LiFePo4 charging process

[mihai_atanasiu]

Hello,

I am new in LiFePo4 technology;

Can anyone tell me how occurs the charge process ? What does the BMS ?

For a 36V 20 AH battery made from 4P12S cells the charging process is like SLA batteries ?
I know that the BMS do something called "balancing";
In the first stage of the charging process the current flows through all the cells ?
When the cells rich 3.65 V the BMS signals to the charger to stop charging and begin the balancing process ?

Can anyone explain to me ?

thanks.

 

[deardancer3]

I think the charger pretty much operates as a power/current suppy, and does not participate in the balancing processs.

As the pack tells the charger to supply current, each cell gets its ration until that cell is full, then it just gets bypassed on the current supplied until all the other cells are full. At some point when the cells are all full, the pack's voltage and current draw are enough to tell the charger/power supply to go into "standby mode".

That is how the simple systems work. Others may have more sophistication.

d

 

[mihai_atanasiu]

Thanks for the answer;

I thought there is something like you say; and when the current drops near zero the charger signals the full charge;

 

[Ypedal]

Most chargers are a simple power supply as DD3 mentioned.. but the BMS handles the rest.

Most ( not all but most ) bms will allow the charger to supply the pack until one cell ( or group of parallel cells ) reaches 3.65v, then , it will start to drain power from that cell or group thru a resistor burning off energy as heat to prevent that cell from going above 3.65~3.8v while the others catch up... if the pack does not reach a specific voltage and the resistors canot disipate enough energy to prevent a cell from going over X voltage the BMS will shut down the charger until the voltage drops below limit... then charging resumes.. repeat until all cells are 3.65v :wink:

 

[dogman dan]

Great job on that explanation.

I'd just like to add, for the noobs, that you have to leave the battery plugged into the charger for awhile for the balancing to take place. If the battery needs a lot of balancing, this could be for all night, or even days. The charger will seem to be shut down, with the green light on. Then later after the bms bleeds down the too high charged cells, the charger may start up again for a breif time. This on and off cycle may need to go on for quite some time if the battey is out of whack much. It may seem like nothing is happening because you never catch the charger running. Some of the new bms have led's on them that tell you when each cell is fully charged. All the lights lit means you can unplug.

But fortunately, lifepo4 stays pretty balanced when treated nicely, so a properly sized pack for the bike and the ride length will balance up quick most of the time.

 

[mihai_atanasiu]

Thanks dogman.

I am another vpower unlucky buyer; the battery came very unbalanced; after two charges and discharges something
happens in the pack: it was some heat and after 10 minutes the charger turns off (the charging light turns from yellow to green);

I checked the cells and one was down; zero volts;

Now I am in discussions with vpower guy to send me a new string of 3.2 V;
I opened the pack and inspect all the cells and the welds seems to be good;

I removed the damaged string and try to measure it by cutting the tabs between the cells and measure each cell;
They are completely damaged; half of them are zero volts and others have 0.04 V and only one had about 1 V.

After I receive the new string of cells what I shall do ? Except that I have to rebuild the pack, to attach the new string to the
battery and put all together ?

Can anyone guide me ?
I have to charge the battery again or simply try to discharge it first and then charge ?
Like you said, Dogman, I have to leave the charger even 24 hours ? or maybe more ?

What can I do to force the battery to balance ?

Yesterday I contacted a guy from Russia that bought from vPower and he told me that his battery - 48V 20AH - also made from
18650 cells - works fine; and he measured that the capacity is over 20AH; he made 80 km with his 48V 500W ebike.

Sorry, I didn't introduce myself:


I have an Chinese ebike scooter with 36V 350W hub motor; It came with lead acid batteries;
I am from Romania, Bucharest.
I want to improve my rides with the LiFePo4 battery.

 

[dogman dan]

Oh boy. Ideally you'd get the new cells and the rest of the pack all to the same voltage, assemble it, and then charge it. If possible, by having the new cells charged with a small 3.5v charger if the rest of the pack is fully charged.

