

[dnmun]

so you bot 2 replacement prismatic cells from fox power?

now you inserted them into the pack to replace the 2 dead cells, but since you ruined the BMS, then you are gonna go naked?

why not get a v1 signalab BMS from ping and use that instead of the one you have there? the v2.5 i don't think he sells separate from a pack yet. but it would cost more too since he added the leds to it.

there is no way you are gonna be able to keep the pack balanced without a BMS especially since you now have 2 different types of cells in the middle of your pack.

i think the v1 signalab is a better value anyway. email him at pinping227@hotmail.com and see what he would charge. shipping is always a problem, but on such a small item it may not be so bad.

 

[pschmatz]

Hi Dnmun

It turns out the replacement cells are exactly the same as I had in my pack, so the differnce would only be that the 2 cells are new (compared to 500+ cycles on the old ones).

I don't know how much ping would be charging for his BMS (Does anyone know how much he's is selling the BMS only?)
I've been looking at some cheap ones here (Signalab http://www.all-battery.com/protectioncircuitmodulefor3cellslifepo16batterypack30A-1.aspx as well as here Ayaa http://www.batteryspace.com/pcmwithequilibriumfunctionfor16cells512vlifepo4batterypackat40alimited.aspx), but it sounds like that these are not that great either

For now I plan to charge up each cell manually, and check after each trip before and after recharging.

Thanks Peter

(Hey: I used to work for HP as well...)

 

[dnmun]

i just bot 3 of the 36 v v1 signalab BMS from ping along with a buncha other stuff and he charged $36 each. i expect a 48V would be $48.

the 2 new cells will have different internal resistance too so it will be hard for any BMS to manage them but i would trust the ping v1 more than others.

 

[pschmatz]

Thanks. Was the $36 incl. shipping?

Thanks Peter

 

[pschmatz]

OK, then I have a Ping V1 BMS 48v on order ($47 + $7 shipping) - Ping was very fast to respond.
Ping mentioned that I need to charge at 60V, but I think that Andy's charger can be adjusted.

Thanks Peter

 

[dnmun]

the reason ping uses the higher setpoint is to make sure that any of the slow cells under the BMS get a chance to catch up if the high cells get to the high voltage cutoff point too soon.

you can monitor your pack and adjust it up to 60V if needed, but you may get them all to balance and the minumum cell up to 3.65V-3.68V, without having to go all the way to 60V.

since you know you have problems, you should keep a record of the voltages on each cell in your pack so you can refer to that when you check them later, so if one starts going out you will know it quicker.