Battery Balancing question

[Alan Quay]

Hi All,

Trying to balance up a 10ah lithium battery that has been stored unused for a long time. I have 10 cells, with voltages ranging between 3.99v and 4.16v. The one at 3.99 started at 3.97 v, but I have had a 5.2v mobile phone charger on it for an hour or so and its rising.

Question is, what sort of tolerance is acceptable before the BMS will allow it to charge normally? Or do I need to charge each cell to max voltage with the mobile phone charger to get it all balanced?

Here's the voltages:

Cell No	Volts
1	4.05
2	4.03
3	3.99
4	4.15
5	4.11
6	4.05
7	4.02
8	4.16
9	4.00
10	4.03

 

[Deleted member 4366]

It should allow charging as long as no cell is above 4.2v or below 2.9. Your problem is the high ones (4.16v and 4.15v), which will be at 4.2v as soon as you switch the charger on. It would speed things up if you could discharge them down to about 4.05v.

 

[Alan Quay]

Would I be better to connect to a bike and go and ride it a bit, then charge it and see?

 

[banbury frank]

Hi Mike each cell needs fully charging at 4.2 volts not 5.2 volts is to high

it is NOT the voltage it is how full the cell is it can take up to 10 hours to full a 10 AH cell charge each cell with a 1 amp charger

Frank

 

[Alan Quay]

Thanks Frank, but I only used the 5.2 v charger to get a little current flowing, monitoring the voltage all the time.

When I took the battery apart I found a bad weld between two cells. I assume this is why the battery was considered faulty. It was 36v before I fixed the weld, now its just over 40v.

I think the cells are all pretty close to full, so I'm hoping that the BMS will sort it out from there.

 

[jerrysimon]

As I understand it what is important is the state of charge of each cell. Voltages of each cell can be pretty close but the state of charge or how much capacity is in each one can vary wildly.

I always break my packs down into two blocks of 5sXp or 6sXp so I can then use an RC charger to bring each cell up to the same capacity.

If the capacity of your cells are very different then the BMS will struggle to equalize them.

You may have to tap onto each parallel block to bring them up to the same capacity or state of charge.

Regards

Jerry

 

[Deleted member 4366]

These chargers are perfect for dealing with such problems because they charge and balance through the balance leads. I have a connector with flying leads, so I can just tack them to whichever cells need sorting. You can make them whatever voltage you want automatically, and it'll also discharge cells to whatever voltage you want. You can do six in a row or any three random cells at a time.
BC168 Balance charger Charger super speed Li-ion/Li Pol | eBay
You can also use it to charge your lipos!

 

[Alan Quay]

Thanks all for the advice. I went with the 'run and see what happens' option, and after a three mile hard run the charger has kicked in. As ever, d8veh spot on. Will test and report back voltages shortly.

 

[Alan Quay]

After allowing the battery to fully charge, voltages are pretty much the same as before. Tomorrow I will put a load on the high cells, and bring them down to 4.05v each, and then put back on charge.

What would people consider a reasonable tolerance on balanced cells to be?

 

[hech]

Fully charged sic preferably within 0.1v, though 0.2v is prob fine. Remember the difference between 4v and 4.2v is less than 15% capacity whereas between 3.85v and 3.65v is over 35% - with lipo that is (ie not with lifepo), methinks.