Do I have a weak cell?

[cwah]

Hello there,

I'm playing with my hyperion and tried a charge discharge test to see how my lipo zippy 20S8AH is performing after 40 cycles. And if I shall invest in A123 or keep my lipo (depends on the capacity loss).

It seems to be very balanced when I'm charging/discharging from 4V to 3.75V. But over that I've one cell that peak a lot over 4V:

This is the charging graph until 4.12V. I didn't go over to preserve the battery life. But when I go over the gap from the red cell is higher and higher.


What's strange is that during discharge, the cell quickly align with other cells:

During discharge the cells gap become minimal (around 0.014 mA gap), and capacity between my 4 * 5S8AH zippy pack are similar.

I've done something like 10 cycles of charges/discharge without balance if I keep my cells between 3.75V and 4V and they'll remain greatly balanced. All bulk charge no hassle! 8)

Under 3.65V, I have the red cell that's falling down like crazy:

The other pack performs better, but it's still a cliff under 3.65V:

However I'm still a bit concerned. Is it a weak cell that may fry over time?

Shall I..
- Replace the cell
- Or add an additional pack on top to compensate this cell
- Or maybe it's normal and I don't need to do anything?


ps: I have around 6AH available on discharge between 4.07V and 3.65V. Not sure if it's good for a pack.

 

[dogman dan]

Not one worth worrying about yet. When you get one that drops overnight more than .1v than the others you'd have one worth keeping an eye on for awhile. Then ditch it if it gets much worse.

 

[cwah]

Thanks Dogman, I can keep cycling with confidence then

 

[cwah]

This weak cell keeps worrying me:

It gets out of sync with other cells from 4V onward, so no way I can safely charge more than 4.15V...

 

[NeilP]

That is not a weak cell, it looks fairly normal to me.

The drop over the cliff voltage is about normal, they all vary a bit It is stable along most of the discharge curve..nothing at all to worry about

I have seen much much worse

 

[cwah]

ok thanks then. Too bad they are not all aligned to the same voltage. That would be simpler and I'd be able to charge up to 4.15V without much trouble.

 

[Jeremy Harris]

I've seen far worse during charge, even from really good packs. Just yesterday I charged a 10S pack that I'd let get down way lower than normal. I charged it as two 5 S packs, with two chargers, and had variations between cells of up to 0.2V at times during charge. One of the two packs took 9.3 Ah to charge (they're 10 Ah packs) the other only took 8.8 Ah to charge, just down to differences during the balance phase. Doesn't worry me unduly, as when they'd finished charging all the cells were within about 20 mV.

Sometimes looking at data graphically like this can make you concerned about things when there's no need, as these Chinese cells will have a fair tolerance on capacity, which manifests itself as fluctuating cell voltages during charge. It's another reason that thinking that cell voltage is a measure of balance at anything other than full state of charge is unwise.

 

[cwah]

Ok thanks guys. No more worry for me until there's a huge gap and fluctuation. That's my first daily lipo pack that's why I'm a bit over-worried :lol:

 

[cwah]

I maybe don't have a weak cell, but it's still quite annoying to have a cell that top to the maximum too early:

The blue cell reaches his maximum too quickly, so in 3 hours charge none of the others cells can't reach their maximum

 

[icecube57]

http://endless-sphere.com/forums/viewtopic.php?f=9&t=41452#p606263

Here is a charge and discharge graph of two 4s packs with kinda close AH and both have a weak cell in them one comes up with the other cells and one overcharges and hits 4.2v first...

 

[motomech]

I maybe don't have a weak cell, but it's still quite annoying to have a cell that top to the maximum too early:

The blue cell reaches his maximum too quickly, so in 3 hours charge none of the others cells can't reach their maximum

Pull that cell individually. I made and adapter using a JXT 2-pin male plug with 4 mm bullets on the ends. I plug it into the corresponding holes in the balance connector and use the 1 cell discharge function my my cheap Thunder balance charger. Don't blink, as it comes down pretty fast at -4.5 Amps
Then bulk chaRGE the group and see where you are at. You may have to repete several times till that cel gets the idea and stays with the group.

 

[cwah]

Thanks Momotech.