I'm puzzled though, how a string got that low if the bms was working. So you may have an underlying problem with the bms too. Perhaps the bad string was just discharged by some kind of short?

 

[mihai_atanasiu]

I think it was damaged from the beginning;
Now I have to wait about 10 days for the string cell;

 

[cell_man]

The BMS is not designed to balance cells that are at significantly different states of charge, if it was it would have a shunt current of at least 1A instead of the likely less than 100mA it has. It's best to charge each of the series cells to say 3.65V when you go in there if at all possible. The pack will likely not have the highest quality BMS and it is very hard on the BMS to be constantly shunting current for a very long time as it will have to. Another option is to either supply the charge from a current limited PSU and keep the current really low once the first of the cells gets up to charge, or you can put a resistor in line with the charger, this will current limit the charge voltage and take some of the strain off the BMS. Balancing the pack through the BMS will likely take a long time. To shunt 1Ah of difference could take 10hrs or more, all of which will be generating a nice little heater on that BMS...

 

[mihai_atanasiu]

cell_man you said to charge each string of cells; I have to buy an 3.6 LiFePo4 charger;
I will look to hobyking web site to find something;

 

[Ypedal]

https://p10.secure.hostingprod.com/@www.voltphreaks.com/ssl/catalog/product_info.php?cPath=3&products_id=28

 

[cell_man]

cell_man you said to charge each string of cells; I have to buy an 3.6 LiFePo4 charger;
I will look to hobyking web site to find something;

If you can borrow a small adjustable PSU from someone that would do. You can get small single cell chargers quite cheap also. A small RC charger is a handy thing to have if you are planning to do some messing about with batteries, but you'll need a PSU to go with it too.

There are other ways you can reduce the bulk of the state of charge differences. With a suitable sized resistor you can pull current out of the high cells quite quickly as they get up towards the balance voltage. A 1ohm resistor across a cell group will pull 3A and will need be over 10W rated, 20W would be better. A higher or lower resistance would be fine too, just make sure it can handle the pwoer. An hour or 2 doing this as the pack charges will get it done quite quickly and just stop the charger if too many cells get too high.

I just think that it's not always such a good idea to let a BMS do the job of balancing a very out of balance pack. A good quality BMS should be fine, but the V power stuff is very much built to a price, that's all.

 

[999zip999]

Mihai Ataanassic, Hay dude there mite be an easy way to find a single cell chargher to fit your needs. The trick is not read the infro on the side of the little charghers your have around your house. Yes those cell phone charghers and stuff. !00% checked the voltage of the chargher perfect would be 3.85v , ThisI is the level my cells hit before the bms drains then down I think ? If this is to high for V-power please chime in. I think you have 7 days before your cells arrive so you will have the answer. At this point chargh the new cell to the level of the pack. Do not go over 4.0v. This a point of no return . No alawys some by accident Have hit 4.2v and more and still work. Mine hit 4.1v. but knot the plan. Also after the cell hits full chargh it will over chargh very rapidly . That's if your bms is woking if knot I think the you should chargh to 3.6v and you are the bms. I throw all this out there so the experts can help. Good luck I'm from So Cal. can see the ocean from the living room.

 

[mihai_atanasiu]

cell_man can you draw a schematic with the PSU and resistors ?
I am not sure that I understand what you wrote;

I have a a 48V PSU with adjustable voltage and it can be down to 44V;
I also have some 1 ohm resistors but only 5 pieces;

 

[cell_man]

Well if you have 1ohm resistors and they have around 10W power handling each (a bit more would be better) you can simply use those to pull down the voltage on any cell that is high whilst charging. What I have done in the past is just use a multimeter and check the voltage on each series cell group in turn looking for any cells that are high and put a resistor across it to pull some charge out of it. I would use a much lower resistance, like 0.1ohm and this would pull about 30A out of a single cell so I wouldn't need to keep it on there continuously. If for example your charger is about 3 or 4A, if you put a 1ohm resistor in parallel with a single cell, all the charge current will be diverted into the resistor and the cell will not charge any more, giving the low cells some time to catch up.

