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What would be considered a bad cell in a lipo pack? - 2/8/2010 7:35 PM   
c6z06


 

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Status: offline 		I just bought two 3s 5000mah turnigy lipo battery packs for my vorza and after a full charge the first pack read 4.19v, 4.19v 4.18v and the other pack read 4.20v 4.20v and 4.11v. Is 0.1 difference in voltage anything to be worried about? The pack was still 0.08v difference after 10 minutes of runtime so I think it's just the charger failed to balance but I'm still interested in how big a difference would be a concern., I have my cutoffs on my cars set at 3.2v per cell anyways though.

Also I purchased a 5000mah 20c discharge 2s turnigy pack for my brother's rustler to replace the 7 cell 4000mah ELITE Ni-mh pack he was using and I was hoping the pack would be as fast as the 7 cell even tough the voltage fully charged is about 1.5 volts less and it's only a 20c lipo. Well the new pack is 160-170 grams lighter than the Ni-mh and now the truck can do wheelies and tops out around 43-44mph where before it would just do 40mph. The Ni-MH pack was $35 and the lipo was $19.95! Lipo's continue to impress me.

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RE: What would be considered a bad cell in a lipo pack? - 2/8/2010 7:44 PM   
Access


 

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Status: offline 		You need to use a balancer before it gets too far out of balance. If any of the individual cells is > 4.25 (after charging) or < 2.8 (after discharging) they are getting damaged.

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RE: What would be considered a bad cell in a lipo pack? - 2/8/2010 10:51 PM   
c6z06


 

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Status: offline 		just charged it again and it was 4.19 4.19 4.13 this time, It is a balance charger though, Accucell 6

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RE: What would be considered a bad cell in a lipo pack? - 2/9/2010 12:44 AM   
Access


 

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Status: offline 		Evidently not a very good balance charger if you measure the cells right after charging and they are out of balance.

Some so-called 'balancing' chargers just stop if any cell > 4.20V which doesn't really help on the discharging end.

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RE: What would be considered a bad cell in a lipo pack? - 2/10/2010 3:18 AM   
BigTb17


 

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quote:

ORIGINAL: c6z06

just charged it again and it was 4.19 4.19 4.13 this time, It is a balance charger though, Accucell 6


Pay attention to the individual voltages during the charge. Are they all around 4.2 just before it stops and then one drops down to 4.13, or was that all the higher that one got during charge?

As Access said, it's possible the charger is stopping because one or more cells is getting above 4.2 v and it can't do enough to keep them from going over and so it stops. If this is the case try charging it slowly (an amp or less) and see if it becomes more balanced.

It's also possible that it simply has a different internal resistance (that one cell) than the other cells (or even possibly a bad solder joint) and will constantly be getting out of balance. If this is the case you will have to be careful not to overdischarge that one cell. The LVC watches total voltage and if one cell becomes drained very much before the others it could "dump" and drop below the safe voltage quickly. Check the resting voltage after you run it and compare the difference then.

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RE: What would be considered a bad cell in a lipo pack? - 2/10/2010 7:48 PM   
Access


 

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quote:

ORIGINAL: BigTb17
It's also possible that it simply has a different internal resistance (that one cell) than the other cells (or even possibly a bad solder joint) and will constantly be getting out of balance. If this is the case you will have to be careful not to overdischarge that one cell. The LVC watches total voltage and if one cell becomes drained very much before the others it could ''dump'' and drop below the safe voltage quickly. Check the resting voltage after you run it and compare the difference then.

No, internal resistance alone should not affect balance. Once they are balanced, they should stay balanced. Likewise, once they are out of balance, they will stay out of balance. This is different than with NIMH or other batteries b'cos of what happens when the batteries reach the fully charged state. The NIMH would just not accept the extra charge (to a point) and the voltage would peak and then start to drop. With LiPo the overcharge state must be avoided, so the charge has to be stopped then and there if the balancer cannot do the job.

From elementary electronics, series and parallel connections: Elements of a circuit in series must have the same current flow, so even if they have different internal resistances, the current flow that passes through any element will be equal to the current flow that passes through all other elements. Hence they will always charge and discharge in sync.

The main influence with going out of balance is self-discharge over time, self-discharge and/or cell damage due to temperature, and so on. If you have cells that were not well matched in terms of capacity, or cells that have lost some capacity due to uneven aging, heat damage, overcharging, etc., they may be out of balance on the discharge end. Also, when you first get a battery that has been sitting on a store or warehouse shelf for 6 months to a year, it may be out of balance.

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RE: What would be considered a bad cell in a lipo pack? - 2/10/2010 8:38 PM   
BigTb17


 

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Status: offline 		Amperage is the same everywhere in the circuit, but not voltage. If the voltage drop is greater across the battery with the higher resistance during discharge it could drop below the safe level before the others do. Also, while charging the voltage could also be different, causing an issue like he is seeing. This is what I was talking about.

I've read enough of your posts to know not to argue with you . If my above reasoning is somehow flawed you can let me know.

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RE: What would be considered a bad cell in a lipo pack? - 2/10/2010 8:42 PM   
HJJFFFAA



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Status: offline 		Are you sure you are in the balance charger mode? And not just LiPo charge? Mine balances my packs just fine.

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RE: What would be considered a bad cell in a lipo pack? - 2/10/2010 9:58 PM   
Access


 

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quote:

ORIGINAL: BigTb17
Amperage is the same everywhere in the circuit, but not voltage. If the voltage drop is greater across the battery with the higher resistance during discharge it could drop below the safe level before the others do. Also, while charging the voltage could also be different, causing an issue like he is seeing. This is what I was talking about.
know not to argue with you . If my above reasoning is somehow flawed you can let me know.

No I know what you're saying but I don't think it should be that significant when charging. Since by the end of the CC-CV charge, charge current is typically pretty low, just a few hundred mA down to 100mA. If it's the charger that is displaying cell voltages realtime, the voltage pull-up towards the end of the charge should be minimal. Sometimes this can be significant when discharging (ie. one cell drops below 3.0V before the others due to differences in internal resistance, once the discharge is stopped they will all measure the same).

I guess it goes back to how balancers work, there are a few different types and the best (ie. 'blinky') will balance dynamically throughout the whole charge or the majority of it. Then there is the type that just act like a voltage clamp, like based on the kit design I posted in an early thread, and finally the cheapest type is just a set of closely matched resistors. But all these types typically have a sink limit of 100-200mA, so if you have a battery that is pretty far out of balace, you can't depend on the balancer to equalize the voltages over a single charge and end up having to charge each cell individually, or charge at a very low rate 100-200mA.

And I've been wrong before no one is (or should be considered to be) an absolute authority on anything.

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