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12v DC Li-ion- Voltage drops per second!!!

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spitso

Member
Hi all

I have purchase a 12v DC Li-ion battery pack awhile ago. http://cgi.ebay.com.au/ws/eBayISAPI.dll?ViewItem&item=220425276858.
Its current limited, as soon as short occurs it only puts out 2.98v and to reset it, it must be conected to recharger for couple seconds.

The problem im having is its not holding its charge, ive conected a Multimeter across to measure V after recharge and i get around 12V then gradually drops 0.01v every couple seconds. And all that its really running is an LED to show that the unit is on.
I ripped it open an found that the battery on the right has voltage drops every couple seconds and drains down to 2 or so volts. The other two dont change from 3.98v(left) and 4.16v(middle) during use, or after recharge.
Each battery conects to an little IC then to next battery and so on.

below is rough idea of electrical layout. aswell as some photos.

if anyone could explain whats happening here and how i could fix it woulld be most appreciated.

thankyou ;)
 

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gerty

Member
What is the voltage across each individual cell.
what are you using to charge it, and at what voltage.
 

spitso

Member
left 3.98v middle 4.16v right 3.9-2.3(only one that varies).

Charging is done with supplied wall charger which outputs 12.6v @200mA
 

gerty

Member
I second what Bill said about the charger. The reason I asked about the individual voltages was to see if you had a weak cell. Apparently you do, but then again, without a proper charger it's just a guess.
Have you tried the led on the two cells that hold the charge, to see if they drop in voltage.
 

audioguru

Well-Known Member
Most Helpful Member
Your battery has one cell bad or the protection circuit is bad.
 

bountyhunter

Well-Known Member
Either one cell is bad or that cell is simply dischrged deeply and your pack needs to be balanced. Li's in series have to be balanced periodically to compensate for differences in cell capacity since they are charged from a constant voltage source. If you have a good power supply you can set to 4.200V, you can charge each cell individually by connecting directly to it. Then retest the pack and see if all the cells work OK. If not, the cell is hosed.
 

spitso

Member
the charger was supplied with the unit.
So you believe it might be the right cell. OK ill try my benchtop power supply to supply each cell with 4.2v, at what amps should i limit it to? Do you think i might need to disconect the cell from each individual circuit board?
 

bountyhunter

Well-Known Member
the charger was supplied with the unit.
So you believe it might be the right cell. OK ill try my benchtop power supply to supply each cell with 4.2v, at what amps should i limit it to? Do you think i might need to disconect the cell from each individual circuit board?
I would limit it to about 0.5A. I don't know if each cell has to be disconnected, I doubt it if you connect the PS directly to each cell.
 

Mr RB

Well-Known Member
You got it from Ebay? They are probably factory "seconds". The factory sell off bad batteries for peanuts, the typical fault is one bad cell. Then unscrupulous ebay resellers sell the factory seconds as working batteries. :(
 

bountyhunter

Well-Known Member
Another point: some of the Li cells (older chemistry) die if let to discharge too far. The original Sony cells did, but they changed the cathode chemistry to fix it. Last I heard, sanyo cells died if allowed to go down too far. There is a non-reversible "plating" that occurs on the cathode which eats up most of the cell capacity. Not sure if that's your cell, but if it won't take a charge, it's hosed.
 

spitso

Member
All of your assumptions seems correct especially Mr RB with concern to factory seconds, i believe this is the case because im pretty sure this has been happening from when i first got them. Ive connected my PSU straight on the right cell and set it on 4.2v at 0.5amps for around 3-4hours and it seems to be holding its charge while running the LED now, when i get some time ill see how it goes with a more current requiring job.
What im thinking is the charger might be possibly defective, theres a red(shows chargers on) and green LED(Shows charging is occuring) inside charger which only illuminates the green charging LED for about 30secs???? possibly the charger isnt fully charging the batteries capacity.
 

bountyhunter

Well-Known Member
Lithium battery cells are ruined if discharged below 3.0V.
Depends on the chemistry. When we worked with Sony in the mid 90's, I know they changed the chemistry of the cathode so the cells were no longer damaged by deep discharge. The reason I remember was that meant we needed two different protection IC's, one that shut off at both over V and under V, and the Sony one which shut off only at over V. I don't know whose cells do what these days, but I know some LI cells handle over discharge and some are destroyed.
 

bountyhunter

Well-Known Member
What im thinking is the charger might be possibly defective, theres a red(shows chargers on) and green LED(Shows charging is occuring) inside charger which only illuminates the green charging LED for about 30secs???? possibly the charger isnt fully charging the batteries capacity.
The charger can't possibly charge a series string correctly using constant voltage if the cells are unbalanced, ie one or more cell is significantly less charged than the others.
 

Diver300

Well-Known Member
Most Helpful Member
What im thinking is the charger might be possibly defective, theres a red(shows chargers on) and green LED(Shows charging is occuring) inside charger which only illuminates the green charging LED for about 30secs???? possibly the charger isnt fully charging the batteries capacity.
If the cells have got unbalanced, the protection circuitry for each cell should stop the cell that is on maximum from being charged, and should slow the charging of any really flat cell down to really slow. Either of these could cause the charger to say the batteries are charged as they have stopped taking current.

If the circuit is good, leaving it on charge for a day or so will eventually balance the cells.
 

bountyhunter

Well-Known Member
If the cells have got unbalanced, the protection circuitry for each cell should stop the cell that is on maximum from being charged, and should slow the charging of any really flat cell down to really slow. Either of these could cause the charger to say the batteries are charged as they have stopped taking current.

If the circuit is good, leaving it on charge for a day or so will eventually balance the cells.
I would be surprised if they included balancing in the protection ICs. We were asked to do that and there was no way because of power dissipation: The cell voltage is 4.2V full charge, for the IC connected around it to "shunt the current" to not overcharge the cell whil another cell takes charge means that little IC will dissipate :

P = 4.2 x I charge

Even at relatively low charge rates like 100 mA, that was way too much power dissipation for the tiny ICs that were small enough to go in the battery pack.

maybe they have a fix for that, but I suspect if there is a "balance" mode, it will take a long time because the charging current will be really low.
 

audioguru

Well-Known Member
Most Helpful Member
Buy a new battery from a real electronics parts distibutor, not a manufacturing reject that is sold on E-Bay.
 
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