I purchased a 5000mA Turnigy Nano-Teck 4S 35C, at Hobbyking for my quad-copter in build.
After a small error, i fried out one the "ground side" cell of my battery with 5 or 6 recharge cycles.
So i decided that i dont need right now a 3S battery, so i purchased a 1S Turnigy Battery bundle to repair my damaged one.
So first thing first, remove the damaged cell, was easy with my 80W iron.
The new cell, fortunately was almost the same size,just a little wider and shorter.
Is a Turnigy 5000mA 40C
Cheking Battery Health - OK!
All ok, and alight, now is soldering time.
A final Check.
And the final result.
After 30min, and some tape i finally restore the battery, i spend 12€ in the end.
The battery is working, and i know that is not a long life battery, after a over discharge, the renaming 3 cells are stressed, but for a low price and only 2 battery's in my gear that worth the risk.
I have some doubts, the cover is insulator tape, and i dont know how she will do in heat. and the C rate, the original turnigy as 35C and the cell that i brought is 40C .
I hope that you enjoyed my little article.
I make the frist test.
I arrest my quad-copter and continuously make measurement of the voltage total and by cell.
C1 - Positive Side (ORGANGE), C2 (GRAY), C3 (YELLOW), C4 new cell (BLUE)
I stop the test at 3,1 volt in one of the cell's
Bill Bonnie – actually not correct. The discharge rate of a cell is not determined solely by current, it is by power, or total energy (watts/watt-hours) which also must take into account the voltage of a cell. A good way to analyze this is by imagining the cells as capacitors. The cell with the lower rating would have the equivalent of a lower microfarad rating. You are correct that the current would be the same in series connected cells; however the capacitor with the lower (or higher) microfarad rating would have a different voltage as the pack charged or discharged. In the case of charging, the balancer circuit in the charger would do its best to balance the pack; in the case of discharge, the mismatched cell would go to a different voltage (lower if the ma rating was smaller), and the voltage of the remaining cells would maintain current flow without regard to balance. Since the low voltage cutoff point is determined by a controller which assumes a matched cell configuration, it would allow discharge to continue until the lower ma rated cell falls below the damage threshold. As long as the cells are well matched in ma and C rating (as well as remaining life), this battery fix should be fine.
With regard to safety considerations with soldering cells, assuming that the person doing the work uses the same level of caution he does when dealing with multiple props spinning at thousands of RPM, this is really not much more dangerous than other aspects of the this hobby.
Wow that was a little extreme, my worst accident was with a 19V 120W Laptop charger, i put the finger on a giant Capacitor and he lovely make a good electric burn line, with entrance point and out point (on the other side of the finger).
About the article.
When i make that article was not to be a Tutorial, i didn't shown all steps, and didn't make the warning's. That was simply a sharing (?).
About the imbalance by difference C rates, i will monitor upclose the cell's and later if possible i will share with you a spreadsheet with the result's, i always recharge my battery's in balance mode.
Again thank you by the suport, not from the task, but by helping-me show that what i have done was not a CRIME but just one more in many's dangerous things to do.
Quazi: a cell will not charge faster, discharge quicker, and who does anything other than balance charge?
Batteries in series discharge at the same current, and in parallel will have the same voltage. If a cell in series reaches a low voltage 3.5v before the others it limits the pack to that amount of power. This is why cells are 'matched' If you use a 5000mAh cell in this case it's a good match.
I agree, if you don't understand, discharge a battery using a bulb to really flat and discard appropriately.
There are safe handling procedures for all batteries, and it's good to know them.
So I took the latter option, and scrubbed out the wound with a nail brush in the shower. Hurt like hell, bled like crazy, but worked a treat. Barely a scare to brag about, even under the right light!
Ooops, Which option did you choose?
One evening, about 2 decades ago, when I was a wide eyed 13-year-old rc buggy fan, I was putting the final touches on a new high capacity high current 7.2V battery using state-of-the-art Sanyo 3300Mah NiCd Sub-C cells (cost a small fortune, weighed a tonne, peaked at something like 200A, lasted about 2-3minutes... worthless junk by today's standards).
I had series'd the cells, but had yet to attach the battery cables. While wrapping the cells up in electrical tape, I inadvertently compressed the pack against my chest for leverage. Bang, I got the fright of my life. The sound, smoke and smell was dreadful!
I looked down to see a 8-10mm circle burnt into my tee shirt... and my belly felt itchy. When I went to scratch the itch, my finger sunk into a freshly formed void in the centre of my rather lean frame. No blood, no real pain even, just an approximately pinky sized hole in the dead centre of my midsection. Oops... Worse was yet too come though!
Later that evening, when my dad saw my perfectly cauterised wound, he presented me with two options. (A) I could throw some antiseptic on it and hop off to bed, and grow up with a pinky sized hole in my chest, or (B) I could unseal the wound, deal with the pain, and let the hole heal itself closed...
Some great observations that should be kept in mind.
Based on your observations it seems that the ROI on attempting to repair a damaged cell might not be worth the effort involved and the possible failure of the repair.
The tabs on these prismatic cells are very thin and quite fragile, and can become damaged relatively easily if over stressed. Over stressing is not difficult either, due to the fact that tabs are often crippled or welded together, with the solder only used for attachment of power and balanced connections. This inevitably leads tinkerers to heat the joint while using force to pull tabs apart, which can easily lead to damage or injury (cuts, burns and cauterised wounds are all possible).
In is also worth noting that simply bending these tabs is enough to increase resistance across the cell.
Combining cells of different types, sizes, or discharge ratings, or even just identical cells with unmatched service lives, has it's issues. For example, the 40C rated cell added to this 35C pack will not only (likely) charge quicker (mandating balanced charging only), it will also likely discharge quicker (mandating a multi-cell battery monitoring system, over a series voltage sensor) while in the air.
Considering the price we pay for these packs, along with the inherent risks associated with their use, we really should be thinking twice about opening them up.
It is also important to understand whether your home insurance covers a fire caused by a lipo. Depending on where you live in the world, having a compliant fire extinguisher might be mandatory (it is for me).
Nannies gotta Nanny. How did we survive without them.