So I have to buy an adapter and discharge the higher voltage cell?

But I think this cell is still remain higher than other as during the discharge it has a similar voltage to other cells.

 

[Jeremy Harris]

It looks to me as if the charger just isn't balancing the pack correctly, but shutting off too soon. Ideally a charger should clamp the voltage of the cell that reaches cutoff of first (usually the lowest capacity cell in the pack - there will always be one), then continue to allow the other cells in the pack to catch up. Your charger seems to be cutting off before going into the balancing phase properly.

My cheapo LiPo charger has two charge settings, a LiPo charge mode and a LiPo balancing charge mode. If I use the plain vanilla charge mode it does what yours seems to, shut off half an hour or so after the first cell group reaches 4.2 V. If I use the balancing charge mode then it can sit for a couple of hours after this first cell threshold point, charging and discharging to get the pack in balance. The balance charge mode does take a long time to charge, but pretty much does as motomech has suggested and drags the higher capacity cells in the pack up to a proper full charge. His method is just as good, and probably quicker, but it might be worth exploring the menu on your charger to check a couple of things, like whether you're in balancing charge mode and whether you have some safety time limit set.

I've had my chargers cut off prematurely when balancing because I had the safety charge time limit set to too short a time, so the charger didn't have a chance to get the pack properly balanced. It still happens on my 30 Ah pack from time to time, even with the maximum time limit of 240 minutes set. I just restart the charger and let it have another go and it works just fine then, it just goes straight into balance charge mode and sits there doing its thing for an hour or so until all the cell groups are up to voltage.

 

[cwah]

Thanks Jeremy for this insight.

I think my Hyperion 1420i (14s) is only able to do charge or balance at once. It can't do balance charge.

What happen during the charge process is that the hyperion charge the cells to the maximum capacity. Then it stops and start balancing. Or if I select balancing first it doesn't charge.

I think I'd need to charge it to the max. Then balance. Then charge again. Not so convenient the hyperion.. :/

 

[cwah]

After balancing the cell, I tried to charge again:

Wasn't very useful

 

[NeilP]

Not had a Hyperion charger so no idea of the menu system..but go thought the settings.

Maybe there is an always balance and/or slow balance setting. try various menu settings to see if you can find better setings

 

[cwah]

I think I found what's happening: The balancing is so slow that it was hardly noticeable:

If you look carefully, all other cells are very very slowly catching up. If think in half a day my pack will be balanced... :?

 

[docnjoj]

What about using dnmun's suggestion to put a resistor or load across that one high cell to draw it down until the others catch up? I don't remember the resistor size but maybe he can jump in here.
otherDoc

 

[cwah]

hmm... you mean to add a resistor in the balance plug? Because I can't see where else I can put my resistor when my pack is balance charging on the hyperion.

 

[docnjoj]

You need either a resistor or even a flashlight bulb with 2 leads,probably on an old JST connector to that one cell to discharge the individual cell. Could go through the back of the present charge plug, Small but a helpful device. Or just put it across the tabs while charging using alligator clips for the tabs of that cell.
otherDoc

 

[cwah]

Put a resistor between the balance lead with another one is a nice idea.cI'm going to try that. Thanks

 

[docnjoj]

Just remember those JST-XH connectors really are fragile. But everyone uses them. The pins are tiny.
otherDoc

 

[cwah]

I know, I bought some JST-HX connectors from ebay and every time I try to solder them the cable just doesn't stay. I'm trying again with the wires from Hobby king to see if I have better results

 

[docnjoj]

The ones that come with the Hyperion and the ones agniusm supplies are quite a bit more robust . Gary used valu-loc on his and Fechters BMS kit which are much better. If you are only using the JST ones a few times to determine voltage or amps charging it is probably ok. Someone recommended straight brass pins, like for sewing and small alligator clips for one or 2 tries. Stick 'em in the back of the connectors and do not let them touch!
otherDoc

 

[cwah]

Thanks Doc, going to check agniusm and straight brass pins for the JST connectors. I do balance charging everyday but I always have problems with these JST connectors and wires. They disconnect from the battery (have to resolder) or are not recognized by the hyperion. Maybe will solve my problem