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[Nige52]

Discharge a single cell in a 3 cell Lipo pack? Is it possible to connect something that will do this, without interfering with the other 2 cells?
Please don't say get a balancer, I already have one that cost quite a bit, but doesn't work. I am aiming to attack a single cell that is way above the other 2....if I can of course 

[andyn]

Won't make the blindest bit of difference if you do discharge it, cos when you charge them up the same thing will happen again

It's more than likely if two cells are showing around 2v and the other one is showing around 4 your pack is on its way out.

[nick_75au]

You can do it, you need to attack the problem through the balance tap of the battery.

you need to put a low amp load on the high cell, sompthing as simple as a  flashlite bulb with wires soldered onto it, best wire would be solid core say from a telephone cable or the insulated wire supplied for electronics bread boarding. A 2 pin PCB header would be better for a more pemanent setup.
 
A 3.7 ohm 10 watt resistor would be better as a load, this will discharge that cell at around 1 amp. the resistor will be a ceramic type and will get quite warm during the discharge

You need to be carefull not to short out the taps as well.

You need to Identy the wires on the balance tap that corespond to the high cell and insert each wire into the plug at the appropriate place.



i.e. if it was the centre cell of the example then the bulb or resistor would be placed accross the two center connections in the tap.

You will have to constantly moniter progress of the discharge with a meter to ensure the volts do not drop below 3 volts, I would let it discharge untill about half a volt difference then try again on the balancer.

Nick

[Nige52]

The high cell is 4.23, then 3.89 then 3.90 when the battery is fully charged up  

Edit: Thanks both for your advice, appreciated  So.....it's putting a load across 2 terminals on the balance lead eh? I have a 12v car light bulb, would this do it? I have no resistors anywhere....

[Nige52]

Nick,
You are suburb!! I have knocked up a simple 12v car bulb to 2 metal probes and am now discharging each cell individually! It works like a dream and extremely quickly, so you have to keep an eye on it and check every minute.
What's happening is, when connected the bulb glows and it rapidly pulls the voltage down from say, 4.23 to perhaps 3.59 in less than 2 minutes. But if you leave it unconnected for a couple of minutes, the voltage in that cell creeps back up to a stable level, maybe 3.80 say.
So it's a bit like a yo yo, or a ball bouncing, it bounces back up, but not as high. So I think around 4 goes will bring the errant cell back down to where the others are, I'm still playing with it, and will post a pic of the probes as soon as I'm sure it works.

Andy, I don't know yet about about charging the battery back up, you could be right and the cell will continue to be mis-matched, but now at least, I know how to do it 

[nick_75au]

Good news then, the car bulb imight have a little bit to low resistance but it looks like your on top of it

What wattage is the bulb?

Cheers
Nick

[Nige52]

The bulb is a car indicator bulb, 12v and 21w.
I have found that on another battery I'm doing right now, it's taking 30 - 40 minutes to pull one cell down from a high of 4.28 to 3.75, maybe because it's a higher capacity battery. Anyway, it works perfectly, so thank you Nick 

[nick_75au]

21 watt bulb, 6.8 ohms, should be fine, gives about 600 mAH draw. If it's doing what you describe then that cell might be crook. Let us know how you go .

Cheers
Nick

[CF-FZG]

The high cell is 4.23, then 3.89 then 3.90 when the battery is fully charged up  :-)resistors anywhere....

If that's the voltages for the 3 cells, then it's not charging properly, and sounds as though it's cutting out when 1 cell is fully charged, but not balancing to the other 2 cells. 

Is it just this battery the chatger has a problem with, or do others have the same problem??


Mark

[grasshopper]

I use a 'battery doctor' made by Etronix for my race packs - it's slow but works without me worrying about over discharging Li-po cells. When they are all down to the same set voltage I then balance charge them at 1C using my fast charger and everything's hunky dory for a month of twice weekly battery abuse!

[Nige52]

Mark, it does this on all my batteries, if the cells are about even, it charges them up at the same rate, but appears to do the same if they are uneven as well. I can see that if I set the charger at 1a for example, when the battery is nearly charged, the indicator shows a charging level of 0.2 for example, then flickers around 0.0 - 0.1, then beeps to tell me the battery is charged.
After I've let it rested for a while to recuperate, I put my tester on the cells and on the wayward batteries show the different voltages in the cells, even though I set the charger to balance charge. It's a B5, without the discharging option.
Cheers
Nige

[essex2visuvesi]

If its a Chinese copy one then this is quite normal.  It seems from the information posted on various RC forums that the balance part of the "Clones" is not very well calibrated.
Good news is tho there are some well documented mods for this and the B6 charger to improve the accuracy of the balance part of the charger

[Nige52]

I've had this one for about two and a half years. It's an imax B5 and the header says LiPro Balance Charger.
How can I tell if it's a clone or the real thing?

[essex2visuvesi]

The iMAX B5 and Turnigy Accucel-6 are Chinese clones of the South Korean BANTAM e-STATION chargers and apparently use lower-quality, less-accurate components.  Much of the time the lower quality is fine and accurate enough, especially if there is a significant price difference.  However, it's a good idea to compare the cell voltages reported by any charger against those of a decent (accurate to within 1%) multimeter a couple times a year.  If the charger is out of spec and significantly reducing the life of your batteries, then a higher quality charger might pay for itself pretty quickly in battery replacement savings.

Here are a few discussions to review:
http://www.rcgroups.com/forums/showthread.php?t=777717
http://www.rcgroups.com/forums/showthread.php?t=886698

[Nige52]

Ah, I see now....
Okay dokay, so what recommendations are there for a new charger? I want a reasonably priced multi function charger with discharge facility that has a good track record? This one is probably fine, but when I have doubts, I buy something new......

[essex2visuvesi]

These are getting good reviews and are what the Chinese are copying
http://bantamtek.com/00_eng/02_pro/sub01.asp

UK supplier here
http://www.energonsolutions.co.uk/

[Nige52]

I think the B5 charger is faulty......it won't balance charge using just the white balance plug as the manual says it will. If I try to do a balance charge without the main red/black deans plug connected as well, I get the error message and beeps, CONNECTION BREAK. It has worked this way from day 1, and I always thought this was normal and BOTH leads had to be connected.............DOH!!!!

[Nige52]

I like that....but £108 will have the other half cursing a bit................

[nick_75au]

Thats normal, 95 % of the balance chargers charge through the main leads and discharge (bleed off excess) voltage through the balance tap.

I Use an Icharger 208B , excellent calibration from the factory, and if it drifts you can recalibrate it. They do several others with different capabilities like the 106B for up to 6 cell etc
Cheers
Nick

[essex2visuvesi]

I think the B5 charger is faulty......it won't balance charge using just the white balance plug as the manual says it will. If I try to do a balance charge without the main red/black deans plug connected as well, I get the error message and beeps, CONNECTION BREAK. It has worked this way from day 1, and I always thought this was normal and BOTH leads had to be connected.............DOH!!!!

I have the Imax B6 I have to connect the main battery connector and also the balance plug

[John Mk2]

The B6 is VERY good for the money Had mine for about 3 year's now  Never missed a beat.

[Nige52]

I looked here on the Astec site at the manual. If you scroll down to page 13, it actually says that the mains lead need not be connected.......

http://www.astecmodels.co.uk/pdfs/Imax-B5%20manual.pdf

Chinglish? 

[John Mk2]

 

[grasshopper]

This is mine, http://www.venom-group.com/itemdesc.asp?ic=0657 not cheap when I bought it - I think this is the one that is now the ImaxB6 for a lot less than I paid!

And I've always had to have both leads connected to charges Li-po's