dead cell?

dodjob

dodjob

1 kW
Joined
Jun 30, 2010
Messages
320
Hi there,
The "adventure" continue with my headways :(
I have had some cut-out yesterday with my pack under load.
I have measured the cells tension after charge (one night) and with charger plugged: (in V )
3.67
3.6
3.66
3.65
3.68
3.68
3.66
3.64
3.64
3.67
3.68
3.65
3.68
3.63
3.35
3.67
The Bms shouldn't try to solve this? Or it cannot because the cell is damaged?
Under load the difference of tension seems to reduce itself. Can I check this without unmounting everything?
Gruß,
H.
 
I wouldn't call 3.35 dead, but it's definitely undercharged enough to make the bms work for days and days to bring it up.

I'd try putting a very small 4v cell phone charger on just those two wires you read 3.35 on, and try to bring it up manualy. Then see if the problem repeats after a ride and recharge. If it does, then the cell should be replaced.
 
You say the charger was on, but is it putting out any current? It could be the pack voltage is high enough to keep it off, or making it cycle on/off every few seconds. Are the shunts on the high cells activated, with the resistors getting warm? Did that charger ever get high enough to balance one low cell? I normally don't balance my 36 volt pack and get 80-100 cycles before getting a low voltage disconnect, then I have to charge at a higher voltage, through a resistor to limit the current to 100 ma, to correct a 2 amp hour low cell.

An ammeter to measure the charging current would be useful; if it's only 50 milliamps that would take 40 hours to put in 2 amp hours. You could get some indication of the charge current ny powering the charger through a killawatt meter; 10 watts on the AC input would indicate something like 100 milliamps on the output (the charger probably only 50% efficient at low current outputs). If it's only 2 watts it will take forever.
 
how far are you getting before the cutout ?

Do you have a CA on the bike ? can you confirm how many amp hours are delivered before that cell drops off ?

Sometimes it's the BMS that is causing the imballance, a fried channel can drain a cell causing issues sometimes..

I'd tap that cell with a single cell charger ( like dogman said, a cell phone charger will work, just keep an eye on it and pull the plug once you hit 3.6~3.8v ! )
 
Hi thanks for the help :).
In the past the charger, after complete charge was starting and stopping (fan) for about 10min (I think that was the bms). But now it doesn't...
@dak664: The charger is on, recharge the cells and the voltage rise up at the end 'til 58.2V then the fan stop and the green light appear. This is a 400W charger, there is a fan, when it's charging the fan blows and the light becomes green.
After reading your comments I have plugged a standard AC killwatt meter. It's oscillating between 8W and 11W I think it does make something. Unfortunately the cells voltage is the same for the last 12 Hours... I will let it plugged for two days and see if something change. Then try the solution of recharging directly with a cellphone charger. *edit* or maybe now ^^
@Ypedals: Yeap, I have a CA. 8Ah were sucked out of the 12Ah pack. But with some... quite hard load 40-60amps very oft to test the mac motor :) And yes I will definitively try to charge this cell alone :)
Thanks a lot again for your help, this forum is the best ever and this because of you guys ;)
Gruß,
H.
 
oooooooook,
I have use a cellphone charger to "repump" my weak cell. It has worked. The cell's voltage went up and reach the same voltage as the other ones (3.68V). As soon as I disconnect the charger, the cell's voltage dropped quickly to 3.54V (the main charger was plugged all the time)
I have then replugged the cellphone charger a couple of times and every times the voltage drop was slower. It's now stabilise to 3.62V.
And here come my questions:
-Is it normal?
-Shouldn't the bms keep the voltage at the same level as the other ones? (or try to do it?)
-Should I see any voltage change on this cell or on the other ones when the bms works?
-When the charger is on I noticed that the bms was warm (not hot but a little bit warm) is it normal?
Thanks again for the previous help!
Gruß,
H.
 
Looks like you have a cell that is weakening. No telling how long it might continue to be usable, if you can live with the shorter range. But getting a new cell on the way looks like the thing to do.
 
Hi,
the pack has about 7 recharge and less than 100Km.. I think I won't live with the shorter range :/
I will request a new cell.
I have made a test today and dry 5Ah out of the pack. The cell seems to good again. I will see if everything is back to normal when they are recharged.
Gruß,
H.
 
Yeah that sucker is probably a dud.
Seems like all lithium batts have some element of quality control issues really. Better to replace 1 big cell though than a bunch of tiny ones, know what i mean?
 
Check that your charger is actually allowing a small current after charge has finished to feed the BMS. Open up the Charger and have a look at the daughter board...it's a small rectangular PCB board with a few components on it. If at the top of the board there is a 3 legged thingamijig with a black 3mm long bit of substance on it he he...then that needs to come off as it stops the charger when pack reaches it's voltage regardless if some cells are high and some are low.

I have to do this a lot to some of the KP chargers I get in. Otherwise your pack will get a lot worse, then you need to leave it on over night to balance the pack.
 