My other suggestion of putting a resistor in series with the charger or using a PSU set to a low current limit will still require the bms to shunt alot of excess capacity, will take a long time and put the BMS under some strain. Applying a resistor across individual cells as they reach a high voltage, as I detailed above will be a quicker solution and will avoid the BMS having to shunt the excess capacity as you will be doing it manually with your resistor/resistors.

 

[mihai_atanasiu]

I know the concept with the resistors;

But I ordered a charger from HobbyKing:

http://www.hobbyking.com/hobbyking/store/uh_viewItem.asp?idProduct=7523

I don't want to have problems with the cells; For each string of cells that fault I have to wait about two weeks;
I already spent a lot of money;
With the Accucel8150 I will charge each cell;

 

[cell_man]

That looks fine. If you are gonna be messing around with batteries, an RC charger is a handy thing to have around for sure.

 

[mihai_atanasiu]

Hello,

I got the charger from post office; it was ordered Thursday and today arrived;
Now I have a problem: the power supply; I have an 18.5 V laptop charger; it is too much ? in the manual they say between 11 and 18 V;

have anyone tested ?

Also when I begin to charge the LiFe cells I want to charge each 3.2 cell instead of putting 8 cells in series;
The charger has a temperature sensor port but no sensor in the package; also the USB port is missing;

 

[BikeFanatic]

I have the Accucell charger also, I tried 3 PC supplies and none of them worked, I got the error code Over voltage I believe.
I did find a power supply around the house that worked, also Any 12 volt lead acid battery will run the charger as well.
Dogman runs it off an old 12 volt lead acid battery that is about dead However he keeps the lead battery hooked up to it's own charger and that works as a supply. ( got it?) Also your car battery could do it, if you have a car.

I use my accucell to charge the first cell group in a LiFePo4 battery - there is a setting for lifepo4 and the charger asks how many cells and you select one cell.

 

[mihai_atanasiu]

Problem solved.

I borrow a power supply 12V 10A and the charger works fine;
I made a test with a string of 18 cells; the charger pushed about 14AH in the cells;

After a charge what is the normal voltage of LiFePo4 cell ? Mine is 3.5 V; I will check the voltage after 12 hours;
before the charge the voltage was about 2.95 V;

The Accucell 8150 seems to be a interesting device; I want to test the discharge function also;

 

[dnmun]

what cell man recommended, is exactly what i do. i have a small 5 ohm 5W ceramic resistor with spade ends and i use jumpers and clip the alligator clips right on the terminal of the cell that is getting to 3.9V first, it doesn't take long, and then you can take the resistor back off. measuring all the cells at the same time to find the next one to reach 3.9V, and so on.

 

[mihai_atanasiu]

what cell man recommended, is exactly what i do. i have a small 5 ohm 5W ceramic resistor with spade ends and i use jumpers and clip the alligator clips right on the terminal of the cell that is getting to 3.9V first, it doesn't take long, and then you can take the resistor back off. measuring all the cells at the same time to find the next one to reach 3.9V, and so on.

3.9 V ? Isn't too much ? The Accucell charger has 3.6 V for Life cells;

 

[dnmun]

the signalab v1 and v2.5, and the headway, and the O2micro BMS all use the 3.9V HVC from my experience.

 

[mihai_atanasiu]

I made a discharge test with the Accucell 8150 on a 18650 string of 18 cells in parallel;
The start voltage was about 3.45 V;
The discharge current was 5A;
After 3 hours and a half I saw about 16 AH consumed from the cells; This is about 0.88 AH for each cell;
The theoretical capacity is 1.1 AH;
Now I am trying to charge the cells again and make then a second discharge test;

 

[mihai_atanasiu]

Yesterday vPower gave me the tracking number for the new cells; I have to wait almost two weeks for them;
Until then I want to charge and discharge each 3.2 V cell to see the real capacity; also I want to balance a group of 6 cells with
Accucell 8150;

I made also a charge and discharge to another cell and the result was: 18 AH charged; I have to check each cell see which has the low capacity; hope that the cells have about the same capacity;