Cell dropped to 2.5V over night?
 

So after a good days worth of racing I packed up and went home. I didn't get home until late so I decided I would recharge my packs in the morning. This morning I hooked my TrakPower 2S pack to the charger and to my surprise one cell had dropped to 2.5V. The other was perfectly fine at 3.4V.

I know your not supposed to let LiPo's sit fully discharged, and it wasn't fully discharged. I never hit the LVC, although i probably got close to hitting the LVC so maybe that point is moot. So after I set the charger to NiMH mode and set it to under 1 amp and after about 30 seconds to a minute the second cell reported 3.4V just like the first one. Now I'm charging in LiPo mode at 1A which is around 1/5C on this pack.

Should I consider this to be a defect in my pack or just a result of me letting the pack sit overnight almost at the LVC? This is pretty much a brand new pack with only 5 or so charges through it.

And just as a disclaimer... I'm not new to this. I fully know the dangers of trying to recharge a cell under 3V. I've done this exact process a few times on packs that I accidentally ran down too far. I'm not asking whether it's safe or not. I know the risk that I'm taking and I am surely keeping a very close eye on the pack as it charges.

What I’m concerned about is that a single cell became so unbalanced and went so low overnight on it's own behalf. It was not plugged into the car during this time, and the pack itself never went below 6.4V while running. It was just stored overnight at a voltage of probably around 6.6V if i had to guess.

I'm not sure what the exact threshold is, but I know that once lithium batteries fall under a certain point, they continue to drop in voltage until one or both cells are under 1V. Storage voltage is 7.6V-- I wouldn't store for any duration any lower than that. If you know you ran hard before you left the track you'd best hit storage mode before turning in for the night.

Sounds like a dead cell. I highly doubt it happened from sitting for under 24h.every time I've seen this and you do a full balance charge and let it sit it'll do it again. I play with lipo batteries too much...

This is what I was thinking too, and I hope it's not the case and it's just a fluke considering its nearly brand new. It was a pretty expensive pack too. I've charged it to 7.6V and I'm going to let it sit for a few days and keep an eye on it.

That's way too low of a discharge for a pack to have any reasonable longevity, even for the higher voltage cell. Apparently it was fully discharged.

The typical 3V/cell minimum, preferably 3.2-3.4V per recommendation, is for the lipo under load while running. Under load the voltage will drop/sag, especially towards the end of charge. But it should rise back up when rested for a bit. Preferably above 3.7V/cell at least for good life.


A typical resting voltage vs capacity table, based on discharging at 1C to 3V. Of course this will vary some by cell type but most tests I have seen over the years are fairly close.

4.20v = 100%
4.03v = 76%
3.86v = 52%
3.83v = 42%
3.79v = 30%
3.70v = 11%
3.6?v = 0%


Suggest checking your LVC for proper operation. In any event stop as soon as the power drops off and don't try to coax a little more out of it. Be sure to unplug the pack too.

The LVC is build into my Tekin RS Gen 2 and the only applicable setting is 6.4V. The next setting higher is 9.6V for 3S.

You can also set a custom cutoff voltage on the tekin stuff.

In all likelyhood it is a bad cell. Nothing you did, it happens. Get ahold of TP customer service and the should get you fixed up. No biggie.

Thread/

I've tried "rescuing" lipos that had a cell that had fallen under the norm. But in every case, that cell never performed the same and would cause the battery not only have a shorter run time, but it didn't have the punch and would end up with that cell significantly lower voltage.

I personally think 3.2v per cell is too low. This is based on load tests I do every once in a while to determine what batteries should be disposed of. I saw that many of my batteries had a voltage drop off somewhere around 3.4v. Obviously it depended on the individual battery. So just to be safe, I run my LVC at 3.5v. Now granted, I'm running in races usually, so getting the most run time is not my priority.

Search for "lipo battery dischart chart", and if you find one with a realistic load, you'll see that the voltage drop off happens in many cases before 3.2v

Here is a typical discharge chart for a lipo cell at fairly high discharge levels, up to 50C. Under load not a lot there after 3.4V or so as mentioned. Note this is measured at the cell, there is some voltage drop across connectors and wiring to the ESC where the LVC is.

Chart credit to Everydayflyer on RCG.
http://www.rctech.net/forum/attachme...0-amps.cor.jpg