I have recharge them. The cell remain at 3.595V when the other ones reach 3.68V I think it's damaged. I know it's not completly dead. But for a pack with 7 recharge this is not ok. I will change it.
@neptronix: You're right, changing one cell is still acceptable.And the system to hold them together is quite easy. I just have to be "respectfull" with my BMS :wink:
@Spacey: I don't have such thing in my charger :) it's a single mother board. When I diconnect the charge port (with charger on and charge ended) the killawatt go from 11W to 4W. I do think that the charger works. My question is why the BMS is not trying to put the same voltage evrywhere.. I thougt it equilibrate the voltage. Did I miss something? (surely :mrgreen: )
Gruß,
H.
 
The cell remain at 3.595V when the other ones reach 3.68V I
Click to expand...

This is perfectly ok, as long as all the cells make it to 3.50 or more, they are all fully charged, or as full as they need to be.

Now, there may still be a problem with that cell if it drops below the BMS cutoff limit way before the others but you will only know this by testing the pack capacity.
 
Ok I have make a capacity check today. I have slowly (20amps max) put 10Ah out. The lvc wasn't hitted. That's the good point. The bad one, is that after recharging them I have 4 of the 16 cells which are under 3.5V.
I start to think that the bms is the culprit. I will let it plugged for the night but I'm not making me too much hope.
If the bms is working, I should be able to see some change in the single voltage of each cell didn't I?
I'm quite sure that the charger's fan (cf: the charger) was starting and stopping at the end of the charge.
Gruß,
H.
 
Hi there,
sorry to insist but i really would like an answer :)
If the balancing worked shouldn't I see something like this at the end of the charge?
[youtube]dtnI7OH9DQk[/youtube]
I mean If I connect my voltmeter at one cell with High voltage I should see something "moving". :?:
Gruß,
H.
 
nope this is only a video to illustrate what I think I should see. I know that lifepo4 should get more about 3.6V and really not 4V ;)
When I plug my voltmeter on one cell I see nothing changing or maybe on some cell a very slow drain like 0.01V every 30-45s
Like I said before, in the past the charger was blowing and stopping at the end of the charge but now, it doesn't. Can a bms work but not balance? If yes, is there a way to check this?
I miss reference on this topic :/
Gruß,
H.
 
You really need to measure the current going out the charger. The BMS wants 100-150 milliamps continuous (or whatever the shunts can handle)for as long as it takes to bring up the low cells. A charger (particularly a high current charger) may consider its job done when the current drops to 200 milliamps. The charger also might cut off at a voltage that is too low for all cells to be at 3.65 (actually more complicated than this; at 150 ma bypassed cells will be at ~3.8 volts so a charger set to cut off at 12*3.65=42.8 volts would also cut off at 11*3.8 + only 1 volt for a low cell. The question is has the charger/BMS combination ever done a balance? It doesn't sound like it.
 
A lot of these balancing problems are because chargers not set up properly just cut off and do not supply the 100 to 200ma needed for the BMS to do it's thing.

You really need to buy a Turnigy Watt meter and place it in line between the charger and the battery pack, when the charger has supposedly finished charging you should see about 2 to 8W of bleeding charge. If not then the charger is just shutting off when it sees the packs rated voltage and is not letting the pack balance.

So your pack will get more and more out of balance with the high cells getting more and more voltage when making up the packs total voltage with the low cells getting less voltage etc...after a while some of the high cells will just break due to overcharging.
 
Yep! That is what I am doing now with my Ping. You can feel individual resistors being warm and see the small amp flow on the turnigy or Whattmeter. I'm gonna give a two day balance and then go in and check individual cells or maybe draw the batt down with a load untill BMS cutoff. Of course you have to power your meter separately with a 9 volt so it keeps the cut off settings when the bms shuts off.
otherDoc
 
ok,
I have plugged my wattmetter and seen that 0.15Amp are drawn. The bms seems to work then. I will manually recharge the "bad" cells and let the bms finish the work for some days.
Now I have some more questions :)
My cell are 12Ah lifepo4 from Headways. The best end-charge voltage is 3.6V right? I should then have 3.6 x 16 = 57.6V at the charger or can I allow me more? I ahve found a little screw that allow me to set the voltage quite precisely. Should I change it?
When they mean 12Ah it's at which C ? 1- 2-3? if my cells are getting that much unbalance at 10Ah under 2-3C thats a quite annoying :(
Gruß,
H.
 
Amp-hours is 1C; how much at higher rates depends on internal resistance which will increase over time, but should not be far from the 1C for a while at least.

3.6 volts per cell is OK for slightly lower capacity and longer life but the BMS likely kicks in at 3v65 where the 12AH capacity is specified. Probably at 100 ma shunt current the high cells would be reaching 3.8 volts, x16 = 60.8 volts max on the charger. The current taper is critical though, if it is putting out an amp at 58 volts the shunts would be swamped and whichever high cell rose to the 3.9 volt BMS cutoff first would repeatedly get banged as the BMS kicked off and on again in a bleeding hysteresis loop.

A microprocessor could apply an arbitrary taper, but since you have a pot the taper will probably shift with the endpoint voltage. A current meter on the DC side is the safest way to make adjustments. A $3 HF meter on the 10 amp range would do. A cable that plugs between charger and battery with pigtails for the ammeter is a good investment, it's also useful for adding a series resistor to limit current. Those are easy to make with Anderson connectors, a pain with XLR.
 
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