Lipo experts - I need you!

[Xanda2260]

Hey guys, i'm really sorry to be a pain, but I've encountered something with my new lipo that I have never read about.
I'm balance charging a 6s Turnigy pack and one cell will not come above 3.90v. All others have reached 4.18-4.20.
It's been charging for over 90 mins with the charge current reading about 0.5A and falling.

Am I being impatient? I realise it will take a while to bring the cell up, but it flew up from the 3.72v it arrived at.
Im just worried the cell is bust

Thanks in advance

Alex

[miuan]

If the cell came badly out of balance, you can't expect it to equalize during a single balance charge. Not all chargers are able to do so in short time. Either be patient or top the cell manually using a cell phone charger or other 5V source, sticking two thin wires into the appropriate holes in the balance plug.
Based on the voltages you stated, it seems the cell takes and retains charge well in short term, it was just the immense difference in state of charge between the cells that caused your problem.
Watch out for the rare occasion of a cell refusing to get over a certain voltage. Don't force it with too much current so you prevent the cell from shorting and going thermal. Charge at a safe place.
The cell came in low most likely because it likes to discharge slowly on its own, and will probably keep doing so. Once you reach uniform voltage, keep an eye on the cell and see how fast it loses balance. I'd lived with such cells for months, but the constant need of topping them had been a pita. I ended up getting rid of them in favour of tens of cycles between balancing.

[Xanda2260]

Awesome, thanks for the quick reply! Voltage has risen to 3.92v now so I guess I am just being impatient. Thanks for the reassurance!

Alex

[wojtek]

best would be a Variable DC Power Supply..

very practical for Lipo users. You can solder the wires from power supply to 2 balance connector pins [if you have some spare] and the just plug into the battery balance leads - to charge one cell within this bettery pack. That is how i do it.

alternatively you can just sacrifice one balance leads extension, cut and keep the male side with the wires coming out, and then use crocodile connector out of power supply to connect to whatever 2 wires out of the 7 you will have for 6s pack..

you can do it with balancing charging but it will take ages. With power supply it is very quick

[MadRhino]

It could happen that you have a weak cell, all Lipo bricks are not 100% good.
It is also possible that this cell is just lower, wakes up with cycling, and become reliable after.

First, lower your top charge voltage to cycle your new Lipo, let's say 4.1v
Set your RC charger to balance cycle 3.7 to 4.1 and let it work.
If you find it slow to top, you can use the storage mode to lower the cells, then start to cycle again.
Eventually, after a few cycles the weak cell will catch up with the others and you could save this brick.
If you succeed charging all cells to 4.1 after a few cycles, you can raise to 4.15 and do a few more cycles.

Best is to cycle all your Lipo bricks and make sure to assemble equal and reliable bricks to make your battery.

[Xanda2260]

Cheers guys. I do have a variable dc supply, so may give that a shot.
My rc charger does not appear to allow me to select the termimation voltage beyond 6s, so it's finishing at 4.2. Serves me right for buying cheap! Still, once the packs are balanced properly I'll be bulk charging most of the time anyhoo.

Many thanks fellas!

Alex

[smudger1956]

I have had a few 6s1p 5000mah packs with a 'low cell' from new, I connect the balancer a leave till all the cells are equal, them bring up with a low amp charge, repeat if necessary, a bit anal,these packs are used for RC Aircraft so I want them 'spot on'.

ATB
Tony