Bad 4S Packs ???

[FloatDaBoat]

I just received two Turnigy 4S 3000 mAh packs from HKUSA, that I planned on running in parallel. I wanted to cycle them a few times to ensure that they were OK before replacing the 4 mm bullets with 5.5 mms. During discharge, one pack was terminated by the charger due to a “Low cell voltage level”. I checked the pack with my EOS Pack Sentry, & cell #2 was ~ 0.7 volts lower than cell #1. On the other pack (which terminated ‘normally’ according to the charger), cell #2 was ~ 0.6 volts lower than cell #1.
Discharge cut-off voltage on the charger for 4S is 12 volts total (3 volts per cell), and the discharge rate reached 1.8 Amps maximum just before discharge ended. Both packs charged normally [?] back to 16.8 volts after being discharged - - no charger warning issues. I decided to re-discharge both packs again, this time with my camera ready to verify individual cell readings … see pic.
Both packs terminated ‘normally', but according to the photo & EOS Sentry, cells 2 & 3 read significantly lower than cells 1 & 4 (as in the photo). Do I have bad packs, or would cell readings be more balanced pulling a heavier load ??? I could build a Load Bank if anyone thinks that would be worthwhile ???
Any thoughts, opinions, personal experiences are welcome . . . .

[kookie_guy]

Personally, I think 3v/cell is too low of a cutoff point. I generally set everything to 3.4v/cell, NEVER lower than 3.2v/cell. When you charge the pack, does it balance back out? Does your charger tell you how much mah went back into the battery pack?

[Fluid]

Never charge packs which are not in balance!!! The maximum difference in voltage between cells in the same pack should be 0.02 volts. The charger should have terminated the charge when the pack voltage was 16.8 volts. Ideally every cell should read exactly 4.2 volts. Something is seriously wrong with either the cells, the charger or both. At this point it is hard to tell which is bad. Put the packs on a balancer until all cells in the pack read the same, then charge and see what happens. Charge questionable cells in a safe place!



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[FloatDaBoat]

Kookie: The packs balanced OK upon charging. The one in question took 2907 mAh. The EOS Sentry read ~ 3.1-3.2 volts for cells 1 & 4, not the 3.4 volts as shown in the photo after discharge. Maybe the charger program is faulty. The only option available for discharge mode voltage is 3.0. The cells were fairly evenly balanced until the discharge voltage got down to ~ 3.2, when the strange stuff started happening.

Fluid: After a few minutes, cells 2 & 3 self recovered to above 2.9 volts. I then storage balance charged the packs back to 30% according to the EOS Sentry, & all cells were within 2 millivolts on one pack & 7 millivolts on the other before being balance charged back to 16.8 volts. A LiPo fire is the last thing I need !!!

[Fluid]

Why are you discharging them all the way in the first place? This is not necessary, unless you want to see what happens when you over-discharge them. Just discharge them to 3.6 volts and charge them up. You may be doing more damage than help.


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[FloatDaBoat]

This is my first experience with 4S packs. All my previous LiPos have been 2S.

I wanted to check if there was anything obviously faulty with the packs before modifying them with 5.5 mm bullets, in order to avoid any warranty issues if I had to return one or both of them. Being Financially Challenged (Dirt Poor - - HAW !!!) no doubt contributed to my folly.

I respect your advice Fluid, & will modify my inept methodology concerning LiPos. Thank You !!! Time to plug-in my 100 watt iron to replace those 4 mm bullets & fabricate some parallel adaptors.

[kookie_guy]

You are really overdischarging your packs. Golden rule is to discharge til there's 20% capacity left. So that would mean your charger should be putting 2400mah back into the pack. You are discharging them to 3%, WAYYYYY over the magic number.

[FloatDaBoat]

Thanks for helping to set me straight, kookie_guy !!!