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Nov 08, 2012, 07:14 PM
Registered User
Cavokk's Avatar
Thread OP
Discussion

New 4s lipo with 1 cell at 0,7 volts restored to life


Hi

I have just recieved a 2200 4s 40 C with one cell at 0,7 volts. With the help of a good PSU I managed to raise the voltage on the bad cell to3,85 volts with a chargerate of 0,1 A. Then I did several normal ballance charges /discharges from 3.85v to 4.20v.(per cell)
I am quite surprised that the battey now seems to be ok. - it charges and discharges normal with all cells ballanced to withing 0,02 volts between each cell and the IR is about 3-4 mohms on all cells.

I thougt that a lipo at 0,7v was dead, - it seems not to be in this case, - I will try to load it beyond 2C tomorrow and see if the battery holds up


Cheers

C
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Nov 08, 2012, 07:20 PM
Registered User
If this was a new pack, return it for refund.

While it may seem ok for now, it most assuredly has a very short useful life ahead of it, especially if the one cell has been below 3 volts for any period of time.

Mark
Nov 09, 2012, 11:07 AM
Registered User
agreed it's not going to last any where near the cycles a good one would, however now that you've raised the voltage you may have a hard time returning it.
Nov 09, 2012, 11:42 AM
Registered User
Cavokk's Avatar
Thread OP
Hi

I did get a refund before charging it so its ok- time will tell how much damage has occurred to the cell. I won't use it on my birds yet - only on my cheap models.

Cheers

C
Nov 09, 2012, 11:44 AM
Registered User
Cavokk's Avatar
Thread OP
By the way - it was a Turnigy batt from HK - send them a picture showing the cell voltages on my iCharger.

My first support case with HK - hope it stays that way
Nov 09, 2012, 01:01 PM
Southern Pride
everydayflyer's Avatar
Snip from OP

Quote:
I have just recieved a 2200 4s 40 C with one cell at 0,7 volts.
Follow up post

Quote:
By the way - it was a Turnigy batt from HK -

Very good they replaced it but still would like an explanation from HK how a customer can receive such a LiPoly when they claim to check every LiPoly before shipping.

Charles
Nov 09, 2012, 10:38 PM
Space Coast USA
hoppy's Avatar
Easy. The cell was charged and tested right after charging, measured 3.85V, but the cell had a manufacturing defect which would cause it to lose voltage over time. As the delivery time can be 3-4 weeks or more, the cell had lots of time to autodischarge. Didn't TP use to charge them and then wait several days or more to check them again to see if they had any bummers in the lot? HK might be shipping them shortly after they leave the plant......who knows how many they reject during the "check before shipment" step. If one assumes that the cells have been sitting around for weeks before testing then I agree with you that it should have been caught but I think they don't spend a lot of time on the shelf.
Nov 10, 2012, 08:22 AM
Southern Pride
everydayflyer's Avatar
Hoppy I agree with your possible explanation and Rick and I have posted same numerious times however HK says they check LiPolys prior to shipping. Now that could mean they chech cells before they are assembled but it reads as though they check the finished product before shipping. They (Hobby King) do not manufacture cells to the best of my knowledge and I do not believe they assemble batteries either. They purchase and sell as most vendors do but on a very large scale compared to most in the hobby trade.

To maintain a high quality controll standard LiPoly cells should be aged for six weeks then checked prior to battery assembly and my guess is most HK LiPloy cells are days old when assembled and never checked after assembly until received by the end user.

Charles
Nov 10, 2012, 11:19 AM
Space Coast USA
hoppy's Avatar
And even if checked after assembly they still may be only days old.

Of all the HK packs I've got, they've all been right on the money. My feeling is that the failure rate is pretty low overall but someday, my turn in the barrel will come.
Nov 10, 2012, 04:24 PM
Frankenstein recycled packs
rampman's Avatar
Magic number is 30 as in 30 cycles till death. : D
Always balance charge to be safe.

Rick
Dec 17, 2012, 06:11 PM
Registered User
Cavokk's Avatar
Thread OP
Hi

18 cycles later on my bat and still going strong NO noticeable difference between cells in IR !! Cycles have been a mix of approx 25 C and approx 40 C (hot)

It will be interesting to see if it changes over time - I will let you know.

Cheers

C
Feb 28, 2013, 05:51 PM
Registered User
Cavokk's Avatar
Thread OP
Hi

A little update on the battery.

I have now long exceeded the 30th cycle and its still going strong. IR on the "bad" cell is surprisingly the slightly lowest of the 4 cells...

Flying is mostly done in cold freezing temps nowadays and I run them at about 25-30C

Makes me wonder that if the cell lost its voltage VERY SLOW and the fact that I restored the cell voltage VERY SLOW at 0,1A then there might not be any damage done to the lipo at all??

Cheers

C
Mar 29, 2013, 04:08 AM
Registered User
Cavokk's Avatar
Thread OP
Hi again

The battery is still going strong and the bad cell still has the lowest IR of the 4cells.

What I have found out is that the bad cell slowly goes down in voltage at a rate about 0,1 volts per 2 weeks when battey is in storage and there must be a manufacturing defect in the cell.

Anyhow my conclusion is that the lipo cell it self besides slowly loosing voltage is FULLY as good as a lipo that has'nt been low on voltage IF the discharge- and chargerate at the time when they are under 3 volts are VERY LOW.

I am fully confident in using the battery along side my other lipos, - it performs perfectly (30-40 C discharge rate in my funjet ultra)

Cheers

C.
Mar 29, 2013, 08:00 AM
Space Coast USA
hoppy's Avatar
Over discharging like you had with that one cell causes copper dendrites to form in the cell causing internal shorts which is why the pack loses voltage and has a low IR. If the shorts get big enough the battery can go thermal so they say.
My opinion.

From http://www.mpoweruk.com/lithium_failures.htm
Quote:
Under-voltage / Over-discharge
Rechargeable Lithium cells suffer from under-voltage as well as over-voltage. Allowing the cell voltage to fall below about 2 Volts by over-discharging or storage for extended periods results in progressive breakdown of the electrode materials.
Anodes

First the anode copper current collector is dissolved into the electrolyte. This increases the self discharge rate of the cell however, as the voltage is increased again above 2 volts, the copper ions which are dispersed throughout the electrolyte are precipitated as metallic copper wherever they happen to be, not necessarily back on the current collector foil. This is a dangerous situation which can ultimately cause a short circuit between the electrodes.
Cathodes

Keeping the cells for prolonged periods at voltages below 2 Volts results in the gradual breakdown of the cathode over many cycles with the release of Oxygen by the Lithium Cobalt Oxide and Lithium Manganese Oxide cathodes and a consequent permanent capacity loss. With Lithium Iron Phosphate cells this can happen over a few cycles .
Mar 29, 2013, 08:07 AM
Registered User
Cavokk's Avatar
Thread OP
Quote:
Originally Posted by hoppy
Over discharging like you had with that one cell causes copper dendrites to form in the cell causing internal shorts which is why the pack loses voltage and has a low IR. If the shorts get big enough the battery can go thermal so they say.
My opinion.

From http://www.mpoweruk.com/lithium_failures.htm
Thanks for the info, - it sounds plausible

Perhaps for safetyreasons I should remove the bad cell and convert it into a 3S battery.

Cheers

C.


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