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Jan 28, 2004, 05:59 PM
Registered User

bad cell in my e-tec pack


i crashed with my e-tec 3s2p pack and after charging it it didn't seem to have any power, i decided to open my pack and check the voltage of the one cell that was slightly nicked and it was reading 2.89v...not good. now my question is what is the max voltage the rest of the cells can be at before they are damaged. when i charged that pack i think my charger has overcharged the rest of the cells trying to get it at the correct voltage
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Jan 28, 2004, 06:07 PM
Registered User
WJ Birmingham's Avatar
4.2V per cell is max. 2.7V is min. If a cell is exposed to air, it's junk 99% of the time.

WJ
Jan 28, 2004, 06:18 PM
Registered User
thats not good some of them are reading 4.89v
Jan 28, 2004, 06:32 PM
Space Coast USA
hoppy's Avatar
"Lipo Incident" in the making Consider yourself lucky that they didn't self destruct. Anything over 4.3V starts to do non-reversable damage. They may work but will probably have deminished capacity which infers pack unbalance problems and subsequently, more cell damage. Probably a write off for piece of mind. Are the high V cells swollen?
hoppy
Jan 28, 2004, 06:39 PM
Registered User
no not at all, only damage seen is one little corner of one cell just little pushed in, i rechecked the cells now they are reading 4.5v except for the one reading 2.89v. actually one other question. the e-tec 11.1v 2400 pack has 6 cells in it. the 3 first cells are reaing 3.85v and the other 2 are reading 4.5v and the one is reading 2.89 are the first 3 cells still good?
Jan 28, 2004, 06:56 PM
Space Coast USA
hoppy's Avatar
That makes sense Jeff - I assume this is a 3s2p pack. So you have 2 sets of 3 cells in series. The set with the damaged cell is likely the problem. Again I'm assuming that the 2.89V and the 2 at 4.5V are in the same series arrangement. That explains why the 2 'good' cells went high. Assuming that the charge ended at 12.6V, and like you said, the 2 cells were at 4.89V, that would be 4.89 + 4.89 + 2.89 = 12.67V, just like it should have ended. So if we assume again that the "good set of 3" represents the actual starting voltage at the start of charging, we can subtract 3.85V form 4.89V which equals 1.04V. Since we have 2 of those that equals 2.08V. Subtracting this from 3.85V gives us the voltage of the "bad cell" at the start of the charge. It comes out to be 1.77V. NOT GOOD.

1.77V + 3.85V + 3.85V charged to 12.6V adds about 1.04V to each cell.

Excuse me Jeff, I get carried away with failure analysis calculations.....

Anyway, in answer to your question, the one set should have 3 cells all at 3.85V and should be good.
hoppy
Jan 28, 2004, 08:42 PM
Hard Wired

Etech and carnage


I had a 1200 cell go bad in a 3S pack after a crash in a flat plate foamy. It wasn't that hard of a thump.. by my standards. LOL No real physical damage.
Anyway, when I tried to fly it again, after a little glue application to the fuse, the the motor wouldn't rev cause the low voltage cutout was shuting it down . I checked the pack and one cell was reading about .8 volts. I de-soldered it from the pack and managed to charge it by cheating it. It actually took a charge but it decayed to under a volt in 24 hours. Contrast. I have a PCW with a 2S 1200 Etch that has hammered into the ground, the shop roof, fences and several inanimate objects. The battery is severly contorted and humped up where the plywood keel has tried to cut it in two upon impact. The battery is ugly but seems unharmed electrically. My emperical data is confusing me.
Jan 29, 2004, 09:37 AM
WickedFunRC's Avatar
All my lipo cells read over 4.5 volts per cell with my cheap multimeter. But with my good meter they read 4.15 volts. In other words make sure your meter is a good one that is calibrated correctly.
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Jan 29, 2004, 10:33 AM
Registered User
Jeff, it looks like some of the cells in your pack are over charged.

Your discription of the event makes it sound like, crash damage caused one or more cells to have less voltage than the others.

When the pack got charged, the higher voltage cells (probably the good ones ), actually received too much voltage, and got over charged.
As you know, over charging is one of the causes of lipo fires, so I think you were lucky not to have had a fire.

If it were my pack, I would read the voltage of each cell. Discharge any that are above 4.2v, with a 10 ohm 5 watt resistor. A #1157 auto light bulb will work too.

Then you need to balance the cells, before you can charge the pack.. ( you can do this by discharging the high volyage cells, but not below 3volts)... 3.8 volts, ( 1/2 charge) is a good place to start.

Cells that are obviously damaged, should not go back into the pack.
Cells that are only low in voltage can be trickle charged back up to nominal values by charging at 100/200 milivolts...good luck.
Jan 29, 2004, 05:30 PM
Registered User
i have heard that replacing cells in a pack will cause a cell impalance. the pack has only 10 charges to it so i don't know if that matters. i'm still not sure i assume i have 800mah cells since i have 11.1v and 2400mah in a 3s3p pack but i have only seen 1200 and 700 mah e-tec cells
Jan 29, 2004, 05:51 PM
Registered User
ops..sorry it is a 3s2p pack...but still not sure if it is ok to replace the bad cell
Jan 29, 2004, 08:38 PM
Space Coast USA
hoppy's Avatar
bad cell (s) - I wouldn't trust those 2 that had been run up to 4.89V. If you do use them I'd sure keep a close eye on the individual cell voltages and treat them as if they might ignite at any time. Just my $.02. If you replace all 3 you can add that 3s to the other 3s and be back in business since they will be be charged in parallel with the hopefully good 3s that wasn't damaged. I like to have all the cells in a series pack to be within 0.01V of each other.
hoppy
Jan 29, 2004, 08:53 PM
Registered User
yeah i think your right, even if i don't decide to replace the cells i still have a good 1200mah 3s pack, don't really see the need in trying to replace that one cell and risking the other 3 good cells by starting them on fire
Feb 01, 2004, 10:41 PM
Registered User
just wanna ask one last thing here, if i decide to connect this good set of cell to a new pack there wont be any harm in connecting a new set of cells with the old set of cells that wern't damaged correct...IE no cell imbalanced or reduced life
thanks jeff
Feb 02, 2004, 08:00 AM
Space Coast USA
hoppy's Avatar
If you connect the new set with the old set in parallel, you will not have any imbalance problems. Imbalance is primarily a problem with series connected cells. The cells in series connections should be within 0.05V of each other, the cells in parallel will self balance.
hoppy